MD-5060
9
APPLICATION NOTE # 32
USE OF
GUARD
TERMINAL IN MEGOHMMETERS
When insulation resistance measurements are performed with megohmmeters, 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
megohmmeter basic circuit diagram of Fig. 1.
Ri
+V
Rx
-
R
A
I
Guard
Fig. 1
Vt
Where:
V
: DC high-voltage generator
Ri
: Generator internal resistance
A
: Indicator meter (microammeter)
