How to measure cable insulation resistance

One of the most important parameters that characterize any element of an electrical circuit is its electrical resistance. Its physical meaning is in the properties of the conductor to resist the passage of an 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 easily be calculated by knowing the voltage at the ends of the measured gap and the current flowing through it. To find out the resistance, it is enough to divide the voltage value by the current. This is the famous formula of Ohm (its generalized application).

In addition to direct calculations, in practical work with electricity, direct measurement of resistance is more often used. Special instruments are used for this. One of the most famous, allowing even an unfamiliar person with electrical engineering to perform a resistance measurement, is represented by a multimeter (digital or analog). The stylus of the device creates a certain voltage and current, knowing which, it is easy to determine the resistance and display its value in the form of digits on the display or the deflection of the indicating arrow.

The measurement of resistance is not a theoretical, but a practical one. Work related to the maintenance of electrical circuits requires frequent use of measuring instruments. Performing a periodic measurement of the resistance of their elements, you can ensure the continued operation of the entire installation.

Every person who is exposed to electrical installations is very familiar with the methods by which the cable insulation resistance is measured . Among them, the simplest and most accessible is the use of a special measuring device - an ohmmeter. It is more correct to call it a "megger" (megaohmmeter), since the insulation resistance of the cable, after all, is measured in thousands of ohms.

The insulation resistance test must be carried out under any suspicion of a change in its condition. If there is a local damage (mechanical, thermal, chemical), then a direct inspection is not enough. Checking insulation is necessary in case of water penetration between the insulating layers of the cable, as well as for prolonged non-use. A megohmmeter is required to perform the measurement, 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 the cable is used to transmit thousands of volts, then it can be considered normal (minimum allowable values), if the condition is 1 megaohm per 1 kilovolt. Lower voltages change the correspondence: 1 kilo to 1 volt.

Most meggerometers use a dynamo as the test voltage source, so you need to rotate the knob at the desired speed for the measurement. Insulation of the cable insulation is started from the spread of the cores on both sides of the section, if possible (in the case of connecting them on one side, the total resistance is checked, which is permissible, though less accurately). 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 is shown relative to the measured core and ground. If it is within the permissible limits, the following measurement is carried out: the resistance between the cores is checked. To do this, one probe touches an arbitrary vein, and the second - alternately to all the following. Rotating the knob, check the resistance value. When testing 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 it to a grounded metal object.