Installation transformers fall into one of two separate categories. The first is the potential transformer which supplies low value voltages to voltmeters. The PT is typically equipped with four terminals; two for the incoming high voltage and two for the reduced instrument voltage. The PT's incoming or primary leads are connected in parallel across the high tension supply. This means that it may be located remotely in control rooms although it can often be mounted in high tension starter panels where local voltage indication is required.
The second type is the current transformer. This instrument transformer variant measures current and is used to drive ammeters and overcurrent protection relays. The typical CT differs from its voltage-reliant sibling in that it is connected in series with the supply and typically utilizes an induced primary feed and not a direct connection. To achieve this, a CT is produced with a hole in its center through which a high tension cable runs. The current passing through the cable induces an electromagnetic field in the CT's primary winding which is then stepped down by its secondary winding and fed to the instruments via the CT's two output terminals.
Instrument transformers are available in a large range of designs and ratings and may be custom built to suit all user requirements. Common PTs and CTs are fairly small components, seldom measuring more than a couple of inches square. Large, oil bath instrument transformers are, however, commonly used to facilitate low voltage measurement in primary distribution grid applications.
