With any switchboard or switchgear lineup, you can order
ammeters, voltmeters, frequency meters, etc. These meters come
with factory-installed instrument transformers, which are needed
to convert the measured parameter to that suitable for the meter
itself. Knowing what these transformers are and how they work
will help you when specifying and/or installing them in existing
equipment.  In this first part of this multi-part series, we'll
talk about the potential transformer, or PT, as it's more
commonly known.  What's a PT?  Basically, a PT is a
conventional, single-phase transformer that has its primary and
secondary windings on a common core. The most popular type of PT
provides 120V at the secondary when nameplate-rated voltage is
applied to its primary. It's important to note that a PT is
designed so that its secondary voltage has a fixed relationship,
or ratio, with its primary voltage.  Table 1 (click here to see
Table 1) lists commercially available Pts for application with
100% of rated primary voltage