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FACT-1 DESCRIPTION
Alternating Current (AC) Ammeter Clear tests are used to indicate the presence of either
grounds or short circuits on a primary feeder's three phase conductors or either grounds
or short circuits on transformer secondaries.
Intentional grounding of the feeder cables may occur during some types of maintenance.
Failure to remove these grounds at the end of the maintenance procedure is unintentional
but nevertheless could cause potential damage if an attempt is made to put the feeder
back in service.
The Feeder Ammeter Clear Test Set has been specifically designed for utilities that do not
or cannot disconnect their transformers from the primary cable under test. A 120V AC,
60Hz source is used to check for grounds or short circuits. The AC signal is also adequate
to energize the transformers connected to the feeder sufficiently enough to detect
grounds left on the secondary side of a transformer.
In the primary phase-to-ground fault scenario, the LCD ammeter display will indicate a
fault current of approximately 5 Amps with a nominal line voltage of 120V. A maximum of
5 Amps is provided by the Test Set and is limited by an internal 24 ohm power resistor. In
the primary phase-to-phase fault scenario, the LCD ammeter display will also indicate a
fault current of approximately 5 Amps with a nominal line voltage of 120V.
In the secondary side fault scenario, a short circuit fault from a feeder transformer
secondary conductor to ground occurs. This allows more current to flow through the
primary windings of this transformer which current then flows through the primary
feeder. The value of the current flowing will depend on the nominal voltage of the
feeder being tested since the turns ratio of the transformer will vary. For example,
when a 13.8kV primary feeder connected to a transformer with a 120V secondary
is tested, the LCD ammeter display reading would be about four amperes while on
a 27kV feeder with a transformer to 120V, the LCD ammeter display reading would
be about 1.4 amperes. These are only estimates and can vary depending on feeder
voltage and the type of wire (impedance) used to ground the feeder.
Under a "No Fault" condition, the LCD ammeter display will generally indicate a fault
current of less than about O.3A and will indicate that No Fault has occurred. This ammeter
reading would then represent the sum of transformer excitation currents on a given typical
feeder should said transformers not be disconnected from feeder on the primary side.
