Megohmmeter Models 5000N/5100/5110
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Utilizing the Guard Terminal
Guard terminals are useful when measuring high resistance values and
for stabilizing readings.
Conductor
to
Line (-)
Terminal
Insulation
to
Guard
Terminal
Shield
to
Earth (+)
Terminal
Exposed
Surface
The guard terminal
is useful when
measuring very
high resistance
values.
A
B
C
Outer jacket
Figure 7
When testing the insulation at the end of a cable, it is necessary to
eliminate the error from surface leakage that occurs, particularly at high
resistance values. The guard terminal provides a third terminal within the
path of the surface leakage. Connect the instrument as shown in Figure
7.
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 state).
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 8.
Figure 8
A
C
B
Ry
Rz
Rx
i1
i
To EARTH
Terminal
To LINE
Terminal
No Connection to
Guard Terminal
Summary of Contents for 5000N
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