Insulation resistance: the need for measurements

If you are familiar with the topic of electricity (at least a little), then this feature of electrical wiring should be known as the resistance of insulation of metal wires. The quality of insulation determines the reliability of the wiring, ensures the operability of the operated system for the required object. The operating rules indicate a mandatory periodic check of the required insulation level of the operating wiring, the procedure of which is to measure the resistance when using grounding.

The norms of regular inspections of the existing insulation of electrical equipment used in everyday life and at production are determined by special normative materials. Among them are GOST, PUE (rules for electrical installation devices) and others. In any case, the insulation resistance should be measured by a mega-meter. The device has in its composition a voltage source, a source of additional resistances and a magnetoelectric logometer. As a source of voltage take the generator, but the option is suitable and with a manual drive.

Since megaohmmeters work in the presence of a constant current source, it is possible to determine the insulation resistance at an overestimated voltage. It should be remembered that in case of connecting a megaohmometer to the unit under test, the resistance index of which is significantly underestimated, then the voltage at the output of the measuring device is also lowered.

Measure insulation resistance in stages:

1. Voltage in the tested circuit must be absent.
2. If the resistance value of the circuit is unknown, you must set the measurement threshold to the maximum value.
3. To short-circuit or completely disconnect all operating circuit elements at a low level of insulation, including capacitors, devices with semiconductors.
4. Ground the test circuit during the measurement work.
5. Apply voltage to the megger for a minute. Take the reading on the scale of the device.
6. After completing the measurements, disconnect the ends of the device from the circuit, remove the accumulated charge from the circuit by grounding.

Measurements of insulation resistance of wiring sections with a larger value of capacity should be made after the instrument has completely stopped oscillating. Measurement in lighting and power networks is recommended to be carried out in the mode of their inclusion, removed fusible inserts and with the electric receivers disconnected from the influence of the network.

It is forbidden to carry out measurements on lines laid near a line under high voltage, and it is forbidden to work during a thunderstorm. The temperature regime is very noticeable in the insulation resistance. Measure should be at a temperature of +5 degrees and above.

There are such direct current installations where as a monitoring device the voltmeter acts as an insulation resistance meter with a high degree of internal resistance. In this case, we look at the indices of three kinds of stresses: between the poles, between the earth and each pole.

Experienced electricians use the following models of electronic megger: F4101, F4102; They are adapted to work at voltages of 100, 500 and 1000 V. Also in the course of the old types of megger: from M4100 / 1 to M4100 / 5 and MS-05.