Utilizing The Guard Terminal - AEMC 1000N User Manual

Utilizing the Guard Terminal

Guard terminals are useful when measuring high resistance values and
for stabilizing readings.
When testing the insulation at the end of a cable, it is necessary to
eliminate the error from surface leakage which occurs, particularly at
high resistance values. The guard terminal provides a third terminal
within the path of the surface leakage "E". Connect the instrument as
shown in Figure 3.
A
If there is no shield at "B", use a copper wire wound several times around
the exposed surface "B". (Note: If a shield is not available and you do not
make up a shield around
"B" and connect to the
guard terminal (-), the
measurement
erroneous and lead to
confusion as
cable's condition.)
If the guard terminal is
not connected at "B", the
instrument measures the
current "I" flowing through
the insulation
surface leakage current
1
"i ". See Figure 4.
B C
Exposed
Surface
Conductor Insulation
to to
Line (-) Guard
Terminal Terminal
will be
to the
and a To EARTH
Terminal
Megohmmeter Model 1000N/1100N
D
The guard terminal
is useful when
measureing very
high resistance
Shield
values.
to
Earth (+)
Terminal
Figure 3
A
- 13 -
No Connection to
Guard Terminal
B
i 1
Ry
Rz
Rx
C
i
Figure 4
To LINE
Terminal