MEGABRAS MD-5060x Guia Del Usuario página 24

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 megohm-
meter 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 deter-
mines the current passing through the circuit, which in turn is indicated by the microam-
meter. 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.
V
t
R
i
-R
+V
A
V
-
Rx = Ri
I
R R R
1 x 2
A B
26
R
x
Guard
Fig. 1
Fig. 2