S&C Instruction Sheet 682-510 35
Dielectric Testing
De-energize the Vista Underground Distribution
Switchgear before performing the resistance
measurements described in this procedure� Follow all
applicable safety procedures� Failure to de-energize
the Vista Underground Distribution Switchgear
before taking the resistance measurements will
result in serious injury or death�
DANGER
Figure 57� Take the measurement from the flat current-
carrying surface of the bushing�
●
Resistance Measurement
Resistance measurements are used to look for areas of
the gear that may exhibit poor contact between current-
carrying parts.
Resistance measurements are taken using a four-termi-
nal measuring device that provides at least 100 amperes
of current to the main circuit. Resistance measurements
should be taken from the bushing conductor across each
way to the same phase on each way of the unit. For example,
a measurement would be taken from Way 1 Phase A to Way
2 Phase A, from Way 2 Phase A to Way 3 Phase A, from
Way 1 Phase A to Way 3 Phase A, from Way 1 Phase B to
Way 2 Phase B, etc.
To measure resistance, perform the following procedure:
STEP 1�
Clamp the two current-carrying probes of the
resistance-measuring device to the bushing
conductors of the current-carrying path to be
measured. See Figure 56. In this example, the
resistance is being taken between Way 1 Phase
A and Way 2 Phase A.
STEP 2�
NOTICE
DO NOT take resistance measurements
from the threaded area of the bushing stud�
Resistance measurements taken through
the threads of the bushing stud will be
inaccurate� See Figure 57�
Clamp or touch the voltage-carrying probes
of the resistance-measuring device to the flat
conductive surface of the bushings that make
up the current-carrying path. Make sure the
measurement probe is in contact with the current-
carrying flat face of the bushing conductor rod.
See Figure 57. If using clamp-style probes, slide
the clamp all the way up against the current-
carrying face to get a good connection.
● Adhere to your company’s standards regarding use of hand PPE
when taking resistance measurements�
STEP 3�
Record the resistance measurement. Acceptable
resistance values are:
• Less than 500 microohms
• Less than 600 microohms for tie switches
Figure 56� Connecting the resistance measuring device�●
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