Application Note 32 - Circutor MI-15KVe Guia Del Usuario

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.
Where:
+V : DC high-voltage generator
Ri : Generator internal resistance
A : Indicator meter (microammeter)
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 resis-
tances which influence on Rx should be minimized.
A typical example of this situation is when the insulation resistance between pri-
mary and secondary windings of a transformer mounted inside a metal housing is
to be measured.
V
t
R
i
-R
+V
A
V
-
Rx = Ri
I
30
R
x
Guard
Fig. 1