AEMC 1000N - Utilizing the Guard Terminal

Megohmmeter Model 1000N/1100N
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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.
Conductor
to
Line (-)
Terminal
Insulation
to
Guard
Terminal
Shield
to
Earth (+)
Terminal
Exposed
Surface
The guard terminal
is useful when
measureing very
high resistance
values.
ABCD
Figure 3
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 will be
erroneous and lead to
confusion as to the
cable’s condition.)
If the guard terminal is
not connected at “B”, the
instrument measures the
current “I” flowing through
the insulation and a
surface leakage current
“i
1
”. See Figure 4.
Figure 4
AC
B
Ry
Rz
Rx
i
1
i
To EARTH
Terminal
To LINE
Terminal
No Connection to
Guard Terminal