Insulation resistance measurement. General technique

The principle of measuring insulation resistance with a megohmmeter.

The operation of the device is based on the famous Ohm's law for the circuit section I=U/R. To implement it, any modification has the following built in inside the case:

• source of constant, calibrated voltage
• current meter
• output terminals

The design of the voltage generator can vary significantly and can be created on the basis of simple hand-held dynamos, as in older models, or by using power from a built-in or external source. The output power of the generator, as well as the value of its voltage, can include several ranges or be performed with a single, fixed value. Connecting wires are connected to the terminals of the device, the other end of which is connected to the circuit being measured. Alligator clips are usually used for these purposes. An ammeter built into the electrical circuit measures the current flowing through the circuit. Taking into account the fact that the generator voltage is already known and calibrated, the scale of the measuring head is immediately calibrated in converted resistance units - megaohms or kiloohms.

This is what the scale of an old analog device, proven by fifty years of operation, looks like. It allows you to take measurements at two scale limits:

If the megohmmeter is created using new digital signal processing technologies, then its display also displays the resistance, but in a more visual form.

How to connect a megohmmeter?

For each model of devices for this purpose, the output voltage value is determined, so in order to effectively test the insulation or measure its resistance, you need to choose the right megohmmeter.

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To check the cable insulation with a megohmmeter, a so-called extreme case is created, in which a voltage higher than the rated one is applied to the test section, but within the permissible standards specified in the technical documentation.

For example: a megohmmeter generator can produce:

• 100V;
• 250V;
• 500V;
• 700V;
• 1000V;
• 2500V.

Accordingly, the voltage supply should be an order of magnitude greater.

The duration of the measurement process usually does not exceed 30 seconds or minutes; this is necessary for more accurate detection of defects, as well as to eliminate their subsequent appearance during voltage surges in the network.

The basis of the technological process of measuring resistance is: preparation for the process, its implementation and the final stage. Each of them includes a certain list of manipulations necessary to achieve the goal without harm to others and, first of all, to oneself.

When preparing for work, you should organize your actions, study the electrical installation diagram to eliminate possible breakdowns, and also ensure your safety.

When starting work, you should first check the device for serviceability. To do this, the leads are connected to the measuring leads. Then their ends are connected to each other in an attempt to short-circuit. After applying voltage, the measurement readings are taken (they should be equal to zero). The next stage involves re-measuring. If there are no malfunctions, the reading should differ from the previous one.

Then they connect the portable grounding to the earth circuit, check and ensure that there is no voltage in the area, install the portable grounding, assemble the measuring circuit of the device, remove the portable voltage, remove the residual charge, disconnect the connecting wire, remove the portable voltage.

The final stage involves restoring the disassembled circuits, removing shunts and short circuits, as well as preparing the circuit for operating mode. The obtained results of measuring the resistance of the insulating layer are documented in the insulation verification report.

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Safety in measurements

Measurements with a megohmmeter always tell insulated conductors the charges, and the better the quality of the insulation, the longer the charge lasts. For safety reasons, be sure to remove these charges using wires with insulated handles. The connection points of the wires from the device are short-circuited and each of the conductors is additionally shorted to ground. There is only one goal - to remove all residual charges for the safety of people.

Measuring the insulation of electrical installations is easier than measuring lines and networks due to the concentration and proximity to personnel. Below is a step-by-step procedure for taking measurements on lines.

Types of megohmmeters

Today there are two types of megohmmeters on the market: analog and digital:

1. Analog (pointer megohmmeter). The main feature of the device is the built-in generator (dynamo), launched by rotating the handle. Analog instruments are equipped with a scale with an arrow. Insulation resistance is measured through magnetoelectric action. The pointer is attached to an axis with a frame coil, which is influenced by the field of a permanent magnet. When current moves through the frame coil, the needle deviates by an angle, the magnitude of which depends on the force and voltage. This type of measurement is possible thanks to the laws of electromagnetic induction. The advantages of analog devices include their simplicity and reliability, the disadvantages are their heavy weight and significant size.
2. Digital (electronic megohmmeter). The most common type of meters. Equipped with a powerful pulse generator operating using field-effect transistors. Such devices convert alternating current into direct current; the current source can be a battery or a network. The measurements themselves are carried out by comparing the voltage drop in the circuit with the resistance of the standard using an amplifier. The measurement results are displayed on the device screen. Modern models provide a function for storing results in memory for further comparison of data. Unlike an analog megohmmeter, an electronic one has compact dimensions and light weight.

Basic rules for the safe use of a megohmmeter

Verification and testing

Any work in electrical installations may only be carried out with working electrical devices. In relation to a megohmmeter, this means that it must simultaneously meet two requirements and be:

Testing means testing the resistance of its own insulation and all components in an electrical testing laboratory with increased voltage. Based on its implementation, the owner of the device is issued a certificate authorizing the operation of the megohmmeter for a certain, limited period. Verification is carried out by metrology laboratory specialists in order to determine the accuracy class of the device and apply a stamp on its body indicating that it has passed control measurements. The owner is obliged to take measures to ensure the safety of the applied stamp with the date and number of the verifier. If it disappears, the device is automatically considered faulty.

Types of jobs

The megohmmeter is selected for each measurement primarily based on the output voltage. It can perform two different types of checks:

2. measurement of the resistance of the dielectric layer

The first method involves creating an extreme case for the test area. For this purpose, it is supplied not with the rated voltage, but with an overestimated voltage provided for in the technical documentation. The test time is also chosen to be quite long. This allows you to timely identify all insulation defects and eliminate their manifestation during operation.

The second method uses a more gentle mode. The voltage for it is selected to a lower value, and the measurement time is determined by the duration of the end of the capacitive charge of the measuring section. For electrodynamic devices, it does not exceed a minute (that’s how long you need to turn the knob at a speed of 120÷140 rpm), and for electronic devices - about 30 seconds (keep the button pressed).

For example, measuring the insulation resistance of a particular electrical circuit must be done with a megohmmeter that produces 500 volts at the output. Then to test it you will need a 1000 V device.

Insulation measurements are carried out by electrical personnel of various professions, and the testing function is provided only to specialists from the insulation service laboratory. Quite often, the capabilities of a megohmmeter for these purposes are not enough for them, and they include in their work additional installations and sources of extraneous voltage that have higher powers and measuring capabilities.

Knowledge of the features of the circuit being tested

Before applying high voltage to the area being measured, it is necessary to take measures to prevent breakdowns and malfunctions of its components. Modern electrical equipment uses many semiconductor elements, various capacitors, measuring and microprocessor devices. They are not designed for the operating conditions created by the megohmmeter generator voltage. All such devices must be protected. To do this, they are removed from the circuit or shunted in a certain way. After the measurements are completed, the entire circuit must be restored and brought into working condition.

