If you are familiar with the topic of electricity (at least a little), then such a feature of electrical wiring as the insulation resistance of metal wires should be known. The quality of insulation determines the reliability of the wiring, ensures the operability of the operating system for the desired object. The operating rules indicate a mandatory periodic check of the required insulation level of the working wiring, the procedure of which is to measure the resistance when using grounding.
Standards for regular checks of existing insulation used in everyday life and in the production of electrical equipment are determined by special regulatory materials. Among them are GOST, PUE (electrical installation device rules) and others. In any case, the investigated insulation resistance should be measured with a megohmmeter. The device incorporates a voltage source, a source of additional resistances and a magnetoelectric logometer. A generator is used as a voltage source, but a variant with a manual drive is also suitable.
Since megaohmmeters work in the presence of a direct current source, it is possible to determine the insulation resistance at an overvoltage. We must not forget that in the case of connecting a megohmmeter to the test unit, the resistance value of which is significantly underestimated, the voltage at the output of the measuring device also decreases.
Measure the insulation resistance in stages:
- There should be no voltage in the circuit under test.
- If the value of the circuit resistance is not known, set the measurement threshold to the maximum value.
- Short-circuit or completely disconnect all working circuit elements with a low level of isolation, including capacitors, devices with semiconductors.
- Ground the circuit under test during measurement work.
- Apply voltage to the megger for a minute. Take readings on the scale of the device.
- After completing the measurements, disconnect the ends of the device from the circuit, remove the accumulated charge from the circuit by grounding.
Measurements of the insulation resistance of wiring sections with a large capacitance value are best done after the oscillation of the arrow of the device is completely stopped. Measurement in lighting and power networks is recommended to be performed in the mode of their inclusion, removed fusible inserts and with power receivers disconnected from the network influence.
The rules prohibit taking measurements on lines laid near a line under high voltage, and it is also forbidden to work during a thunderstorm. The temperature regime is very noticeable on the insulation resistance. It should be measured at a temperature of +5 degrees and above.
There are such direct current installations, where the voltmeter acts as a control device as an insulation resistance meter, which has a large degree of internal resistance. In this case, they look at the indicators of three types of stresses: between the poles, between the earth and each pole.
Experienced electricians use the following models of electronic meggers: F4101, F4102; they are adapted for operation at voltages of 100, 500 and 1000 V. Also old types of megohmmeters are in use: from M4100 / 1 to M4100 / 5 and MS-05.