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11. 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, let us start 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.
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