To evaluate the performance of the cable, the wiring, it is necessary to measure the insulation resistance. For this, there is a special device – a megohmmeter. It provides a high voltage to the measured circuit, measures the current flowing through it and sends the results to a screen or scale. How to use a megohmmeter and consider in this article.
The device and the operating principle
Megohmmeter is a device for testing insulation resistance. There are two types of instruments: electronic and arrow. Whatever the type, any megohmmeter includes:
In arrow devices, the voltage is generated by a dynamo built into the body. It is powered by a meter – it turns the handle of the device with a certain frequency (2 turns per second). The network powers electronic models, but they can run on batteries.
The megohmmeter’s work is based on Ohm’s law: I = U / R. The device measures the current flowing between two connected objects (two cable conductors, conductor-ground, etc.). Measurements are made by calibrated voltage, whose value is known, knowing the current and voltage, you can find the resistance: R = U / I, which makes the device.
Before the test, the probes are placed in the appropriate support of the instrument, then connected to the measuring object. During the test, a high voltage is generated in the device, which is transmitted to the object under test using probes. The measurement results are displayed in mega-ohms (MΩ) on a scale or screen.
Work with a mega-meter
During the test, a megohmmeter produces a very high voltage – 500 V, 1000 V, 2500 V. It is therefore necessary to perform measurements very carefully. In companies to work in the device are allowed to people who have an electrical safety group not inferior to the 3rd.
Before measuring with a megohmmeter, the circuits tested are disconnected from the power supply. If you want to check the wiring status of the house or apartment, you must turn off the buttons or unscrew the plugs. After turning off all semiconductor devices.
If you check the outlet groups, remove the plugs from all the devices included in them. If the lighting circuits are checked, the bulbs are unscrewed. They do not support the test voltage. When checking the motor insulation, they are also completely disconnected from the power supply. After that, the ground circuits are connected to the circuits tested. For this purpose, a stranded cable with a cross-section of at least 1.5 mm 2 is fixed to the ground bus. This is called portable grounding. For safer operation, a free end with a bare conductor is attached to dry wood support. But the stripped end of the wire should be accessible – so that they can touch the wires and cables.
Requirements to ensure safe working conditions
Even if you want to measure the insulation resistance of a cable at home, before using the megohmmeter, it is helpful to become familiar with the safety requirements. There are several basic rules:
The rules are not very complicated, but your safety depends on it.
How to connect the test leads
The device usually has three slots to connect the probes. They are located at the top of the instruments and are signed:
- A is the line;
- З – the 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 hooks on the cable screen (if there is one). On the double-tap of this probe is the letter “E.” The plug that comes from this tap and is installed in the corresponding socket. The second socket is installed in the “L” socket – the line. Only one probe is always connected to the earth electrode.
On the stylets, there are stops. When making measurements by hand, grasp them so that the fingers reach these stops. This is a mandatory condition for safe operation (we remember high voltage).
If only the unshielded insulation resistance is to be verified, two single probes are placed, one in the “Z” terminal, the other in the “L” terminal. With the help of clip-crocodiles on the ends, we connect the probes:
There are no other combinations. The insulation is checked more often and its breakdown, work with the screen is rather rare, since the shielded cables in apartments and private homes are rarely used. The use of a megohmmeter is not particularly difficult. It is important not to forget the presence of high voltage and to remove the residual charge after each measurement. This is done by touching the ground wire to the just measured wire. For added security, this wire can be fixed on a dry wooden support.
We are adjusting the voltage, which will produce a megohmmeter. It is chosen not arbitrarily but from the table. Some megohmmeters work with a single voltage; there are several. The second, of course, is more practical because they can be used to test various devices and circuits. Switching the test voltage is done with a button or button on the front panel of the device.
|Name of the element||Mega-Meter Voltage||Minimum permissible insulation resistance||Notes|
|Electrical products and devices with a voltage up to 50 V||100 V||Must match the passport, but not less than 0.5 MΩ||During measurements, semiconductor devices must be shunted|
|also, but with a voltage of 50 V to 100 V||250 V|
|also, but with a voltage of 100 V to 380 V||500-1000 V|
|more than 380 V, but not more than 1000 V||1000-2500 V|
|Distribution boards, tables, current distributors||1000-2500 V||Not less than 1 MΩ||Measure each section of the equipment|
|Wiring, including the lighting network||1000 V||Not less than 0,5 MΩ||In hazardous areas, measurements are taken once a year, in others – every three years|
|Stationary electric heaters||1000 V||Not less than 1 MΩ||The measurement is performed on a disconnected plate heated at least once a year|
Before using a megohmmeter, make sure there is no voltage on the line – a tester or an indicator screwdriver. Then, after preparing the instrument (adjust the voltage and set the scale on the switches) and connect the test leads, remove the ground of the cable tested (if you remember, it is connected before starting the work ).