Rules for taking measurements with a megohmmeter

A megohmmeter refers to devices that measure the characteristics of electrical equipment related to determining the possibility of its safe operation. And at its conclusions during measurements there is a life-threatening voltage. Therefore, its use is possible in the following cases:

1. The device must undergo metrological verification once a year.
2. Only trained personnel are allowed to use the megohmmeter.
3. Only a licensed electrical laboratory has the right to issue a protocol with a conclusion on the suitability of electrical wiring for further use. Measurements taken by other persons have no legal force.

If you have a megohmmeter at your disposal, then you can measure the insulation resistance only on your own initiative. We finished installing the electrical wiring for our neighbor, measured it and made sure there were no defects. But if, when connecting your neighbor’s house to the network, the energy supply organization requires a measurement protocol, your work will not be counted. The neighbor will have to call specialists and pay them money for the same work.

In kindergartens, schools, institutions and enterprises, the insulation resistance of electrical wiring is measured regularly. The results are documented in protocols required by representatives of the fire department and energy supervision. The registration documents of the laboratory that performed the measurements are attached to the protocols. Without them, they are a useless piece of paper.

Insulation resistance measurement protocol

If a fire occurs in an organization’s premises, the first thing that is required from its managers is insulation measurement protocols. If there are none, the culprits are determined automatically. The same thing happens when an employee is struck by an electric shock. Even if he himself inserted a screwdriver into the socket, holding onto its rod. If, during the investigation of an accident, an insulation measurement protocol is not discovered, problems are guaranteed for management.

However, a megohmmeter is a useful device for people involved in electrical wiring. It is better to find the defect immediately, before specially trained personnel arrive. Otherwise, they will come again, after eliminating the defect. Laboratory personnel are not required to search for it themselves. When they return, they will force the owner to pay an additional amount for the work. Most likely, he will deduct it from your fee.

After replacing the electrical wiring in the apartment, insulation measurements are not officially required. Therefore, it doesn’t hurt to do them to reassure yourself, and in the eyes of the client, your rating will only increase in the end.

What to pay attention to when working with a megameter

Increased device voltage

The output power of the megohmmeter generator is quite enough to not only detect the appearance of microcracks in the insulation layer, but also cause serious electrical injury. For this reason, safety regulations permit the use of the device only by trained and well-trained personnel authorized to work in live electrical installations. And this is at least the third group for TB. During measurement, increased voltage of the device is present on the circuit under test, connecting wires and terminals. To protect against it, special probes are used, installed on measuring wires with a reinforced insulation surface. At the ends of the probes, a restricted area is marked with safety rings. It should not be touched with exposed parts of the body. Otherwise, you may be exposed to voltage. To manipulate the measuring probes, grasp the surface of the working area with your hands. During measurements, well-insulated alligator clips are used to connect to the circuit. The use of other wires and probes is prohibited.

There should be no people in the entire test area during the measurement. This is especially true when measuring the insulation resistance of long cables, the length of which can be several kilometers.

Induced voltage

The energy passing through the wires of power lines has a large magnetic field, which, changing according to a sinusoidal law, induces a secondary EMF and current in all metal conductors. Its value on extended products can reach large values.

This factor must be taken into account for two reasons:

2. safety of working personnel.

The first reason is that when assembling a circuit for measuring insulation resistance, a current of unknown magnitude and direction will flow through the measuring organ of the megohmmeter, caused by the induction of electrical energy. Its value will be added to the meter reading from the calibrated generator voltage. As a result, two unknown current values ​​are summed up in an arbitrary manner and create an insoluble metrological problem. Measuring the resistance of electrical circuits under any voltage, and not just induced voltage, is therefore generally meaningless.

The second reason is that working under induced voltage can lead to electrical injuries and requires strict adherence to safety rules.

Residual charge

When the generator of the device supplies voltage to the network being measured, a potential difference is created between the electrical equipment bus or line wire and the ground circuit and a capacitance is formed that receives a charge. After the megohmmeter circuit is broken by disconnecting the measuring wire, part of this potential is retained: the bus or wire has a capacitive charge. As soon as a person touches this area, he receives electrical injury from the discharge current through his body. For this reason, it is necessary to take additional safety precautions and always use a portable grounding device with an insulated handle to safely remove capacitive voltage. Before connecting a megohmmeter to a circuit whose insulation will be measured, it is always necessary to check that there is no voltage or residual charge on it. This is done with a tested indicator or a verified voltmeter of appropriate ratings. After each measurement, the capacitive charge is removed by portable grounding using an insulating rod and other additional protective equipment.

Typically, you need to take a lot of measurements with a megohmmeter. For example, in order to draw a conclusion about the quality of the insulation of a ten-core control cable, it is necessary to check it against the ground and each core and between all the cores in turn. For each measurement, it is necessary to use a portable grounding connection. For quick and safe work, one end of the grounding conductor is initially connected to the ground loop and left in this position until the work is completed. The second end of the wire is attached to an insulating rod and with its help, grounding is applied each time to remove the residual charge.

Working with a megohmmeter

During testing, the megohmmeter produces a very high voltage - 500 V, 1000 V, 2500 V. In this regard, measurements must be taken very carefully. At enterprises, persons with an electrical safety group of at least 3 are allowed to work with the device.

Before taking measurements with a megohmmeter, the circuits under test are disconnected from the power supply. If you are going to check the condition of the wiring in a house or apartment, you need to turn off the switches on the panel or unscrew the plugs. Then turn off all semiconductor devices.

One of the options for modern megaohmmeters

If you check the socket groups, remove the plugs of all devices that are included in them. If the lighting circuits are checked, the light bulbs are unscrewed. They will not withstand the test voltage. When checking the insulation of the motors, they are also completely disconnected from the power supply. After this, grounding is connected to the circuits being tested. To do this, a stranded wire in a sheath with a cross-section of at least 1.5 mm2 is attached to the “ground” bus. This is the so-called portable grounding. For safer operation, the free end with the exposed conductor is attached to a dry wooden holder. But the bare end of the wire must be accessible so that it can touch wires and cables.

Requirements for ensuring safe working conditions

Even if you want to measure the cable insulation resistance at home, before using a megohmmeter you should familiarize yourself with the safety requirements. There are several basic rules:

1. Hold the probes only by the insulated part limited by stops.
2. Before connecting the device, turn off the voltage and make sure that there are no people nearby (along the entire route being measured, if we are talking about cables). How to use a megohmmeter: electrical safety rules
3. Before connecting the probes, remove residual voltage by connecting a portable ground. And turn it off after the probes are installed.
4. After each measurement, remove residual voltage from the probes by connecting their bare parts together.
5. After the measurement, connect a portable ground to the measured core, removing the residual charge.
6. Wear gloves.

The rules are not very complicated, but your safety depends on their implementation.

