22
PM5A2
13. Application Note 32
Use of “Guard” terminal in insulation testers:
When insulation resistance measurements are performed with insulation testers, especially with high sensitivity
instruments measuring high resistance values, the use of the
GUARD
terminal avoids the harmful influence of
stray resistances.
In order to better explain the function of this terminal, start by reviewing the insulation tester basic circuit diagram of
fig. 1.
Where:
Vt
:
DC high-voltage generator
Ri
:
Generator internal resistance
A
:
Indicator meter (micro-ammeter)
The unknown resistance (Rx) is connected between V and R terminals. Its value determines the current passing
through the circuit, which in turn is indicated by the micro-ammeter. The value of Rx can be determined as follows:
In many cases the resistance to be measured is in parallel with other stray resistances which influence on Rx
should be minimized.
A typical example of this situation occurs when measuring the insulation resistance between primary and
secondary windings of a transformer mounted inside a metal housing.
Rx: Insulation resistance between primary and secondary winding.
R1: Insulation resistance between primary winding and housing.
R2: Insulation resistance between secondary winding and housing.
