Let us distinguish the components of the megohmmeter:
- the generator of direct current;
- a measuring head assembled by the interaction principle of two frames (work and counter-action);
- toggle switch for measuring limits, making it possible to switch different resistance circuits to change the output voltage and the operating mode of the head;
- current limiting resistors.
Pretty simple construction contains no unnecessary elements. On the waterproof and robust dielectric housing of such a device are placed:
- handle for easy transport;
- the portable folding handle of the generator, which must be turned to produce a voltage;
- the lever of a switch for switching measurement modes;
- output terminals for wiring the connection circuit.
The megohmmeter is equipped with three output terminals, called:
- З – the earth;
- A is a line;
Grounding terminals and lines are used for all insulation resistance measurements about the ground loop and the screen terminal is designed to eliminate leakage currents between two parallel conductors of a cable or other similar active parts. For its inclusion in work, it is necessary to use a special construction measuring wire with shielded ends. They always complete the device in the factory. It has two terminals at one end, one of which is marked with the letter E. This pin is connected to the corresponding terminal of the megohmmeter.
The voltage generated by the mega-ohm generator from different models can be one of the following values: 100, 250, 500, 700, 1000, 2500 volts. And some devices work on the same band, while others have more than one. The output power of devices designed to control the insulation of high voltage industrial equipment can be many times greater than the characteristics of models designed to operate in domestic electrical installations. The dimensions of such devices will also be different.
For this reason, focusing on small models that can be kept in the jacket pocket cannot be justified in any case.
What to look for when working with a megohmmeter
The increased voltage of the device
The power output of the megohmmeter generator is sufficient not only to determine the occurrence of microcracks in the insulation layer but also to obtain severe electrical trauma. For this reason, the safety regulations allow the use of the device only by qualified and well-trained personnel authorized to work in live electrical installations. And that’s at least the third group on TB. The increased voltage of the device during the measurement is present on the test circuit, the connection wires and the terminals. To protect against this, special probes are installed on the measuring leads with a reinforced insulating surface. At the ends of the test leads, a safety zone is assigned with safety rings. You can not touch it with open parts of the body. Otherwise, you may find yourself under the influence of tension. To manipulate the probes, the hands are taken on the work surface. During the measurements, well-insulated alligator clips are used to connect to the circuit. Do not use other wires or probes.
During measurement, there should be no people on the entire test section. This is especially true when measuring the insulation resistance of long cables, which may be several kilometers long.
The energy that passes through the wires of the power lines has a large magnetic field which, by changing sinusoidally, induces an electromotive force and a secondary current in all the metallic conductors. Its value on long products can reach great value.
This factor must be taken into account for two reasons related to:
1. accuracy of the measurement;
2. security of the work staff.
The first reason is that when assembling a circuit for measuring the insulation resistance across a measurement megohmmeter, a current of unknown intensity and direction flows into the meter’s power supply. Its value will be added to the reading of the instrument from the voltage of the calibrated generator. Consequently, two unknown values of the current are added arbitrarily and create an insoluble metrological problem. The measurement of the resistance of electrical circuits under any voltage, and not only under that induced is therefore meaningless.
The second reason is that induced live work can lead to electrical injuries and require strict compliance with safety rules.
When the generator of the device supplies a voltage to the measured network, a potential difference is created between the electrical equipment bus or the line wire and the ground contour and a capacitance is formed which receives a load. After breaking the megohmmeter circuit by cutting the test lead, some of this potential is retained: the bus or wire has a capacitive load. It is only for a person to touch this area because he receives electrical trauma from the discharge current through his body. For this reason, it is necessary to take extra safety measures and constantly use a portable mass with an insulated handle to safely remove the capacitive voltage. Before connecting the megohmmeter to the circuit, whose insulation will be measured, it is always necessary to check the absence of voltage or residual load on it. Make it a tested indicator or a certified voltmeter of the appropriate names. After each measurement, the capacitive load is removed by a portable earthing using an insulating rod and other additional protective equipment.
Usually, a megohmmeter has to perform many measurements. For example, to draw a conclusion on the quality of the insulation of the cable with ten control conductors, it is necessary to check it concerning the ground and each core and between all the strands in turn. For each measurement, it is necessary to use a portable earthing. For fast and safe operation, one end of the grounding conductor is initially connected to the earth loop and left in this position until the job is completed. The second end of the wire is attached to the insulating rod and each time it is grounded to remove the residual charge.