How to connect probes

The device usually has three sockets for connecting probes. They are located at the top of the instruments and are labeled:

• E - screen;
• L-line;
• Z - earth;

There are also three probes, one of which has two tips on one side. It is used when it is necessary to exclude leakage currents and clings to the cable screen (if there is one). There is an “E” on the double tap of this probe. The plug that comes from this outlet and is installed in the corresponding socket. Its second plug is installed in the “L” socket - line. A single probe is always connected to the ground socket.

Probes for megaohmmeter

There are stops on the probes. When taking measurements, grasp them with your hands so that your fingers reach these stops. This is a prerequisite for safe operation (remember about high voltage).

If you only need to check the insulation resistance without a screen, place two single probes - one in the “Z” terminal, the other in the “L” terminal. Using crocodile clips at the ends, we connect the probes:

• To the wires being tested, if you need to check the breakdown between the wires in the cable.
• To the core and the “ground”, if we check the “breakdown to ground”. There is a letter “E” - this end is inserted into a socket with the same letter

There are no other combinations. The insulation and its breakdown are checked more often; working with the screen is quite rare, since shielded cables themselves are rarely used in apartments and private houses. Actually, using a megohmmeter is not particularly difficult. It is only important not to forget about the presence of high voltage and the need to remove the residual charge after each measurement. This is done by touching the ground wire to the wire you just measured. For safety, this wire can be secured to a dry wooden holder.

Measurement process

We set the voltage that the megohmmeter will produce. It is not selected randomly, but from a table. There are megohmmeters that work with only one voltage, and there are those that work with several. The latter, of course, are more convenient, since they can be used to test various devices and circuits. The test voltage is switched using a knob or button on the front panel of the device.

Before using a megohmmeter, we make sure that there is no voltage on the line using a tester or an indicator screwdriver. Then, having prepared the device (set the voltage and set the measurement scale on the dials) and connected the probes, remove the grounding from the cable being tested (if you remember, it is connected before starting work).

The next stage is to turn on the megohmmeter: on the electronic ones we press the Test button, on the pointer ones we turn the dynamo handle. We turn the switches until the lamp on the body lights up - this means the necessary voltage has been created in the circuit. In digital, at some point the value on the screen stabilizes. The numbers on the screen indicate the insulation resistance. If it is not less than the norm (the averages are indicated in the table, and the exact ones are in the product data sheet), then everything is normal.

How to take measurements with a megohmmeter

After the measurement is completed, we stop turning the knob of the megohmmeter or press the end of measurement button on the electronic model. After this, you can disconnect the probe and remove the residual voltage.

In short, these are all the rules for using a megohmmeter. Let's look at some measurement options in more detail.

Effect of residual stress

When the megohmmeter generator produces voltage entering the measured network, a voltage occurs between the wire and the ground loop. This leads to the formation of a capacitance endowed with a certain charge.

Once the test lead is disconnected, the megohmmeter circuit becomes open. Due to this, the potential is partially preserved, since a capacitive charge is created in the wire or bus. If this area is touched, a person may receive electrical injury from a discharge of current passing through the body. In order to avoid such troubles, you should. Its handle must be insulated, which makes it possible to safely remove capacitive voltage.

Before connecting a megohmmeter to measure insulation, it is necessary that there is no residual charge or voltage in the circuit being tested. For this, there are special indicators or a voltmeter with the appropriate rating. Using a megohmmeter you can perform a wide variety of measurements. For example, the insulation in a ten-core cable is first checked against ground, and then each core is measured. The quality of insulation is determined in turn between all cores. A portable ground should be used during each measurement.

To ensure fast and safe operation, the grounding conductor is initially connected at one end to. He remains in this position until the end of work. The other end of the conductor is in contact with the insulating rod. It is with her direct participation that grounding is applied to remove the residual charge.

Checking cable insulation using a megohmmeter

Insulation resistance is the most important parameter of cable performance, and as soon as the resistance drops below a certain level, the cable is considered unusable and must be replaced immediately. In this article I will talk about the reasons leading to the deterioration of insulation and how to correctly check its level using a megohmmeter.

Table of contents

Why is insulation getting worse?

Safety precautions when working with a megohmmeter.

Checking the performance of the megohmmeter.

How to understand that the insulation has become unusable.

Why is insulation getting worse?

There are a number of factors that influence the value of insulation resistance, namely:

1. Atmospheric conditions. If the cable is constantly surrounded by moisture, then even a microcrack in the insulating material will cause the insulation resistance to deteriorate sharply. That is why, in rainy weather, electrical appliances connected via a cable with poor insulation may simply not work.

2. Incorrect cable installation. If the insulating material is damaged when laying the cable, then even a new cable (if dampness forms) may show a low insulation resistance.

3. Insulation obsolescence. Whatever one may say, even the highest quality wire with extremely reliable insulation will become unusable over time due to constant exposure to the environment.

In order to identify a problematic cable in time and prevent an emergency, a device such as a megohmmeter is used to periodically check the condition.

There are both mechanical and electronic measuring instruments. Next, I will talk about the process of checking the cable with a mechanical Megaohmmeter ES0202/2-G.

Safety precautions when working with a megohmmeter

To carry out a safe inspection, the Labor Safety Rules during the operation of electrical installations (as amended by Order of the Ministry of Labor of Russia dated February 12, 2016 No. 74n) contain the following requirements:

Checking the functionality of the megohmmeter

Before direct insulation measurements, it is necessary to check the functionality of the measuring device itself. To do this, follow these steps:

— Take the device out of the case and carefully inspect its probes. You should not find any damage to the insulating material on them;

— Then we insert the probes, set the regulators as shown in the picture and scroll the knob several times and make sure that the arrow tends to show infinite resistance;

— The next step is to close the probes together (with the help of crocodiles) and also make several turns and make sure that the arrow shows a zero value;

So, having made sure that the measuring apparatus is in full working order, you can proceed to further actions.

Checking the cable insulation

1. Before checking, disconnect the cable from the electrical installation on both sides and ground it.

2. Then we connect the megohmmeter to the core being measured and the ground loop (or to two adjacent cores, if we are checking the insulation resistance between the cores), while the device itself must be installed on a horizontal surface.

Note. Depending on the position of the switch, the ES0202/2-G Megaohmmeter is capable of measuring resistance up to 50 and up to 10,000 MOhm.

3. Next, remove the grounding from the measured conductors.

4. We begin to turn the knob and monitor the indicators of the device. Moreover, if we measure a high-voltage cable, then we set the voltage regulator to 2,500 V.

If at the first limit the readings of the device go off scale, then we transfer it to the second limit and now the upper scale will be included in the readings.

5. Then we record the readings. And then, using a special jumper (an ordinary piece of wire will do), we remove the residual charge from the core being measured (connecting it to the ground) and install the grounding back.

6. All measurements of this particular core or cores are considered completed. Measuring the other ends of the cable occurs in exactly the same way. But according to the operating conditions of this megohmmeter, the break between each measurement should be equal to two minutes.

