Instrument Transformers

Instrument Transformer are used in A.C. system for measurement of electrical quantities like voltage , current, power, energy, P.F. frequency. Instrument transformers are also used with protective relays in power protection of power system.


Basic function of instrumental transformer is to step down the A.C system voltage & current . The voltage & current level of power system is very high. It is very difficult & costly to design the measuring instruments for measurement of such high level voltage & current. Generally measuring instruments are designed foe 5A & 110 volt.



There are two types of Instrument Transformer 

1.  Current Transformer (C.T.)

2. Potential Transformer (P.T.)

P.T (Potential Tansformer)

P.T(Potential Transformer) is used to step down the voltage of power system to low voltage level to make is feasible to be measured by small rating voltmeter 110 or 120 volt. A typical diagram shows in below figure.

Points to be remembered about P.T

• Connected in parallel with power circuit.
• Secondary is connected to volt meter.
• Secondary works almost in open circuit condition.
• Primary current depends on secondary burden. 
• One terminal of secondary is earthed for safty 

Symbol for transformer.

Related topic

C.T (current transformer)