S&C Instruction Sheet 681-510 27
Dielectric Testing
Resistance Measurement
DANGER
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 switchgear before taking resistance
measurements can result in serious injury or death.
Resistance measurements are used to look for areas of
the gear that may exhibit poor contact between current
carrying parts. See Figure 46.
Resistance measurements are taken using a four-
terminal measuring device that provides at least 100
amperes of current to the main circuit. Resistance mea-
surements 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 46. 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.
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 mea-
surement probe is in contact with the current-
carrying flat face of the bushing conductor rod.
If using clamp-style probes, slide the clamp all
the way up against the current-carrying face to
get a good connection. See Figure 47.
STEP 3.
Record the resistance measurement. Acceptable
resistance values are:
• Under 500 microohms
• Under 600 microohms for tie switches
Figure 46. Connecting the resistance measuring device.
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Figure 47. Take the measurement from the flat current-carrying
surface of the bushing.
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Adhere to your company’s standards in regards to using hand PPE
when taking resistance measurements.
Current-carrying surface