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TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
(a) The water-in and water-out temperatures
modify these general directions in accordance with
of water-cooled transformers.
the manufacturer's recommendations.
(b) The oil- in and oil-out temperatures of
a. Readings. Current, voltage, and temperature
forced-oil-cooled transformers with oil-to-air or oil-
readings should be taken at the peak load time.
to-water heat exchangers.
Voltage and liquid level readings should be taken at
(c) The pressure in the nitrogen cylinder for
the end of a low load period.
a transformer equipped with an automatic gas-
(1) Current. Load currents are a very impor-
pressure system. If the pressure drops below the
tant part of recommended regular inspections. If
manufacturer's recommended value, the cylinder
the observed current in any phase exceeds the rated
should be replaced and leaks repaired.
full load value, and the rated maximum tempera-
ture is exceeded, steps should be taken to reduce
(d) Dehydrating breathers should be checked
the load. Trends in load currents should be noted for
to ensure that they are free from restriction and
programming additional transformer capacity or
have not absorbed excessive moisture.
consolidation of loads on lightly loaded transform-
(e) For dry-type transformers, the operation
of integral ventilating fans should be checked. If
(2) Voltage. Overvoltages and undervoltages
installed indoors, the temperature of the room
can be detrimental to the transformer and the load
should be measured regularly and recorded. Proper
it serves. Investigate immediately and take correc-
ventilation is essential, and any material or ob-
tive action to bring the voltage within acceptable
struction that might prevent the free circulation of
air around a transformer should be removed. If the
(3) Temperature. Use integral unit-mounted
room has power-driven ventilating fans, their cor-
temperature gauges and maximum temperature in-
rect operation should be determined. Overtem-
dicators, where available. Record readings, and re-
perature alarms, if provided, should be tested. Ex-
set the maximum temperature indicator. Excessive
cessive air velocity can be as damaging as no
temperature can indicate an overload interference
circulation at all.
with the normal means of cooling. Prolonged opera-
b. Inspections and repairs. The inspection and re-
tion at an overtemperature will accelerate liquid
pair recommendations given are general in nature.
deterioration; result in a reduced life expectancy of
For specific directions, the manufacturer's recom-
the solid insulation; and may greatly increase the
mendations should be followed.
risk of failure.  Constant monitoring against
(1) Protective devices. Basic transformer pro-
overtemperature is often provided by special alarm
tection is required by the NEC and is often supple-
contacts on a transformer's temperature gauge.
mented with additional protective relays and de-
(4) Liquid level. Check regularly, and espe-
vices. Inspect and maintain these devices on a
cially after a long low-load period with a low ambi-
regular basis to ensure that they will operate in
ent temperature, as this is when the liquid level
case of failure. Provide an annual maintenance
should be at its lowest point. Liquid must be added
check for sudden pressure relays, undervoltage and
before the level falls below the sight glass or bottom
overvoltage relays, alarm and auxiliary relays, and
reading of the liquid-level indicator. If there is no
wiring and instrument transformers associated
liquid-level indicator, de-energize the transformer
with the protective relays.
and check the liquid level by removing the inspec-
tion plate on the top of the transformer, or by re-
(2) Protective alarms. Transformers come with
moving the top if no inspection plate is available.
various types of alarms, such as overtemperature,
liquid temperature, and pressure-relief devices.
(5) Pressure / vacuum. Pressure/vacuum gauges
These devices usually have open-type contacts con-
are commonly found on sealed-type transformers.
nected to either alarm or to trip the protective cir-
They indicate the integrity of the sealed construc-
cuit breaker. Because of their importance, check
tion and should be added to transformers without
alarm contacts and associated wiring on a monthly
them, if feasible. The readings should be compared
to the recommendations of the manufacturer per-
taining to normal operating ranges. High pressures
(3) Ground connections. A transformer tank is
indicate an overload or internal trouble and a sus-
normally provided with a ground connection to
tained zero pressure reading indicates a leak or a
eliminate electric shock (however, at least one
defective gauge.
state's safety orders do not permit pole-mounted
transformers to have grounded tanks). The ground
(6) Miscellaneous. The features of special types
resistance of a substation may vary from less than 1
of transformer construction that should be included
ohm for very large capacity substations, to 25 ohms
in regular inspections include:


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