Quantcast Section VI- Instrument Transformers

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TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
3-23. Definition of bus structures.
corroded connections, and poor alignment that
might result from short-circuit stresses. Special at-
A bus structure is an assembly of bus conductors
tention should be given to contacts between dissimi-
with associated connection joints and insulating
lar metals. For example, copper salts falling onto
supports. It can have bare or insulated conductors.
aluminum will chemically deteriorate the alumi-
A busway is a grounded metal enclosure, containing
num. This situation will be most severe at locations
factory-mounted, bare or insulated conductors,
subject to salt spray.
which are usually copper or aluminum bars, rods, or
tubes. Each serves as a common connection between
3-25. Cleaning of bus structures.
two or more circuits.
The cleaning of buses is limited primarily to that of
3-24. Maintenance of bus structures.
eliminating excessive contamination from the sup-
porting insulators. It is not necessary to remove
Bus structures need regular scheduling of visual
corrosion from the conductors, except where it ei-
ther affects contact resistance of connections or can
a. Schedule.  Enclosed buses occurring in
lead to deterioration of conductors.
switchgear should be inspected visually, in conjunc-
tion with scheduled outages for circuit breaker,
3-26. Testing of bus structures.
fuse, switch, or other associated equipment main-
tenance. Open-type buses may be visually inspected
Generally, no testing is required in connection
without being de-energized. The frequency of
with a bus structure, except that trouble spots
such inspections depends largely on the local con-
should be detected by checking bus temperature.
tamination problem and will, therefore, vary with
Whenever electric current flows, there is some tem-
each installation. However, each bus should be visu-
perature rise. If this rise becomes excessive, such as
ally inspected at least once each year as a mini-
at a point of poor contact, trouble will develop.
Checking for higher-than-normal temperatures by
b. Visual inspection. Inspect all bus conductors
infrared inspection can reveal these future trouble
and connections for evidence of overheating, loose or
3-27. Definitions of instrument transformers.
3-28. Short-circuiting dangers.
An instrument transformer is designed to reproduce
The basic difference between current and potential
transformers must be observed. A voltage trans-
in its secondary circuit (in a definite and known
proportion) the current or voltage of its primary
former like most constant-voltage devices should
circuit with the phase relations and waveform sub-
never be short-circuited. A current transformer, be-
stantially preserved.
ing a constant-current device, requires that the sec-
a. Current transformers. A current transformer is
ondary circuit always be closed. As long as there is
a constant-current transformer which reduces line
current in the primary winding, there will be cur-
currents into values suitable for standard measur-
rent in the secondary winding. On an open circuit
ing devices such as ammeters and wattmeters and
the voltage will be the secondary current multiplied
standard protective and control devices. It also iso-
by an extremely high open-circuit secondary resis-
lates these devices from line voltages. The primary
tance. This is a voltage which may both damage
winding is connected in series with the circuit car-
insulation and prove dangerous to life. Under no
rying the line current, or as a window-type arrange-
circumstances should the secondary of a current
ment linked magnetically with the line conductor
transformer be opened while the primary circuit of
which eliminates the need for an integral primary
the transformer is energized, unless the terminals
of the current transformer are of the short-
b. Voltage (potential) transformers. A voltage
circuiting type.
transformer is basically a conventional constant-
3-29. Maintenance of instrument transform-
voltage transformer with primary and secondary
windings on a common core connected in shunt or
parallel to the power supply circuit to be measured
Instrument transformers should be scheduled for a
or controlled. The secondary winding insulates de-
maintenance inspection every 2 years. In addition,
vices from the power circuit.
they should be inspected visually any time appara-


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