The next step – we include a mega-ohm meter in work: on the electronic press, the button Test, in the rotary, we twist the handle of the dynamo. In the switch, we turn until the lamp lights on the body – this means that the necessary voltage in the circuit is created. In digital at a certain point, the screen value does not stabilize. The numbers on the screen are the insulation resistance. If it is not less than the norm (the averages are shown in the table, and the exact ones are in the passport of the product), then everything is normal.
After the measurement is complete, stop turning the megohmmeter knob or click the end of the measurement button on the electronic model. After that, you can detach the probe, remove the residual voltage.
In short, these are all the rules for using a megohmmeter. We will consider some alternative measures in more detail.
Measure the insulation resistance of the cable
It is often necessary to measure the insulation resistance of a cable or wire. If you know how to use a megohmmeter, it will not take you more than a minute to test a single-strand cable. The exact time depends on the number of veins – you will need to check each one.
The test voltage is selected according to the network with which voltage the wire will operate. If you plan to use it for 250 or 380 V wiring, you can set 1000 V (see table).
Checking the three-wire cable – you can not twist and measure all pairs
To check the insulation resistance of a single conductor cable, a probe is attached to the core, the second to the armor and the voltage is applied. If there is no armor, attach the second probe to the ground socket and also apply a test voltage. If reading is greater than 0.5 MΩ, everything is OK; the wire can be used. If less – the insulation is perforated and cannot be used.
If it is necessary to check the multicore cable, tests are performed separately for each core. In this case, all other conductors are twisted into a bundle. If in this case, it is necessary to also check the failure at “ground,” a wire connected to the corresponding bus is also added to the common beam.
If the cable has a shield, a metal sheath or armor, they are also added to the harness. When forming the rope, it is important to ensure good contact.
In the same way as the insulation resistance of the bushing groups is measured. Outside outlets, turn off all devices, turn off the screen. A probe is installed on the ground terminal, the second – in one of the phases. The test voltage is 1000 V (according to the table). We include, we check. If the measured resistance is greater than 0.5 MΩ, the wiring is OK. Repeat with the second dwelling.
If the old wiring is only in phase and at zero, testing is done between the two conductors. The parameters are the same.
Check the insulation resistance of the motor
To make measurements, the motor is disconnected from the power supply. Arriving at the winding wires is necessary. Asynchronous motors operating at voltages up to 1000 V are tested with a voltage of 500 V.
To check their insulation, a probe is connected to the motor housing; the second is applied alternately to each of the conductors. You can also check the integrity of the winding connection. For this verification, the test leads must be installed on pairs of windings.
Megohmmeter – n the name of this device is composed of three words: “mega,” indicating the dimensionality of the measurement value (one thousand or 10 6), “ohm” – unit of electrical resistance, “meter” – short for measurement. It immediately becomes clear the technical purpose of the device: the measurement of electrical resistance in the megohmmic range. Often, Russian language experts correct this word, excluding the letter “a” under the pretext that two consecutive vowels are inconsistent. But this method distorts the meaning embedded in the device in the same way as the slang of the individual electricians – “merger.”
The principle of measurement of insulation resistance megohmmeter.
The apparatus is based on the famous Ohm law for the circuit section I = U / R. For its implementation in the case of any integrated modification:
- Constant and calibrated voltage source
- current meter
- output terminals
The design of the voltage generator can vary considerably and be created by simple hand-held dynamo machines, as in older models, or by using the energy of the integrated or external source. The output power of the generator, as well as the amplitude of its voltage, may include several ranges or are performed by a single fixed value. Connect the connecting wires to the terminals of the unit, the other end being connected to the measured circuit. For these purposes, generally, use forceps-like “crocodile.” The ammeter with an integrated electrical circuit measures the current flowing through the circuit. Taking into account that the voltage of the generator is already known and calibrated, the
This is the scale of the old device, tested by the 50-year life of the analog device. It allows measurements on two-scale limits:
If the megohmmeter is created using new technologies to process digital signals, then its display also displays a resistance, but in a more visual form.