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 TM 5-685/NAVFAC MO-912
POTENT I A L
CURRENT
D
LI
A-AMMETER
Figure 5-4. Instrument transformers, typical applications.
cuit. Never open a CT secondary while the primary
(1) Ratings. Toroidal CTs are rated for the size
circuit is energized.
of the primary conductor diameter to be surrounded
d. Polarities. When connection secondaries of PTs
and the primary to secondary current (5A) ratio.
and Cts to metering circuits the correct polarities of
Bus bar type CTs are rated for the size of bus bar,
all leads and connections must be in accordance
primary voltage and the primary to secondary cur-
with the metering circuit design and the devices
rent 5A) ratio.
connected. Wrong polarity connections will give
(2) Application. The primary of a CT is either
false readings and result in inaccurate data, dam-
the line conductor or a section of the line bus. The
age and injury. All conductors and terminations
secondary current, up to 5A, is directly proportional
should carry identification that matches schemat-
to the line current. The ratio of the primary to
ics, diagrams and plans used for construction and
secondary current is inversely proportional to the
maintenance.
ratio of the primary turns to secondary turns.
e. Control circuits. Switchgear control circuits
(3) Safety. A CT, in stepping down the current,
provide control power for the starting circuit of the
also steps up voltage. The voltage across the second-
prime movers and the closing and tripping of the
ary is at a dangerously high level when the primary
switchgear circuit breakers. Additionally, the con-
is energized. The secondary of a CT must either be
trol circuits provide control power to operate the
shorted or connected into the closed metering cir-
5-5
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