6
depends upon changing the internal
arc path from a fair conductor to a
good insulator in the short interval
while the current is passing through
zero. Manufacturer's instructions state
the lowest allowable dielectric
strength for the various circuit break-
ers. It is advisable to maintain the
dielectric strength above 20 kV even
though some manufacturer's
instructions allow 16 kV. Detailed
instructions for oil testing are found in
FIST Volume 3-5.
arc path from a fair conductor to a
good insulator in the short interval
while the current is passing through
zero. Manufacturer's instructions state
the lowest allowable dielectric
strength for the various circuit break-
ers. It is advisable to maintain the
dielectric strength above 20 kV even
though some manufacturer's
instructions allow 16 kV. Detailed
instructions for oil testing are found in
FIST Volume 3-5.
If the oil is carbonized, filtering may
remove the suspended particles, but
the interrupters, bushings, etc., must
be wiped clean. If the dielectric
strength is lowered by moisture, an
inspection of the fiber and wood parts
is advisable and the source of the
moisture should be corrected. For
these reasons, it is rarely worthwhile
to filter the oil in a circuit breaker
while it is in service.
remove the suspended particles, but
the interrupters, bushings, etc., must
be wiped clean. If the dielectric
strength is lowered by moisture, an
inspection of the fiber and wood parts
is advisable and the source of the
moisture should be corrected. For
these reasons, it is rarely worthwhile
to filter the oil in a circuit breaker
while it is in service.
c. Observe breaker operation under
load.
load.
d. Operate breaker manually and
electrically and observe for malfunc-
tion. The presence of excessive
friction in the tripping mechanism and
the margin of safety in the tripping
function should be determined by
making a test of the minimum voltage
required to trip the breaker. This can
be accomplished by connecting a
switch and rheostat in series in the
trip-coil circuit at the breaker (across
the terminals to the remote control
switch) and a voltmeter across the trip
coil. Staring with not over 50 percent
of rated trip-coil voltage, gradually in-
crease the voltage until the trip-coil
plunger picks up and successfully
trips the breaker and record the mini-
mum tripping voltage. Most breakers
electrically and observe for malfunc-
tion. The presence of excessive
friction in the tripping mechanism and
the margin of safety in the tripping
function should be determined by
making a test of the minimum voltage
required to trip the breaker. This can
be accomplished by connecting a
switch and rheostat in series in the
trip-coil circuit at the breaker (across
the terminals to the remote control
switch) and a voltmeter across the trip
coil. Staring with not over 50 percent
of rated trip-coil voltage, gradually in-
crease the voltage until the trip-coil
plunger picks up and successfully
trips the breaker and record the mini-
mum tripping voltage. Most breakers
should trip at about 56 percent of
rated trip-coil voltage. The trip-coil re-
sistance should be measured and
compared with the factor test value to
disclose shorted turns.
rated trip-coil voltage. The trip-coil re-
sistance should be measured and
compared with the factor test value to
disclose shorted turns.
Most modern breakers have trip coils
which will overheat or burn out if left
energized for more than a short pe-
riod. An auxiliary switch is used in
series with the coil to open the circuit
as soon as the breaker has closed.
The auxiliary switch must be properly
adjusted and successfully break the
arc without damage to the contacts.
which will overheat or burn out if left
energized for more than a short pe-
riod. An auxiliary switch is used in
series with the coil to open the circuit
as soon as the breaker has closed.
The auxiliary switch must be properly
adjusted and successfully break the
arc without damage to the contacts.
Tests should also be made to deter-
mine the minimum voltage which will
close the breaker and the closing coil
resistance.
mine the minimum voltage which will
close the breaker and the closing coil
resistance.
e. Trip breaker from protective
relays.
relays.
f. Check operating mechanism
adjustments. Measurements of the
mechanical clearances of the operat-
ing mechanism associated with the
tank or pole should be made. Appre-
ciable variation between the value
found and the setting when erected or
after the last maintenance overhaul is
erected or after the last maintenance
overhaul is usually an indication of
mechanical trouble. Temperature and
difference of temperature between
different parts of the mechanism
effect the clearances some. The
manufacturers' recommended
tolerances usually allow for these
effects.
adjustments. Measurements of the
mechanical clearances of the operat-
ing mechanism associated with the
tank or pole should be made. Appre-
ciable variation between the value
found and the setting when erected or
after the last maintenance overhaul is
erected or after the last maintenance
overhaul is usually an indication of
mechanical trouble. Temperature and
difference of temperature between
different parts of the mechanism
effect the clearances some. The
manufacturers' recommended
tolerances usually allow for these
effects.
g. Doble test bushings and breaker.
(FIST 3-16 1/92)