Instrument Potential Transformer
The instrument potential transformer (PT) steps down voltage of a circuit to a low value that can
be effectively and safely used for operation of instruments such as ammeters, voltmeters, watt
meters, and relays used for various protective purposes.
Instrument Current Transformer
The instrument current transformer (CT) steps down the current of a circuit to a lower value and
is used in the same types of equipment as a potential transformer. This is done by constructing
the secondary coil consisting of many turns of wire, around the primary coil, which contains only
a few turns of wire. In this manner, measurements of high values of current can be obtained.
A current transformer should always be short-circuited when not connected to an external load.
Because the magnetic circuit of a current transformer is designed for low magnetizing current
when under load, this large increase in magnetizing current will build up a large flux in the
magnetic circuit and cause the transformer to act as a step-up transformer, inducing an
excessively high voltage in the secondary when under no load.