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10. Application note 32
Use of "Guard" terminal in insulation testers
When insulation resistance measurements are performed with insulation testers,
specially 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, let us start reviewing the
insulation tester basic circuit diagram of Fig. 1.
Ri
i
Vt
The unknown resistance (Rx) is connected between Vt and +R terminals. Its value
determines the current passing through the circuit, which in turn is indicated by the
microammeter. 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 is when the insulation resistance between
primary and secondary windings of a transformer mounted inside a metal housing
is to be measured.
R
x
A
R
1
-V
Rx
+R
Where:
A
-V : DC high-voltage generator
Ri : Generator internal resistance
Guard
A : Indicator meter (microammeter)
Rx = V
i
Rx: Insulation resistance between
B
R1: Insulation resistance between
R
R2: Insulation resistance between
2
25
-
Ri
primary and secondary winding.
primary winding and housing.
secondary winding and housing.
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