In this case, the choice of voltage for testing is regulated by the PUE, 7th edition, clause 1.8.7

How to understand that the insulation has become unusable

According to the requirements of technical documentation, the lower insulation limit after which cable replacement is inevitable is 0.5 MOhm

But for better orientation in the degree of cable insulation quality, you can use the following table

This will be quite enough to understand the degree of wear of the insulation of a particular cable.

That's all I wanted to tell you about checking cable insulation using a megohmmeter. If the article was interesting or useful to you, please rate it with a like.

What is a megaohmmeter, scope of application and principle of operation

A megohmmeter is a special meter used to measure high resistance values. The main difference from traditional ohmmeters is that measurements are carried out at a significant voltage level, independently generated by insulation meters.

The functioning of insulation resistance meters is explained by Ohm's law, which operates in a section of an electrical circuit: I=U/R. The main components installed inside the housing are represented by a voltage source having a constant and calibrated value, as well as a current meter and terminal outputs.

The connecting wires are fixed to the terminals using conventional alligator clips, and the current values ​​of the electrical circuit are measured using the ammeter present. Some models are characterized by the presence of a scale with two types of values ​​or numbers displayed on the screen.

Operating principle of a megohmmeter

Megohmmeters are used to measure insulation resistance, as well as to determine the insulation absorption coefficient of electrical equipment that is not under operating voltage conditions. Insulation resistance meters are classified depending on the typical features of the circuit and indication method.

Digital models are cheaper devices, while analog devices are expensive, but are distinguished by high accuracy of measurements. The main area of ​​application is currently represented by production and distribution systems of electrical energy, control systems for the operation of electrical equipment in industry, laboratories and in the field. In everyday life, such devices are not in great demand.

Test conditions

Requirements for the environment and device

Checking the resistance of cable sheaths should be carried out in enclosed spaces, where the air temperature is at least +15-+35 °C. At the same time, the humidity inside the building should not exceed 80 percent.

These requirements are determined by the general provisions of regulations and in each specific case may have slightly different meanings. The parameter measured during testing (leakage resistance) can be determined in several ways. But in any case, its measured value must significantly exceed the normalized value (at least 20 times).

Device and principle of operation

A megohmmeter is a device for checking insulation resistance. There are two types of devices - electronic and pointer. Regardless of the type, any megohmmeter consists of:

• Constant voltage source.
• Current meter.
• Digital screen or measurement scale.
• Probes, through which voltage from the device is transmitted to the object being measured.

This is what a pointer megaohmmeter looks like (on the left) and an electronic one (on the right)

In pointer instruments, the voltage is generated by a dynamo built into the housing. It is driven by a meter - it rotates the handle of the device with a certain frequency (2 revolutions per second). Electronic models take power from the mains, but can also run on batteries.

The operation of the megohmmeter is based on Ohm's law: I=U/R. The device measures the current that flows between two connected objects (two cable cores, core-ground, etc.). Measurements are made with a calibrated voltage, the value of which is known; knowing the current and voltage, you can find the resistance: R=U/I, which is what the device does.

Approximate diagram of a magaohmmeter

Before testing, the probes are installed in the appropriate sockets on the device, and then connected to the object being measured. During testing, a high voltage is generated in the device, which is transmitted to the object being tested using probes. Measurement results are displayed in mega ohms (MΩ) on a scale or screen.

The main types and brands of megohmmeter devices from my practice (design and principle of operation)

Megaohmmeter ESO-210

4. Let's start with the simple ones. So, the first participants in today's parade are the Ukrainian devices ESO 210/3 and ESO 210/3G. The letter “G” indicates that the device is powered by an internal generator and has a handle. The model without a handle operates from a 220V network and from a button. They are small in size and easy to use. These are faithful assistants to energy workers. They are convenient for testing any electrical equipment. You can also take one of the ends after the test and ground it with it, because the ends on both sides have metal tips. In models with a handle, an alternating current generator acts as a voltage source, in models with a button - a transformer that converts alternating voltage into direct voltage.

So, let's go through the device settings. The device can be tested by applying a constant voltage of 500, 1000 or 2500 Volts. The readings appear on a dial scale, which has several limits that are switched by a switch. This is a scale of “I”, “II” and “IIx10”.

Scale “I” – lower numbers of the upper scale. The countdown goes from right to left. Values ​​from 0 to 50 MOhm.

Scale “II” – the upper numbers of the upper scale. The countdown goes from left to right. Values ​​from 50 MΩ to 10 GΩ.

Scale “IIx10” – similar to scale “II”, however, values ​​​​from 500 MOhm to 100 GOhm.

The device also has a lower scale from 0 to 600 V. This scale is available in the ESO-210/3 device and, when the voltage supply button is not pressed, shows the voltage at the ends. In general, we brought the ends of the megohmmeter to the socket, and the needle rose to 220V. But you just need to connect them correctly to measure voltage, not insulation resistance. One on the zipper and the other on the Ux.

When voltage is applied, the red light on the scale lights up, which indicates the presence of voltage at the ends of the device.

How to connect the probes of the device? We have three holes for connecting probes - a screen, a high voltage and a third measuring hole (rx, u). In general, two probes are paired and one of them is signed. It is not easy for an attentive person to make a mistake.

Megaohmmeter sonel mic-2510

Let's step further and focus our attention on a powerful Polish device called Sonel - megohmmeter mic-2510. This megohmmeter is digital. Outwardly, it is very nice, the kit includes a bag in which alligator type probes (quite powerful and reliable) and plug-in ones are folded. In addition, the kit includes a charger. The device itself runs on a battery, which is quite convenient. No network connection is required and no rotation of the knob is required, as with older models of domestic megohmmeters. There is also a tape for comfortable placement on the neck. At first it didn’t seem very convenient to me, but eventually you get used to it and realize all the advantages. In addition to a reliable battery, the advantages include the ability to supply voltage without holding the button. To do this, first press start, then “enter” and that’s it – watch the readings and don’t let anyone get under voltage.

This device can measure the following quantities using a two-wire and three-wire method. The three-wire method is used for measurements where it is necessary to exclude the influence of surface currents - transformers, cables with a screen.

The device can also measure temperature using temperature sensors, voltage up to 600 volts, and low-resistance contact resistance.

The device scale has values ​​of 100, 250, 500, 1000, 2500 Volts. This is a wide enough range to suit engineers' needs for a wide range of testing needs. From absorption coefficient to polarization coefficient. The maximum measurable insulation resistance that the device can measure is 2000 GOhm - an impressive value.

The polarization coefficient characterizes the degree of aging of the insulation. The smaller it is, the more worn the insulation is. The polarization coefficient is 2500V and we measure the insulation resistance after 60 and 600s or after 1 and 10 minutes. If it is more than two, then everything is fine, if from 1 to 2, then the insulation is questionable, but if the polarization coefficient is less than 1, it’s time to sound the alarm. Western chief engineers do not welcome high-voltage tests, by the same AID, but are happy to conduct a vigilante test at 5 kV or 2.5 kV with the measurement of this coefficient.

