Current transformers (CTs) operate in series with the current being measured and cannot have an open secondary circuit, while potential transformers (PTs) operate in parallel across the voltage being measured and can have an open secondary circuit without damage. CTs treat the primary current as independent of the secondary circuit conditions, while PTs have primary current that depends on the secondary circuit conditions. CTs are used with low-voltage ammeters to measure high currents, and PTs are used with low-voltage voltmeters to measure high voltages.

1. Difference between C.T. and P.T.
Current Transformer Potential transformer
The secondary of a C.T can not The secondary of a P.T can be
be open circuited on any open circuited without any
circumstance when it is under damage being caused either to the
service. operator or the transformer.
A CT may be considered as a P.T may be considered as a
series transformer. parallel transformer.
The primary current in a C.T is The primary current of a P.T
independent of the secondary depends upon the secondary
circuit conditions (burden). circuit conditions (burden).
The primary winding of the CT The primary winding P.T is
is connected in series with the connected across the line of
line carring the current to be voltage to be measured. Hence the
measured. Hence it carries of full line voltage is impressed
the full line current. across its terminal.
With the help of CT, a 5A With the help of P.T, a 120V
ammeter can be used measure a voltmeter can be used to measure
high current like 200A. very high voltages like 11KV.