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To assess the performance of the cable, wiring, it is necessary to measure the insulation resistance. To do this, there is a special device – a megohmmeter. It applies a high voltage to the measured circuit, measures the current flowing through it, and displays the results on a screen or scale. How to use a megaohmmeter and consider in this article.
Design and function
Megaohmmeter – a device for testing insulation resistance. There are two types of devices – electronic and pointer. Regardless of the type, any megohmmeter consists of:
- DC voltage source.
- The current meter.
- Digital screen or measurement scale.
- Probes, through which the voltage from the device is transmitted to the object being measured.
It looks like a pointer megohmmeter (left) and electronic (right)
In pointer instruments, the voltage is generated by a dynamo built into the housing. It is driven by a meter – it turns the handle of the device with a certain frequency (2 revolutions per second). Electronic models take mains power, but can also run on batteries.
The operation of the megaohmmeter 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 uXNUMXd U / I, which is what the device does.
Before checking, the probes are installed in the corresponding sockets on the device, after which they are connected to the measurement object. During testing, a high voltage is generated in the device, which is transmitted to the object under test using probes. Measurement results are displayed in mega ohms (MΩ) on a scale or screen.
Working with a megaohmmeter
During testing, the megohmmeter generates 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 3rd are allowed to work in 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 shield or unscrew the plugs. Then turn off all semiconductor devices.
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 that, ground is connected to the tested circuits. To do this, a stranded wire in a sheath with a cross section of at least 1,5 mm2 is attached to the “earth” bus. This is the so-called portable grounding. For safer work, the free end with the bare 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 insulation resistance of a cable at home, you should familiarize yourself with the safety requirements before using a megohmmeter. There are several basic rules:
- Hold the probes only by the insulated and limited part.
- Before connecting the device, turn off the voltage, make sure that there are no people nearby (along the entire measured route, if we are talking about cables).
How to use a megaohmmeter: electrical safety rules - Before connecting the probes, remove residual voltage by connecting a portable ground. And turn it off after the probes are installed.
- After each measurement, remove residual voltage from the probes by connecting their bare parts together.
- After the measurement, connect a portable ground to the measured core, removing the residual charge.
- Work with 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 devices and are signed:
- E – screen;
- L- line;
- Z — land;
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 any). There is an “E” on the double tap of this probe. The plug that comes from this tap and is installed in the appropriate socket. Its second plug is installed in the socket “L” – line. A single probe is always connected to the ground socket.
There are stops on the probes. When taking measurements with your hands, take them so that your fingers are up to 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, two single probes are placed – one in the “Z” terminal, the other in the “L” terminal. Using crocodile clips at the ends, we connect the probes:
- To the wires under test, 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 the ground”.
There is the letter “E” – this end is inserted into the socket with the same letter
There are no other combinations. The insulation and its breakdown are checked more often, work with the screen is quite rare, since the shielded cables themselves are rarely used in apartments and private houses. Actually, using a megaohmmeter is not particularly difficult. It is only important not to forget about the presence of high voltage and the need remove residual charge after each measurement. This is done by touching the ground wire to the wire just measured. For safety, this wire can be fixed on a dry wooden holder.
Measurement process
We set the voltage that the megohmmeter will produce. It is not chosen arbitrarily, but from a table. There are meggers that work with only one voltage, 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. Switching of test voltage is made by the handle or the button on the front panel of the device.
| Item Name | Megger voltage | Minimum allowable insulation resistance | Notes |
|---|---|---|---|
| Electrical products and devices with voltage up to 50 V | In 100 | Must comply with the passport, but not less than 0,5 MΩ | During measurements, semiconductor devices must be shunted |
| too, but with voltage from 50 V to 100 V | In 250 | ||
| too, but with voltage from 100 V to 380 V | In 500-1000 | ||
| over 380 V, but not more than 1000 V | In 1000-2500 | ||
| Distribution devices, shields, conductors | In 1000-2500 | Not less than 1 MΩ | Measure each section of switchgear |
| Wiring, including lighting network | In 1000 | Not less than 0,5 MΩ | In dangerous rooms, measurements are taken once a year, in others – once every 3 years. |
| Stationary electric stoves | In 1000 | Not less than 1 MΩ | The measurement is carried out on a heated switched off stove at least once a year. |
Before using a megohmmeter, we make sure that there is no voltage on the line – with a tester or an indicator screwdriver. Then, having prepared the device (set the voltage and set the measurement scale on the arrows) and connecting the probes, remove the ground from the cable under test (if you remember, it is connected before starting work).
The next stage is to turn on the megaohmmeter: on the electronic ones we press the Test button, in the turnouts we turn the knob of the dynamo machine. In the turnouts, we twist until the lamp lights up on the body – this means the necessary voltage in the circuit has been created. In digital at some point the value on the screen stabilizes. The numbers on the screen are 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 passport for the product), then everything is normal.
After the measurement is completed, stop turning the handle of the megohmmeter or press the button to end the measurement on the electronic model. After that, you can disconnect the probe, remove the residual voltage.
In short – these are all the rules for using a megger. Let’s take a closer look at some measurement options.
Cable insulation resistance measurement
It is often required to measure the insulation resistance of a cable or wire. If you know how to use a megaohmmeter, it will take no more than a minute when checking a single-core cable, it will take longer to fiddle with stranded ones. The exact time depends on the number of veins – you will have to check each one.
You choose the test voltage depending on the voltage in the network with which the wire will work. If you plan to use it for wiring at 250 or 380 V, you can set it to 1000 V (see table).
To check the insulation resistance of a single-core cable, we hook one probe to the core, the second to the armor, apply voltage. If there is no armor, we attach the second probe to the “earth” terminal and also apply a test voltage. Let’s look at the evidence. If the arrow shows more than 0,5 MΩ, everything is normal, the wire can be used. If less, the insulation is broken and it cannot be used.
You can check the multi-core cable. Testing is carried out for each core separately. In this case, all other conductors are twisted into one bundle. If at the same time it is also necessary to check the breakdown to the “ground”, a wire connected to the corresponding bus is also added to the common harness.
If the cable has a screen, metal sheath or armor, these are also added to the bundle. When forming a tourniquet, it is important to ensure good contact.
Approximately the same is the measurement of the insulation resistance of the socket groups. Turn off all appliances from the sockets, turn off the power on the shield. One probe is installed on the ground terminal, the second – in one of the phases. Test voltage – 1000 V (according to the table). Turn on, check. If the measured resistance is greater than 0,5 MΩ, the wiring is OK. We repeat with the second core.
If the wiring is old-style – there is only a phase and zero, testing is carried out between two conductors. The parameters are similar.
Check motor insulation resistance
To carry out measurements, the motor is switched off from the power supply. It is necessary to get to the terminals of the winding. 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 in turn to each of the terminals. You can also check the integrity of the connection between the windings. For this test, probes must be installed on pairs of windings.