|MiCom P63x Protection Relay|
There are a series of protection relays such as MiCom protection relays that are compatible with ANSI/IEEE CTs as specified in the IEEE C57.13 standard. The applicable class for protection is class "C", which specifies a non air-gapped core. The CT design is identical to IEC class P but the rating is specified differently.
Where IEEE standards are used to specify CTs, the C class voltage rating can be checked to determine the equivalent knee-point voltage (Vk) according to IEC.
The equivalence formula is:
Vk = (C x 1.05) + (Ksc x In x Rct)
Vk = (C x 1.05) + (100 x Rct)
Note: IEEE CTs are always 5A secondary rated, i.e. In =5A, and are defined with an accuracy limit factor of 20, i.e. Kssc =20.
The following table allows C57.13 ratings to be converted to a typical IEC knee-point voltage:
- * Assuming 0.002/turn typical secondary winding resistance for 5A CTs