The absorption coefficient is the ratio of the insulation resistance after 60 and 15 seconds. This coefficient characterizes the moisture content of the insulation. If it tends to unity, then it is necessary to raise the issue of drying the insulation. More details about its value for different types of equipment are described in the electrical equipment testing standards of your country.

In the process of work, I came across other devices, but these two show how far progress has come in the production of megaohmmeters. Each of the devices I have seen has its pros and cons.

Check the insulation resistance of the electric motor

To carry out measurements, the engine is disconnected from power. It is necessary to get to the winding terminals. Asynchronous motors operating at voltages up to 1000 V are tested with a voltage of 500 V.

To check their insulation, we connect one probe to the motor housing, and apply the second one to each of the terminals in turn. You can also check the integrity of the connection between the windings. For this check, probes must be installed on pairs of windings.

Electrical networks are characterized by various parameters. One of the most important parameters of networks is electrical insulation. Insulation is any material that prevents electrical current from flowing in the wrong direction. Insulation can be the protective sheath of wires and cables. Devices such as insulators prevent conductive lines from contacting the ground. All these measures to isolate conductive parts are aimed at preventing short circuits, fires, or electric shock to humans.

How to use a megohmmeter correctly?

To carry out testing, it is important to correctly set the measurement ranges and test voltage level. The easiest way to do this is to use special tables that indicate the parameters for the various objects being tested. An example of such a table is given below.

Table 1. Correspondence of the voltage level to the permissible insulation resistance value.

Let's move on to the measurement technique.

Safe operation of the megohmmeter

Any measurements should be made only with a working megohmmeter. The device must be tested in a laboratory where its own insulation and all components are checked. An increased voltage is used for testing, after which the megohmmeter is given permission to operate for a certain, limited period.

For verification purposes, the megohmmeter is sent to a metrology laboratory, where specialists determine its accuracy class. Passage of control measurements is confirmed by a stamp applied to the body of the device. During further operation, the safety and integrity of the mark must be maintained, especially the date and number of the specialist who carried out the verification. Otherwise, the device will automatically fall into the faulty category.

The correct application area also ensures safety when working with the megohmmeter. Before each measurement, the output voltage value is determined. The device is primarily used for insulation testing. For this purpose, extreme conditions are created for the area being tested, when not the rated voltage, but an overestimated voltage is supplied. The time period is also quite long. This contributes to the timely identification of possible defects and their prevention in subsequent operation.

Each circuit to be tested has its own characteristics that affect the safe operation of the megohmmeter. Therefore, before applying high voltage to the desired area, it is necessary to eliminate all malfunctions and breakdowns of the constituent elements. Modern equipment is literally saturated with semiconductors, capacitors, measuring and microprocessor instruments. They are not designed for the high voltage generated by the megohmmeter generator. Before testing, all such devices are bypassed or completely removed from the circuit. At the end of the measurements, the circuit is restored and brought into working condition.

TB and documentation

Luxometer: light measurement

In terms of compliance with safety rules when handling high-voltage measuring equipment, it is necessary to focus on the following important points:

• It is prohibited to begin any test work unless you are completely sure that the object has been completely de-stressed;
• Before starting measuring operations, you should inspect it and make sure that there are no working personnel on the lines connected to this section of the cable;
• “High voltage” warning signs should be placed along the entire cable route undergoing testing;
• During all measurements, touching live parts with exposed parts of the body is strictly prohibited.

At the end of the test, residual electricity should be removed by briefly grounding these parts.

Portable grounding

After checking the cable insulation with a megohmmeter is completely completed, a documentary report should be prepared, which should contain such mandatory items as:

• Date and place of their holding;
• Composition of tested and measuring equipment;
• The results of the measurements taken, documented in a protocol drawn up in a special form.

In conclusion, we note that insulation resistance tests for strength can only be carried out under conditions of constant ambient temperature and the absence of moist fumes. During rain or thunderstorms, working with voltages above 1 kilovolt is prohibited.

How to check a megohmmeter

Before starting the measuring work, an operation is performed to check the serviceable condition of the device and its leads; for this, the wires connected to the device are short-circuited and the generator handle is rotated, the arrow should show “0”; the short circuit is in the “I” switch position. When checking, while connecting the wires, you must not touch them with bare hands, as you may receive an electric shock.

How to use a megohmmeter
or the sequence of measuring work:

1. Connecting a megohmmeter to the resistance measurement sockets.
2. Connecting the grounding conductor to the screen (casing) socket.
3. Setting the switch to the desired measurement range, there are two of them, the higher the power of the equipment, the larger the measurement range.
4. We check the operation of the device by closing the measuring probes while simultaneously rotating the handle.
5. After connecting the measuring cords, rotate the handle of the megohmmeter (power generator), the speed should be at least 120 rpm.
6. Moving the measurement arrow to a specific position is the start of the measurement report.
7. To reduce the time it takes to measure resistance with a megohmmeter on scale II, short-circuit the resistance socket (before starting the measurement) and rotate the device knob for about 5 seconds.
8. After using the megohmmeter, set the switch to the neutral position.

The permissible error in the operation of the megohmmeter is 0.05 Mohm + -15%. The additional error limit associated with the presence of industrial-frequency currents in the form of interference in the measurement circuit is about 500 μA. The device can be operated at temperatures ranging from 30 to +50 o C. At the terminals there is a measuring voltage of the megohmmeter from 500 to 2500V, depending on the measurement range used, therefore, upon completion of the measurement, it is necessary to discharge the generator by touching the “ground” with the measuring probes or short-circuit them for a second, between each other, until an electric discharge occurs.

Evaluation of test results and their frequency

Measuring the resistance of the grounding device

The value of the controlled parameters is determined by the characteristics of the object under study and its functional purpose. According to the requirements of the PUE, the insulation resistance for low-voltage (up to 0.4 kV) cable lines and electric motor wiring should not be less than 0.5 MOhm.

The same parameter for high-voltage equipment (more than 1000 Volts) is 1 MΩ, and for overhead cable lines it cannot be less than 10 MΩ. For a comparative assessment of the insulation state of conventional cable routes, you can use the table below.

Insulation condition assessment

The specified values ​​of standardized indicators are valid for any weather conditions. The frequency of testing procedures is determined by current regulations and depends on the characteristics and condition of the object being examined. All issues related to the tests themselves (maximum stresses, procedure and timing of measurements), as well as assessments of their final results, are discussed in detail in the PTEEP.

According to these standards, the quality of insulation of cables for lighting, crane and elevator equipment must be checked at least once a year. The same procedures for portable welding units and electric motors are supposed to be organized every six months.

Any violations of the inspection frequency specified by the standards may lead to disruption of the normal operation of cable or wire lines, and, as a result, cause damage to the equipment connected to them.

