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 CP-4 series Encapsulated Interposing current transformer

CP4 Encapsulated Interposing Current Transformer


There are C.Ts.Which are interposed between the main C.T.and the burden(Load)so that the secondary current of the main C.T. is adjusted as required by the burden.When used in providing transformer differential protection,these C.Ts are also used to correct the phase shift,which may result due to particular method of connection used for the transformerwindings.

The main fuction of an interposing CT is to balance the current supplied to the relay or others where threre would otherwise be an imbalance due to the ratios of the main CTs.Interposing CTs are equipment with a ratio that can be selected by the user to achieve the balance required.

As the name suggests,an interposing CT is installed betweenthe secondary winding of the main CT and relay.They can be used on the primary side or secondary side of power transformer being protected,or both interposing CTs also provide a convenient method of establishing a delta connection for the elimination of zero sequence currents where this is necessary.

Encapsulated current transformer

Technical parameter

Tough resilient flame retardant UL-94V0

Temperature range -20°C to 70°C

Accuracy class 1.0 ,0.5

Test voltage 4kV 50Hz 1min


Insulation Class E,Max 120°C

Totally enclosed in tough,self extinguishing mouldings

Operating voltage 0.72kV

Summation current transformer

Normal secondary winding 1A or 5A

Compliance:IEC 185 and BS 3938/1973

Burden:5VA or 7.5VA

Accuracy:0.5 or 1.0

Rated voltage:720V 50/60Hz

Dielectric voltage:4kV

Type of material:High impact flame-retardant moulded resin

Input:5/5A,1/1A other by request.


Interposing CT’s are used when the ratio of transformation is very high. It is also used to correct for phase displacement for differential protection of transformer.

An assembly of interposing current

transformer  including Class PX for

differential and unit schemes.

Manual for Encapsulated current transformer

A  When the main C.T.s having rated secondary current of 1Amp.And the burden requires a rated current of 5Amp,an used,at the load end so that the lead burden is fed by 1Amp.Current.The lead burden on the negligible as it placed very close to the burden(Pic.1).

B  The arrangement suggested in a above may also be used when the main having rated secondary of 5Amps.And the burden requires a rated current of 1amp.In this case .However,the I.CT, of ratio 5/1 Amp will have to be placed at the main C.T.end to reduce the effect of lead burden on the main C.T.(Pic. 2)

C  In the case of transformer differential protection when the main C.T. ratios do not match.If phase shift is introduced due to the model of connections of the transformer on the H.T. and L.T.side,I.C. Ts Can be used to correct the phase shift.It is assumed here that all main C.Ts are star connected(Pic.3)

D  For suppressing zero sequence currents from the out of balance current.This is achieved by providing an additional secondary winding on I.C.Ts.Which is connected in closed delta(Pic.4)

E  If high ratio is selected for the main C.T. against a low load current,I.C.Ts may be used across the main C.T. to reduce the ratio effectively  and satisfactorily cater ti the rated current requirement of the burden(Pic.5)

In all above applications it must be borne in mind that an I.C.T. imposes burden on the main C.T.if this burden is required to be restricted to a certain value,the value  must be specific to enable the manufacture to design the I.C.T.suitably.If this value of burden(expressed in VA)is too      low,it may result into an uneconomical design of the I.C.T.

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