MIL-HDBK-1025/10
6.8.1 Voltage (Potential) Transformers. These units provide a means of
obtaining low voltage from a higher voltage circuit. To serve their intended purpose they
are designed and selected within certain accuracy limits and burdens. Units procured as
replacements should have characteristics identical with the original units. There are
certain hazards inherent in the maintenance and removal of these units. A voltage
transformer has a constant voltage maintained on both the primary and secondary,
although there is a fixed difference between the two voltages. If by accident the secondary
is short circuited, a very high current will flow in both windings, causing the windings to
overheat very quickly. The case and one of the windings of the low-voltage side of
voltage transformers should be grounded before energizing the transformer.
6.8.1.1 Fuse Replacement
CAUTION
Replacing a blown primary-winding fuse is potentially dangerous
when the circuit to the voltage transformer is energized. The
secondary fuses should be removed to prevent the possibility of
energizing the voltage transformer from the secondary side. A
thorough investigation should be made in either case to determine
the probable cause of the trouble, before attempting to install a
new primary-winding fuse. Ordinarily, trouble in the voltage
transformer is apparent from visual evidence in the form of a
smoked or burned case, damaged bushing, or the condition of the
fuse.
a. Also, before any inspection or replacement is done, be sure the service to the
primary side of the voltage transformer is disconnected.
b. A dark lamp, connected on the low-voltage side of a voltage transformer, is not
a positive indication of the condition of the high voltage side. Voltmeters, in addition to
lamps, should be connected to the low-voltage side. Lamps should first be connected
while the voltmeter is used as an extra check.
c. On most modern switchgear a drawout arrangement usually automatically
disconnects and grounds the transformers, when access to the fuses is necessary.
6.8.1.2 Supervisory Requirements. A supervisor should give instructions for
replacing a blown primary winding fuse on a distribution-potential voltage transformer
located within switchgear, or where it is impossible to use a standard 1.8-meter (6-foot)
long puller. Whenever a circuit breaker or a sectionalizing switch is not provided to
isolate the voltage transformer, the worker should report the situation to his/her
supervisor immediately. The supervisor should arrange for a feeder breaker opening.
Replacing primary fuses when the potential transformer is energized is not authorized.
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