Application Note N° 32 - Circutor MD-5060e Guia Del Usuario

Application note N° 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:
: DC high-voltage generator
+V
: Generator internal resistance
Ri
: Indicator meter (microammeter)
A
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.
Rx: Insulation resistance between primary and secondary winding.
R1: Insulation resistance between primary winding and housing.
R2: Insulation resistance between secondary winding and housing.
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