How to use a megaohmmeter?

An integral part and indicator of the electrical network is such a thing as insulation. The protective sheath of the wire or cable, the electrical isolator of the overhead line, the insulator of the terminals of the transformer and other devices prevent the electric current from contacting where we do not need. The insulating shell provides protection against short circuit, fire, breakdown on the body of an electrical device or machine, as well as protecting a person from electric shock. Nevertheless, insulation is subject to external factors such as time, sun, frost, water, mechanical wear, contact with aggressive environment. In order to identify a defect in time, there is a device - a megaohmmeter. How to use this device, we will describe further by providing a method for measuring insulation resistance with a megohmmeter.

The principle of operation of the device

The megaohmmeter generates voltage with its own high-voltage converter, and the milliammeter records the current in the measured circuit. From the school physics course we know Ohm's law, and the relationship between the resistance R, which is equal to U divided by I.

Currently, digital meters have gained distribution due to their compactness and lightness, but arrow models with a manual dynamo still go along with them. Now we will consider how to use the megaohmmeter of the old model and the new one.

Digital meter

Switch machine

We draw your attention to the fact that some call the device for measuring insulation resistance a megger. This is not a correct name, as if the word is divided in parts, the prefix is ​​"mega", the unit of measurement is "Ohm" and "meter" (from Greek it is translated as a measure).

User manual

Checking the insulation resistance is carried out on de-energized equipment or cable line, wiring. Remember that the device generates a high voltage and if safety measures are violated regarding the use of the megohmmeter, electrical injuries are possible, as measuring the insulation of a capacitor or a long cable line can cause a hazardous charge to build up. Therefore, the test is carried out by a team of two people who have an idea of ​​the dangers of electric current and have received a safety approval. During the test of the object, no unauthorized persons should be nearby. Remember about high voltage.

Do not include people working

The instrument is inspected for integrity at every use, for the absence of chips and damaged insulation on the test probes. Trial testing is carried out by testing with diluted probes and closed. If the tests are carried out by a mechanical device, then you need to place it on a horizontal level surface so that there is no error in the measurements. When measuring the insulation resistance with an old-type megohmmeter, you need to rotate the handle of the generator with a constant frequency, about 120-140 revolutions per minute.

Handle rotation

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

Insulation resistance is not constant and largely depends on external factors, so it can vary during measurement. The test is performed for at least 60 seconds, starting from 15 seconds the readings are recorded.

For household networks, tests are performed with a voltage of 500 volts. Industrial networks and devices are tested with a voltage in the range of 1000-2000 volts. What exactly is the measurement limit to use, you need to find in the instruction manual. The minimum allowable resistance value for networks up to 1000 volts is 0.5 MΩ. For industrial devices no less - 1MOhm.

As for the measurement technology itself, it is necessary to use a megohmmeter according to the procedure described below. As an example, we took the situation with measuring insulation in the AC (power shield). So, the procedure is as follows:

  1. We bring people out of the tested part of the electrical installation. We warn of danger, we put out warning posters.
  2. We remove the voltage, completely de-energize the shield, the input cable, take measures from erroneous supply of voltage. We post a poster - DO NOT TURN ON, PEOPLE WORK.
  3. We check the absence of voltage. Pre-grounding the conclusions of the test object, install the test probes, as shown in the connection diagram of the megohmmeter, and also remove the ground.This procedure is carried out with each new measurement, since nearby elements can accumulate a charge, introduce an error in the testimony and pose a danger to life. Installation and removal of the probes is done with insulated handles in rubber gloves. We draw your attention to the fact that the insulating layer of the cable must be cleaned from dust and dirt before checking the resistance.Device connection diagramPhase connection
  4. 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.
  5. We turn off all machines, RCDs, turn off the lamps and lighting fixtures, disconnect the zero wires from the zero terminal.
  6. We measure each line between phase and N, phase and PE, N and PE. The results are entered into the measurement protocol.
  7. If a defect is detected, we disassemble the measured part into its constituent elements, look for a malfunction, and eliminate it.

At the end of the test portable grounding remove the residual charge from the object, by short-circuiting, and the measuring device itself, discharging the probes between each other. Here, according to such instructions, it is necessary to use a megaohmmeter when measuring the insulation resistance of cable and other lines. To make the information more understandable, below we have provided a video that clearly demonstrates the measurement procedure when working with certain models of devices.

Video lessons

First of all, we bring to your attention the operating instructions for the ES0202 / 2-G switch megaohmmeter:

Working with an old model

Another popular dial meter, which is an analog of the above model, is the m4100. Using it is also quite simple, as you can see by watching this video:

How to use m4100

Digital megaohmmeters with a display are even easier to use. For example, to measure the insulation resistance of a cable with a modern meterUT512 UNI-T can be using this technology:

Digital Model User Manual

Well, the last instruction concerns another popular device - E6-32. The video below shows in sufficient detail how to use a megohmmeter to measure the insulation resistance of a transformer, cable, and even metal communications:

Application E6-32

Here, by this technique, they measure the insulation resistance with a megohmmeter.As you can see, using this device is not difficult, but you need to take safety measures seriously and take all necessary protective measures.

It will be interesting to read:

Working with an old model

How to use m4100

Digital Model User Manual

Application E6-32

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