How to measure cable insulation resistance

Most often it is necessary to measure the insulation resistance of cables. How to use a megohmmeter in this case? If the cable is already in use, it is disconnected from the power supply and the load connected to it is removed. There are several types of changes:

1. Each cable core in relation to all the others, united in a bundle and connected there with an earth wire.

This is how the cable insulation condition is measured

Points 2 and 3 are performed if the results of the first measurement are below normal. These measurements are simple, but if you have lived a lot, they take a lot of time. It’s good that electricians mainly use three-core wires, and only when supplying a three-phase network can there be more of them.

How to use a megohmmeter: this is how you measure the insulation resistance between two wires in a cable

When measuring on the panel, all machines are switched to the “off” position, the load is removed, and then measurements are taken. In this case, you don’t have to remove the wires from the sockets, but touch the contact screws with probes. Be careful: the input line on the input machine (connected to the upper sockets) cannot be measured without turning off the power at the substation.

If the cable is shielded (there is a metal braid of wire, steel or aluminum tapes), install a probe with a forked tip, and the screen is added to the bundle to the wires and the “ground”.

Preparatory work

Testing of insulation resistance does not proceed until the exact air temperature in the room where the work is to be done is known.

This is explained by the fact that if there are particles of frozen water in the cable, and the ambient temperature is below 0°C, the measurement result will give an incorrect value, since ice is not electrically conductive.

What you will need

Measurements are carried out using a special device - a megohmmeter. According to state regulations, this can be a device from such series as: ESO202, F4 100, F4 102/2, M4 107/1, etc.

Measuring the insulation of an asynchronous motor with a megohmmeter

Before measurements, turn off the power and remove the residual voltage. Then you need to gain access to the winding terminals. We attach one dipstick to the engine body. Make sure that the contact is with clean metal - you need to find an area without paint or rust. When checking, we connect the second probe to each of the windings (you also need to make sure that it is clean under the “crocodile”.

According to the table, asynchronous motors connected to a 220 V or 380 V network are tested at a voltage of 500 V.

Work with a megaohmmeter can be carried out without a work permit only in electrical installations up to 1 kV. In this article we will tell you what safety rules are required when using it.

From the article you will learn:

Resistance control

Checking the insulation of cable products is carried out by measuring its resistance with a device specially designed for this purpose (megaohmmeter).

Before you start working with this tool, you need to become familiar with the reasons for the deterioration of cable insulation, which usually manifest themselves in the following:

• Voltage variability in power supply lines;
• The destructive effect of solar UV radiation (for objects laid openly);
• Sudden temperature fluctuations;
• Exposure to aggressive environments (when hidden in the ground).

Regardless of the condition of the cable's protective sheath, measuring the insulation resistance with a megohmmeter is carried out at a certain frequency, determined by current standards. The results of testing activities, with a certain accuracy, make it possible to determine the causes of damage to the insulating sheath, and, under certain conditions, to detect damaged sections of the cable.

When testing, you should be guided by the current measurement methods, taking into account the conditions of their conduct, as well as the basic methods of working with measuring equipment. In addition, the guidelines stipulate the parameters of the test mode of the megohmmeter (the value of the rated current supplied to the controlled cable in particular).

Megaohmmeter

Megaohmmeter device

A typical megohmmeter consists of a direct current generator, a measuring head, a toggle switch and current-limiting resistors. The operation of the measuring head is based on the interaction of the working and counteracting frames. The toggle switch can be set to certain measurement limits. It switches various resistor chains that change the output voltage and operating mode of the head.

All elements are enclosed in a durable, sealed dielectric case, equipped with a handle for more convenient carrying. There is also a portable folding generator handle located here. To begin generating voltage, it unfolds and rotates. On the case there is a toggle switch control lever and three output terminals to which connecting wires are connected. Each output has its own designation: “G” - ground, “L” - line and “E” - screen.

Terminals “Z” and “L” are used in all cases where it is necessary to measure the insulation resistance in relation to the ground loop. Terminal “E” is necessary to eliminate the impact of leakage currents when measuring between cable conductors located in parallel or similar current-carrying parts. Terminal “E” works in conjunction with a special test lead that has shielded ends. It is usually connected to a casing or screen. This terminal provides the most accurate measurements. In some models, terminals “L” and “Z” are designated by the corresponding markings “rx” and “-”.

The principle of operation of megohmmeters using internal or external generator power supplies is the same as that of designs with a handle. In order to output voltage to the circuit being tested, you must press the button and hold it in this state. There are devices that can produce different combinations of voltages by combining several buttons.

Modern megohmmeters have a more complex internal structure. The voltage produced by generators of different designs is approximately a range of values: 100, 250, 500, 700, 1000 and 2500 V. Some megohmmeters can operate only in one range, while others can operate in several at once.

The output power value of a megohmmeter capable of checking insulation on high-voltage industrial equipment is many times higher than the same parameter for megohmmeter models capable of checking only household wiring. Their sizes also differ markedly from each other.

Megaohmmeter: principle of operation and design of the device

What is a megohmmeter, why is it called that and what is the purpose of its use? If we decipher this word, we will see that its part “mega” means the measurement value, “ohm” means a unit of electrical resistance, and “meter” means to measure. Thus, it becomes clear that a megameter is a device that tests electrical resistance.

Internal structure of the megohmmeter:

• Current generator;
• Measuring head;
• Measuring range switch;
• Current limiting resistors.

To perform a measurement, the device supplies current to the circuit being tested, and it must be constant. Alternating is not suitable here, since cable lines have capacitive reactances, and capacitors can conduct alternating current, which will lead to distortion of measurement results.

Types of megohmmeters, based on voltage:

• 100 volts – needed to check the insulation of low-voltage wires;
• 500 volts – for low-power electric machines;
• 1000 volts – for household lighting fixtures and socket modules;
• 2500 volts – for high-voltage devices and overhead lines.

The most popular models of devices are: ES0202/2G, M1101M, M4100, F4101, ESO 202/2G, electronic ut512UNI-T.

You can also ring an electric motor with a megameter to check the integrity of its windings. But basically, testing the engine or any other equipment is carried out with another device - a multimeter.

However, which device is suitable for what can be read in the technical documentation of electrical equipment.

By the way, some megohmmeters show the result after just a few seconds, while the true result is the resistance shown 60 seconds after the start of the test. Moreover, they do not have the ability to generate voltage over a long period. This is also bad, since in a short time you may not be able to see all the wiring defects.

How to measure resistance with a megohmmeter

Measuring the insulation resistance of any type of cable line with a megohmmeter is almost the same with some specific differences. To understand what differences there are in each case, let’s look at all three of them separately.

So, first of all, the cable is checked to ensure there is no voltage on it. For this purpose, special high voltage indicators are used. After which the measuring device itself is connected to the conductors from the side where the insulation is checked. On the other hand, the conductors are separated to a certain distance, legalized by the PUE. By the way, it is on this side that it is necessary to place a person who will act as a watchman so that the curious do not decide to touch the protruding wires with their bare hands. Be sure to post posters everywhere stating that tests are being carried out.

