Current Transformers

The primary of a current transformer typically has only one turn. This
is not really a turn or wrap around the core but just a conductor or bus
going through the “window.” The primary never has more than a very
few turns, while the secondary may have a great many turns,
depending upon how much the current must be stepped down. In most
cases, the primary of a current transformer is a single wire or bus bar,
and the secondary is wound on a laminated magnetic core, placed
around the conductor in which the current needs to be measured.
If primary current exists and the secondary circuit of a CT is
closed, the winding builds and maintains a counter or back
EMF to the primary magnetizing force. Should the secondary
be opened with current in the primary, the counter EMF is removed;
and the primary magnetizing force builds up an extremely high
voltage in the secondary, which is dangerous to personnel and
can destroy the current transformer.
CAUTION:
For this reason, the secondary of a current transformer should
always be shorted before removing a relay from its case or
removing any other device that the CT operates. This protects the
CT from overvoltage.
Current transformers are used with ammeters, wattmeters, powerfactor
meters, watt-hour meters, compensators, protective and
regulating relays, and trip coils of circuit breakers. One CT can be
used to operate several instruments, provided the combined loads of
the instruments do not exceed that for which the CT is rated.
Secondary windings are usually rated at 5 amperes. A variety of
current transformers are shown in figure 13. Many times, CTs have
several taps on the secondary winding to adjust the range of current
possible to measure on the primary.