Report on

Guard Terminal on Insulation Resistance Test

By Joe Marie Valcarcel

The function of Guard on Insulation Resistance is to shunt out the parallel or unwanted leakage current.

Parallel Leakage Current can be removed thus precise result can be achieved. One practical use of this

Guard terminal is on the testing of Conductors with insulation material. Insulation of conductors

degrades and deteriorates overtime and the insulation resistance test reads the leakage current that

pass thru this deteriorated insulation. But in addition to leakage through the insulating material, there

can also be current drawn across the surface of material. In order to separate out the surface leakage,

positive and earth terminal will be connected to a conductor each. Guard terminal will be connected to

its sheath. Current traveling across the surface from one terminal to the sheath will be intercepted and

shunted back through the guard circuit. This circuit returns to the tester’s transformer, bypassing the

measurement function. It is effectively shunted out of the measurement. Accordingly, the tester

measures only the leakage getting through the insulating material, from conductor to the other

conductor.