Insulation Resistance

Insulation Insulation ResistanceResistance

Insulation ResistanceInsulation Resistance

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

Insulation ResistanceInsulation Resistance

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

Measurement of Insulation Measurement of Insulation ResistanceResistance

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


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


The name comes from the fact that the The name comes from the fact that the insulating resistance of a properly-insulating resistance of a properly-designed appliance is in the range of tens designed appliance is in the range of tens and hundreds of meghoms. and hundreds of meghoms.


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


Meggers are equipped with three connection Meggers are equipped with three connection terminals, labeled terminals, labeled LineLine, , EarthEarth, and , and GuardGuard. .

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


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

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


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


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


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

Insulation Resistance

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