One of the most important parameters characterizingany 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 be easily calculated, 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 practicalWork with electricity is often used to measure the resistance directly. For this purpose special devices are used. 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.
Measurement of resistance is not theoretical, butpractical nature. 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 comes acrossmaintenance of electrical installations, very familiar with the methods by which the insulation resistance of the cable 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" (megahmmeter), since the insulation resistance of the cable, after all, is measured in thousands of ohms.
Insulation resistance testPerform at any suspicion of a change in her condition. If there is local damage (mechanical, thermal, chemical), then direct inspection is not enough. Checking insulation is necessary in case of water entering between the insulating layers of the cable, as well as for prolonged non-use. To measure, 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 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 as a sourceThe test voltage is used by a dynamo, so you need to rotate the knob at the desired speed for measurement. Insulation of the cable insulation is started from the spread of the cores on both sides of the section, if possible (in case of their connection on one side, the total resistance is checked, which is permissible, though less accurately). One probe of the megohmmeter is alternately applied to the conductor of each core, and the second to the ground (mass).
When the knob is turned, theinsulation resistance 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. For this purpose, 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.
