UFC 3-560-01
6 December 2006
Including change 1, 7 December 2006
9-5
INSTRUMENT TRANSFORMERS.
9-5.1
Potential (Voltage) Transformers (PT).
9-5.1.1 PTs provide a means of obtaining a low voltage from a higher voltage circuit.
They are designed and selected to operate within certain accuracy limits and burdens.
9-5.1.2 Replacement transformers must have characteristics identical with the
original units.
9-5.1.3 The case and one of the windings of the low-voltage side of voltage
transformers must always be grounded before energizing the transformer.
9-5.1.4 Be aware of the following hazards inherent in the maintenance and removal
of these units.
9-5.1.4.1 If the secondary windings are inadvertently shorted together when the primary
windings are energized, a very high current will flow causing the windings to quickly
overheat.
9-5.1.4.2 On most modern switchgear, a drawout arrangement automatically
disconnects and grounds the transformers when access to the fuses is necessary.
9-5.1.4.3 On older obsolete switchgear, fuse replacement is potentially dangerous when
the primary circuit to the transformer remains energized. Follow these additional safety
precautions.
9-5.1.4.3.1 The authorized individual-in-charge will give specific instructions for
replacing a blown primary winding fuse on a potential transformer located within
switchgear and whenever it is not possible to use a standard 6 ft (1.8 m) fuse puller.
9-5.1.4.3.2 If a circuit breaker or sectionalizing switch is not installed to isolate a
potential transformer, the worker must report the situation to the authorized individual-
in-charge before replacing the fuse. The authorized individual-in-charge will arrange for
deenergizing the primary circuit. Replacing a primary fuse when the potential
transformer is not isolated is particularly hazardous, and requires specific approval.
9-5.1.4.3.3 When disconnecting the primary service to the transformer, verify the
absence of voltage using a suitably rated voltmeter (a handheld test meter with a high-
voltage probe is not acceptable). Lamps can be used in addition; however, note that a
non-illuminated lamp, connected on the low-voltage side of a voltage transformer, is not
an adequate indication that the primary side of the transformer is deenergized.
9-5.1.4.3.4 The secondary fuses must also be removed before replacing the primary
fuse, and then reinstalled before the transformer is reenergized.
9-8
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