switch ratings. The alarm switch must be manually r�set
after each operation.
Necessary maintenance is the resetting of the sema
phore and alarm switch (if supplied) after each operation.
If the gasket between the transformer boss and the relief
device must be replaced, follow the instructions in Sec
tion 8.7.
CAUTION
Never enter a vault or any other confined area in which a
transformer relief device has been known to operate or in
which a transformer has failed, until the area has been
thoroughly ventilated. Gases released may be toxic and
could cause danger to life. Then enter cautiously, with
another person in attendance.
When requested a vent hood is supplied bolted over the
pressure relief device. The purpose of the vent hood is to
allow the toxic and/or combustible gas released by relief
device operation to be vented outside a vault or building.
It is the owner's responsibility to properly connect the
vent hood.
Should disassembly of the relief device be necessary,
caution must be exercised when removing the protective
cover because the springs are under compression. The
relief device cover could blow off and injure personnel.
5.8 Winding Temperature Gauge
This device, also known as a Hot Spot Thermometer,
simulates the hottest spot temperature of the trans
former windings.
A temperature sensitive bimetal stem is mounted in a
leak-proof well, permitting removal of the instrument and
heating element without lowering the liquid level. The
stem is heated by both the surrounding liquid and a heater
element which is fed from a current source that is propor
tional to load current, in order to simulate the hot spot
winding temperature gradient above top liquid. The com
bination of the two temperatures is indicated on the
gauge. A red pointer is furnished to show the highest tern·
perature attained since last reset and is resettable by
means of
a
push button projecting through the bottom of
the dial bezel.
The gauge has four single-pole, normally open con
tacts with normal settings of 70, 75, 112 and 117°C.
9
Fig. 5
Front View of Winding Temperature Gauge
Should it become necessary to adjust any of these con
tacts from the above factory settings, a
±
17.5 degree ad
justment is possible by turning the calibration screws lo
cated on rear of case. The calibration screws are arranged
in counter-clockwise order (facing the rear of the case)
by ascending temperature with the 70 degree contact in
the 5 o'clock position. Clockwise rotation of the screw
increases the temperature at which the given switch will
close. Should a check on accuracy and calibration be in
dicated send for Instruction Leaflet 48-062-10.
5.9
TR0-2
Thermal Overload Relay
Transformers may be furnished with a Thermal Overload
Relay (type TR0-2) as an optional item for the Hot Spot
Thermometer when registration is required in terms of
Percent Thermal Load rather than winding hot spot tem
perature.
The TR0-2 relay provides no continuous indication of
simulated winding hot spot temperatures but a related
reading in terms of "Percent Thermal Load." It uses
3
single-pole, normally open contacts, the first of which is
for actuation of fans when supplied.
The TR0-2 relay dial has two colored zones. Operation
in the yellow zone will cause moderate loss of transformer
life. Excessive loss of transformer life is caused by opera·
tion in the red zone. Passage into the yellow zone is ac
companied by the closing of an alarm contact, while a
tripout contact closes if the loading continues into the red
zone above 110% thermal load. For long and satisfactory
transformer life, it is recommended that the transformer
be operated at all times below 100% thermal load, with
whatever margin experience shows to be advisable for an-