# Difference between Current Transformer and Potential Transformer

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## Current Transformer

Current transformers (CT) are broadly employed in order to measure high magnitude currents. Such transformers basically step down (lower) the current which is to be measured, so that it can be measured with an average range ammeter. A CT generally possesses one or few primary turns. The primary side winding could be simply a conductor positioned in an empty (hollow) core.

## Fig.1: Current Transformer Circuit Diagram

Whereas the secondary side possesses a large number of turns which are precisely wound for a particular turns ratio. Hence the CTs step the voltage up whilst stepping the current down.

Normally, CTs are expressed in terms of primary to secondary current ratio like Ip/Is. A 200:5 CT rating means that the secondary side current is 5 amperes when primary side current is 200 amperes. Generally, the secondary side current rating is 1 ampere or 5 amperes. Current transformers are represented by the following symbol.

Fig.2: Current Transformer Symbol

## Potential Transformers

Voltage transformers (VTs) are essentially step-down transformers with highly accurate turns ratio. VTs generally step down the higher voltage to a lower voltage so that it can be measured easily with the standard voltmeter. Such transformers possess a higher number of turns on a primary side and smaller number turns on the secondary side.

## Fig.3: Potential Transformer Circuit Diagram

A Voltage transformer is generally represented in terms of primary to secondary voltage ratio like Vp/Vs. For instance, 1000:120 VT means that secondary side voltage is 120 V when the primary side has 1000 V. Voltage transformers are represented by the following symbol.

Fig.4: Potential Transformer Symbol

This article keys out the main differences between Current and Potential Transformer on the basis of several factors such as function, connection, use, primary and secondary windings, excitation current, core, types, and applications.

## Difference between Current Transformer and Potential Transformer

CharacteristicsCurrent TransformerPotential Transformer
FunctionTransforms high current into low currentTransforms high voltage into low voltage
ConnectionConnected in series with the circuit so full line current flows through the windingConnected in parallel with the circuit so full line voltage appears across the winding
Primary currentPrimary current does not depend upon secondary side circuit conditionsPrimary current relies on secondary side circuit conditions.
Secondary sideSecondary side can’t be open circuited when under serviceSecondary side can be open circuited without any damage
UseUsing current transformer, a 5 Ampere ammeter can be utilized to measure high currents such as 200 amperesUsing potential transformer, a 120 V voltmeter can be used to measure high voltages such as 11 KV.
Primary windingIn CT, primary has small number of turnsIn PT, primary has large number of turns
Secondary windingPossesses large number of turns in the secondary sidePossesses small number of turns in the secondary side
Excitation current & flux densityVary over a wide rangeVary over a narrow range
CoreMade up of silicon steelMade up of high quality steel which operates at low flux density
Input valueConstant CurrentConstant Voltage
Secondary winding range1A-5A110V-120V
TypesClosed core and wound coreCapacitor voltage type and electromagnetic type
Step up/downThey are step-up transformersThey are step-down transformers
ApplicationsMeasuring current and operating protective relay in the substationMeasuring voltage and operating protective relay in the substation

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