Now you can carry out testing. To do this, each core is checked. That is, two free ones are grounded, and one terminal of the megohmmeter is connected to the one being tested, and its second terminal is connected to ground (grounding). Next, measure the resistance with a 2500 volt megohmmeter. The duration of the test is one minute. Others are checked in the same way.

The preliminary stages are exactly the same here. But the measurement scheme itself is very different from the one described above. There are several connection and testing schemes in low-voltage lines. Here they are, taking into account the markings of the cores (A; B and C).

• First, the cores are tested against each other. That is, A-C, A-B and C-B.
• Next, a check is made between each core and zero. That is, N-A, N-B and N-C.
• Then between the conductors and the ground loop. That is, PE-A, PE-B, PE-C.
• And the resistance of the zero circuit must be checked. In this case, the megohmmeter is connected according to the N-PE scheme. Do not forget that in this case the zero must be disconnected from grounding.

Testing of control cable systems

The insulation resistance of cable control systems is measured using the same technology with the only difference. That is, first the absence of voltage on the conductors is determined and the megohmmeter is set to check 500-2500 volts.

One end (output) of the device is connected to the end of the cable under test, the other to ground. The remaining conductors are interconnected and connected to the ground loop. You can connect the second output of the megohmmeter to one of the free wires. The check is carried out within one minute. All cable cores are checked in the same way.

The results obtained must be recorded and subsequently compared with the tabulated ones. Tables can be found in the PUE and PTEEP. If the actual value is not lower than the table value, then the cable being tested can continue to be used. By the way, based on the tests carried out, a conclusion must be drawn and a protocol must be drawn up, which indicates the actual testing indicators.

User manual

The insulation resistance is checked on de-energized equipment or cable lines or electrical wiring. Remember that the device generates high voltage and if the safety precautions for using a megohmmeter are violated, electrical injuries are possible, because Measuring the insulation of a capacitor or long cable line can cause a dangerous charge to accumulate. Therefore, the test is carried out by a team of two people who have an understanding of the dangers of electric current and have received safety clearance. During testing of the object, no unauthorized persons should be nearby. Remember about high voltage.

Each time the device is used, it is inspected for integrity, for the absence of chips and damaged insulation on the measuring probes. Trial testing is carried out by testing with open and closed probes. If tests are carried out with a mechanical device, then it must be placed on a horizontal, flat surface so that there is no error in the measurements. When measuring insulation resistance with an old-style megohmmeter, you need to rotate the generator handle at a constant frequency, approximately 120-140 rpm.

If you measure resistance relative to the body or ground, two probes are used. When testing the cable cores relative to each other, you need to use the “E” terminal of the megohmmeter and the cable screen to compensate for leakage currents.

Insulation resistance does not have a constant value and largely depends on external factors, so it can vary during measurement. The check is carried out for at least 60 seconds, starting from 15 seconds the readings are recorded.

For household networks, tests are carried out with a voltage of 500 volts. Industrial networks and devices are tested with voltages in the range of 1000-2000 volts. You need to find out exactly what measurement limit to use in the operating instructions. The minimum permissible resistance value for networks up to 1000 volts is 0.5 MOhm. For industrial devices, no less than 1 MOhm.

As for the measurement technology itself, you need to use a megohmmeter according to the method described below. As an example, we took the situation with measuring insulation in a switchboard (power panel). So, the procedure is as follows:

We remove people from the part of the electrical installation being tested. We warn about the danger and post warning posters. We remove the voltage, completely de-energize the shield and input cable, and take measures against erroneous voltage supply. We hang up a poster - DO NOT TURN ON, PEOPLE ARE WORKING. We check that there is no voltage. Having previously grounded the terminals of the test object, we install the measuring probes, as shown in the megohmmeter connection diagram, and also remove the grounding. This procedure is carried out with each new measurement, since nearby elements can accumulate a charge, introduce an error in the readings and pose a danger to life. Installation and removal of probes is carried out using insulated handles while wearing rubber gloves.

Please note that the insulating layer of the cable must be cleaned of dust and dirt before checking the resistance.

We check the insulation of the input cable between phases A-B, B-C, C-A, A-PEN, B-PEN, C-PEN. The results are recorded in the measurement protocol. We turn off all automatic devices, RCDs, turn off lamps and lighting fixtures, disconnect the neutral wires from the neutral terminal. We measure each line between phase and N, phase and PE, N and PE

The results are entered into the measurement protocol. If a defect is detected, we disassemble the measured part into its component elements, look for the fault and fix it.

At the end of the test, we use portable grounding to remove the residual charge from the object, by short-circuiting, and from the measuring device itself, discharging the probes among themselves. According to these instructions, you need to use a megohmmeter when measuring the insulation resistance of cable and other lines. To make the information more clear to you, below we have provided videos that clearly demonstrate the order of measurements when working with certain models of devices.

Purpose and principle of operation

Topic: “measuring angles with a theodolite and determining distances using a rangefinder”

A megohmmeter is a measuring device designed to measure the insulation resistance of wires and other live parts and circuit elements. Its sensitivity is sufficient to indicate in which part of the line the insulation of the wires (or terminals) has become so old that it can break through under the influence of high voltage. An insulation breakdown in high-voltage networks and electrical circuits is a source of step voltage, the current of which flows into the ground, and a possible fire.

In addition to the measuring head and power supply, the megohmmeter includes a range switch for the resistance being measured and additional resistors, which set this limit. To connect the device, you need terminals through which it is connected to the circuit or line being measured using probes with wires insulated from each other. To ensure reliable connection, the ends of the probes connected to the circuit being measured are equipped with “crocodiles” that vaguely resemble a clothespin. The use of a crocodile makes it possible to establish reliable electrical contact. For power supply, without which the resistance of the medium being measured cannot be measured, either a separate AC adapter or a battery or accumulator is used. It, in turn, produces a voltage that is taken into account in the calculation of resistance.

Unlike (kilo) ohmmeters, where the voltage supplied to the circuit being measured does not exceed one or several volts, this value in megohmmeters is set in the range of 50-5000 V, which forces measurers to use dielectric gloves, a rubber carpet and shoes with the same sole, and securely insulated instrument. The principle of operation of the megohmmeter, like its brother - the ohmmeter, is based on the application of Ohm's law, which guides all electricians and power engineers. According to this law, knowing the voltage (or EMF of the power source) and measuring the leakage current, it is possible to determine the current resistance of the medium through which this current passes. Before measurements are taken, the section of the line or circuit on which the resistance is measured is disconnected from the general circuit.

