19
If the guard lead is connected to ground, then since the negative
terminal is at the same voltage as the guard terminal, the leakage into
the negative terminal will be considerably reduced. This will improve
accuracy because the current flowing into the negative terminal is
measured by the instrument and used to calculate resistance. This
technique is only permissible if the item under test is isolated from
ground. “Isolated” in this context means insulated by a resistance of at
least 5 M
Ω
for the positive terminal, or at least 10 k
Ω
for the negative
terminal.
Conversely, if the positive terminal is grounded, then the negative
terminal will be at a voltage equal to the test voltage relative to ground,
which will result in an increase in leakage current, and worsening of
measurement accuracy.
When making measurements above 100 G
Ω
therefore, the user should
ground the Guard Lead where possible, otherwise parallel leakage paths
may occur.
+ Test V
2
- Test V
2
Ground
+
-
+ Test V
0V
Ground
Guard
+
-
Alternatively, screened leads are available as an optional accessory from
Megger. The lead to the negative terminal is fully screened. The screen
is plugged into the Guard terminal, diverting any stray leakage currents.
This considerably improves measurements made with a floating output,
where the leads might touch each other or anything other than the test
piece.
Circuit block diagram
For 5 kV instruments C1 = 47 nF, R1 = 50 k
Ω
, R2 = 40 k
Ω
For 10 kV instruments C1 = 15 nF, R1 = 156 k
Ω
, R2 = 110 k
Ω
+
-
100 M
Ω
R1
R2
C1
Volts
Fuse
Cable
under
test
+
-
G
Discharge
resistance
Current
Ref
Voltage
control
Current
Limit
High
voltage
source
