The majority of our relays are designed to be used with widely available ‘protection class’ 1A or 5A secondary rated ring type current transformers.
Typically referred to as 5p10, 10p20 or a combination of (10p10 for example) where the first number represents the accuracy at the limit of the linear range, and the second number represents the linear range.

Hence for a motor circuit a 5p10 or 10p10 CT is adequate and for a feeder circuit a 5p20 or 10p20 CT is common.

For core balance protection for motors, a CBCT with a rating 5p3 or 5p5 may also be suitable.

The other critical factor is often the inner diameter of the core to enable the primary conductor to pass. Typically available at 35mm, 55mm, 85mm or 120mm.

Class X CTs are required for restricted earth fault applications and knee point level of a class X in relation to the EF setting becomes the critical value.

For modern relays the input burden is very low, consisting of a short pass-through cable between the S1 and S2 terminals at the relay. A CT with a 1VA rating would be adequate for most installations; a larger CT VA is really only needed in circumstances where the CT is mounted physically far from the relay.

Should you wish P&B to source and supply CTs along with our relays we can do so, providing a magnetisation curve and calculations if required.


P&B is a leading UK innovator of electrical protection, safety and control technologies.

Our specialist expertise and unrivalled experience is relied upon in heavy industries throughout the world to ensure the highest levels of safety and performance.