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 also be known. The quality of the 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 live wiring, the procedure of which is to measure the resistance when using grounding.
The norms for regular checks of the existing insulation used in everyday life and in the production of electrical equipment are determined by special regulatory materials. Among them are GOST, PUE (rules for electrical installations) and others. In any case, the investigated insulation resistance must be measured with a megohmmeter. The device includes a voltage source, a source of additional resistances and a magnetoelectric ratiometer. A generator is taken as a voltage source, but a manually operated option is also suitable.
Since the megohmmeters work with a direct current source, it is possible to determine the insulation resistance at an overvoltage. We must not forgetthat if a megaohmmeter is connected to the unit under test, the resistance index of which is significantly underestimated, then the voltage at the output of the measuring device also decreases.
Insulation resistance should be measured step by step:
- There must be no voltage in the circuit under test.
- If the value of the circuit resistance is unknown, you need to set the measurement threshold to the maximum value.
- Short-circuit or disconnect all operating circuit elements at a low level of insulation, including capacitors, devices with semiconductors.
- Earth the circuit under test during measurement work.
- Apply voltage to the megger for one minute. Take readings on the instrument scale.
- After completing the measurements, disconnect the ends of the device from the circuit, remove the accumulated charge from the circuit by grounding.
It is better to measure the insulation resistance of wiring sections with a large capacitance value after the device needle has completely stopped fluctuating. Measurement in lighting and power networks is recommended to be performed in the mode of their inclusion, removed fuse-links and with power receivers disconnected from the influence of the network.
The rules prohibit measurements on lines laid near a high voltage line, and it is also forbidden to work during a thunderstorm. The temperature regime has a very noticeable effect on the insulation resistance. Measurement should be at a temperature of +5 degrees andabove.
There are direct current installations where the voltmeter acts as a control device as an insulation resistance meter with a large degree of internal resistance. In this case, they look at the indicators of three types of voltage: between the poles, between the ground and each pole.
Experienced electricians use the following models of electronic megohmmeters: F4101, F4102; they are adapted to work at a voltage of 100, 500 and 1000 V. Old types of megohmmeters are also in use: from M4100/1 to M4100/5 and MS-05.
