Central Unit PM866K01

Central Unit PM866K01

Central Unit PM866K01

The voltage at the current transformer secondary terminals directly affects the current transformer saturation. This voltage is developed in a loop containing the secondary wires and the burden of all relays in the circuit. For earth faults the loop includes the phase and neutral wire, normally twice the resistance of the single secondary wire. For three-phase faults the neutral current is zero and it is just necessary to consider the resistance up to the point where the phase wires are connected to the common neutral wire. The most common practice is to use four wires secondary cables so it normally is sufficient to consider just a single secondary wire for the three-phase case.

There are many different standards and a lot of classes but fundamentally there are three different types of CTs:

• High remanence type CT

• Low remanence type CT

• Non remanence type CT The high remanence type has no limit for the remanent flux.

This CT has a magnetic core without any airgaps and a remanent flux might remain almost infinite time. In this type of transformers the remanence can be up to around 80% of the saturation flux.

Typical examples of high remanence type CT are class P, PX, TPX according to IEC, class P, X according to BS (old British Standard) and non gapped class C, K according to ANSI/IEEE.