One of the most important parameters characterizing any element of an electric circuit is its electrical resistance. Its physical meaning consists in the properties of the conductor to resist the passage of electric current. Depends on the structural features of the atomic structure of the material and external conditions. The numerical value of the electrical resistance of any part of the circuit can be easily calculated, knowing the voltage at the ends of the measured gap and the strength of the current flowing through it. To find out the resistance, it is enough to divide the voltage value by current. This is Ohm's famous formula (its generalized application).
In addition to direct calculations, in practical work with electricity, direct measurement of resistance is more often used. For this, special devices are used. One of the most famous, allowing even a person unfamiliar with electrical engineering to measure resistance, is represented by a multimeter (digital or analog). The probes of the device create a certain voltage and current, knowing which, it is easy to determine the resistance and display its value in the form of numbers on the display or deviations of the pointing arrow.
The measurement of resistance is not theoretical, but practical. Work related to the maintenance of electrical circuits requires frequent use of measuring instruments. Performing a periodic measurement of the resistance of their elements, it is possible to ensure continuous operation of the entire installation.
Everyone who encounters maintenance of electrical installations is very familiar with the methods by which the cable insulation resistance is measured . Among them, the most simple and affordable is the use of a special measuring device - an ohmmeter. It is more correct to call it a “megohmmeter” (megaohmmeter), since the cable insulation resistance, after all, is measured in thousands of ohms.
Checking the insulation resistance should be carried out in case of any suspicion of a change in its condition. If local damage occurs (mechanical, thermal, chemical), then a direct inspection is not enough. Insulation testing is necessary in case of water ingress between the insulating layers of the cable, as well as during prolonged non-use. To perform the measurement, a megger is required, the limits of which allow measuring the insulation resistance, and access to the stripped ends of the cable. The normal insulation resistance depends on the voltage at which the element is operated. For example, if a cable is used to transfer thousands of volts, then it can be considered normal (minimum permissible values), if the condition is 1 megaohm per 1 kilovolt. Lower voltages change compliance: 1 kilo-ohm per 1 volt.
Most megger meters use a dynamo as a source of test voltage, so for measurement it is necessary to rotate the knob at the desired speed. The cable insulation test begins with a spread of cores on both sides of the section, if possible (if they are connected on one side, the total resistance is checked, which is acceptable, although less accurate). One probe of the megger is alternately applied to the conductor of each core, and the second to the ground (mass).
When the handle of the device is rotated, the insulation resistance relative to the measured core and earth is displayed. If it is within acceptable limits, then carry out the following measurement: the resistance between the cores themselves is checked. To do this, one probe touch an arbitrary vein, and the second - in turn, to all the following. Turn the knob to check the resistance value. When checking with a megger, a high voltage is applied to the cable , so you can not touch it. After completing the measurements, each core should be discharged by touching a grounded metal object.