Based on the measurement results, the effective value of the electrical strength is calculated (in volts per meter of insulator thickness, but a multiple unit is used - kilovolts per millimeter of dielectric layer). There is an approximate standard for the minimum number of megaohms into which any of the electrical insulators tested for “breakdown” must fit. For example, the material of the kinescope board - textolite, in now outdated televisions did not have a volumetric resistance of less than 100 megaohms per millimeter of thickness (between the printed tracks), since a voltage of 4 to 25 kilovolts was applied to the kinescope electrodes.

The leakage current must be such that it can be neglected, that is, an order of magnitude less than its operating value. However, GOST 183-74 does not dictate clearer values ​​for the volumetric and surface resistance of a dielectric - specific requirements for it are calculated by design engineers at the stage of system and circuit design of a particular device. If the total resistance of the tested section of the line or circuit is less than this value, the insulator must be replaced before the current leakage becomes noticeable or leads to breakdown of the insulator and a short circuit.

Types of testers

When operating electrical devices, digital megohmmeters of the model: F4101/4102 from 100.0 to 1000.0 V are widely used. Installers still work with tester brands M4100/1, 4100/5 and MS-05 m from 100.0 to 2500.0 V. The choice of megger size is based on rated resistance of the device under test: power cables and transformers, machines and insulators.
To determine the insulation condition in electrical installations up to 1000.0 V, it is allowed to use megohmmeters from 100.0-1000.0 V, and in installations over 1000.0 V - 1000.0-2500.0 V. Devices are also classified according to the generated voltage and resistance limits in MOhm:

• 500.0 V - 500.0;
• 1000.0 V - 1000.0;
• 2500.0 V - 2500.0.

Additional Information. The devices also differ in accuracy classes. The popular M4100 model has an error of no more than 1%, and the F4101 brand has an error of up to 2.5%. The selection of electrical installation testing devices is carried out taking into account acceptable performance indicators.

Electronic meter

Electronic meter

A digital or electronic tester is a modern type of equipment, equipped with a powerful generator with field-effect transistors. Measurements are made by comparing the voltage drop in a reference circuit with a fixed resistance. The results are shown on the panel. The test results saving function accumulates data for subsequent analysis. This model differs from analog devices in its compact size and light weight. Advantages of a digital tester:

• High level of accuracy, allows you to determine resistance over large sections of the circuit;
• convenient, easy-to-read digital panel;
• technological accessibility for measurement by one user;
• works great even in very busy spaces;
• convenient and safe to use.

Disadvantages of the electronic type megger:

• Requires external power source;
• high prices for products.

Electromechanical meter

Electromechanical device

These models have an analog display on the front panel of the tester and a hand crank used to rotate and generate voltage that passes through the electrical system.

Advantages of a manual megger:

1. Remains important in today's high-tech world, remaining the oldest method for determining resistance values.
2. No external source is required for operation.
3. Low prices on the market.

Disadvantages of a manual megger:

1. At least 2 people are required for operation, one to rotate the handle, the other to connect the megger to the electrical system being tested.
2. Low measurement accuracy.
3. Requires a large free space for placement.
4. Provides an analog measurement result.
5. High requirements for safety during use.

Circuit design features:

1. Deflector and Control Coil - Connected in parallel with the generator, mounted at right angles to each other and maintained in polarity so as to produce torque in the opposite direction.
2. Permanent magnets create a magnetic field to deflect the pointer using a North-South magnetic pole.
3. Pointer - one end connected to the coil, the other deviates on a scale from infinity to “0”.
4. A scale is provided at the top of the megger from the zero to infinity range and allows the user to read the value.
5. Connecting a direct current (DC) source or battery.
6. The test mode is generated by a generator for a manually controlled megger. A battery or electronic charger is provided for the digital megger for the same purpose.

Note! Current coil resistance helps protect the tester from any damage during testing due to low external electrical resistance

User manual

The insulation resistance is checked on de-energized equipment or cable lines or electrical wiring. Remember that the device generates high voltage and if the safety precautions for using a megohmmeter are violated, electrical injuries are possible, because Measuring the insulation of a capacitor or long cable line can cause a dangerous charge to accumulate. Therefore, the test is carried out by a team of two people who have an understanding of the dangers of electric current and have received safety clearance. During testing of the object, no unauthorized persons should be nearby. Remember about high voltage.

Each time the device is used, it is inspected for integrity, for the absence of chips and damaged insulation on the measuring probes. Trial testing is carried out by testing with open and closed probes. If tests are carried out with a mechanical device, then it must be placed on a horizontal, flat surface so that there is no error in the measurements. When measuring insulation resistance with an old-style megohmmeter, you need to rotate the generator handle at a constant frequency, approximately 120-140 rpm.

If you measure resistance relative to the body or ground, two probes are used. When testing the cable cores relative to each other, you need to use the “E” terminal of the megohmmeter and the cable screen to compensate for leakage currents.

Insulation resistance does not have a constant value and largely depends on external factors, so it can vary during measurement. The check is carried out for at least 60 seconds, starting from 15 seconds the readings are recorded.

For household networks, tests are carried out with a voltage of 500 volts. Industrial networks and devices are tested with voltages in the range of 1000-2000 volts. You need to find out exactly what measurement limit to use in the operating instructions. The minimum permissible resistance value for networks up to 1000 volts is 0.5 MOhm. For industrial devices, no less than 1 MOhm.

As for the measurement technology itself, you need to use a megohmmeter according to the method described below. As an example, we took the situation with measuring insulation in a switchboard (power panel). So, the procedure is as follows:

We remove people from the part of the electrical installation being tested. We warn about the danger and post warning posters. We remove the voltage, completely de-energize the shield and input cable, and take measures against erroneous voltage supply. We hang up a poster - DO NOT TURN ON, PEOPLE ARE WORKING. We check that there is no voltage. Having previously grounded the terminals of the test object, we install the measuring probes, as shown in the megohmmeter connection diagram, and also remove the grounding. This procedure is carried out with each new measurement, since nearby elements can accumulate a charge, introduce an error in the readings and pose a danger to life. Installation and removal of probes is carried out using insulated handles while wearing rubber gloves.

Please note that the insulating layer of the cable must be cleaned of dust and dirt before checking the resistance.

We check the insulation of the input cable between phases A-B, B-C, C-A, A-PEN, B-PEN, C-PEN. The results are recorded in the measurement protocol. We turn off all automatic devices, RCDs, turn off lamps and lighting fixtures, disconnect the neutral wires from the neutral terminal. We measure each line between phase and N, phase and PE, N and PE

The results are entered into the measurement protocol. If a defect is detected, we disassemble the measured part into its component elements, look for the fault and fix it.

At the end of the test, we use portable grounding to remove the residual charge from the object, by short-circuiting, and from the measuring device itself, discharging the probes among themselves. According to these instructions, you need to use a megohmmeter when measuring the insulation resistance of cable and other lines. To make the information more clear to you, below we have provided videos that clearly demonstrate the order of measurements when working with certain models of devices.