Insulation Resistance

Insulation Resistance

Insulation Resistance The electrical resistance of an insulating material to a direct voltage. It is determined by measuring the leakage of current which flows through the insulation.

Insulation Resistance Insulation resistance is defined as the resistance (in megaohms) offered by the insulation to an impressed direct voltage. The resulting current is called insulation current. OR The resistance offered, by the insulation of a cable, in the path of leakage current, is called Insulation resistance of the cable.

Measurement of Insulation Resistance A Megger is an ohmmeter-type instrument by means of which the value of a resistance can be measured and directly indicated by the position of a pointer on a scale. The megger consists of two principal elements: a hand-driven magneto type direct current generator, which supplies the current for making the measurement; and the moving element with pointer, by means of which the value of the resistance under measurement is indicated.

Measurement of Insulation Resistance The megger is a portable instrument used to measure insulation resistance. The megger consists of a hand-driven DC generator and a direct reading ohm meter. A simplified circuit diagram of the instrument is shown in Figure 17.

Measurement of Insulation Resistance

Measurement of Insulation Resistance

Measurement of Insulation Resistance

Measurement of Insulation Resistance The name comes from the fact that the insulating resistance of a properlydesigned appliance is in the range of tens and hundreds of meghoms.

Measurement of Insulation Resistance Meggers are field instruments: that is, they are designed to be portable and operated by a technician on the job site with as much ease as a regular ohmmeter

Measurement of Insulation Resistance Meggers are equipped with three connection terminals, labeled Line, Earth, and Guard. To measure insulation resistance from a conductor to the outside of the cable, we need to connect the "Line" lead of the megger to one of the conductors and connect the "Earth" lead of the megger to a wire wrapped around the sheath of the cable:

Measurement of Insulation Resistance

Measurement of Insulation Resistance Resistance is measured between the Line and Earth terminals, where current will travel through coil 1. The "Guard" terminal is provided for special testing situations where one resistance must be isolated from another. Take for instance this scenario where the insulation resistance is to be tested in a two-wire cable:

To measure insulation resistance from a conductor to the outside of the cable, we need to connect the "Line" lead of the megger to one of the conductors and connect the "Earth" lead of the megger to a wire wrapped around the sheath of the cable:

Measurement of Insulation Resistance

Measurement of Insulation Resistance In this configuration the megger should read the resistance between one conductor and the outside sheath. Or will it? If we draw a schematic diagram showing all insulation resistances as resistor symbols, what we have looks like this:

Measurement of Insulation Resistance

Measurement of Insulation Resistance Rather than just measure the resistance of the second conductor to the sheath (Rc2-s), what we'll actually measure is that resistance in parallel with the series combination of conductor-to-conductor resistance (Rc1-c2) and the first conductor to the sheath (Rc1-s). If we desire to measure only the resistance between the second conductor and the sheath (Rc2-s), then we need to use the megger's "Guard" terminal:

Measurement of Insulation Resistance

Measurement of Insulation Resistance

Measurement of Insulation Resistance Connecting the "Guard" terminal to the first conductor places the two conductors at almost equal potential. With little or no voltage between them, the insulation resistance is nearly infinite, and thus there will be no current between the two conductors. Consequently, the megger's resistance indication will be based exclusively on the current through the second conductor's insulation, through the cable sheath, and to the wire wrapped around, not the current leaking through the first conductor's insulation.