Differential Protection and Electromechanical Relays

Why not???Even if it's numerical relay one has to have PT class CT for the differential protection,otherwise it may cause maloperation if my knowledge is correct.I have not came across any numerical relays fed from with CTs other than PS class.

One MUST go for a Class PS CT for Differential Protection irrespective of an Electromechanical Relay or a Numerical Relay, because selection of PS Class is not with respect to the type of relay but with respect to differential CT Satauration if convetional (Class 5P, 10P etc.) CTs are used.

Thank you for your reply. Can not I use CT of class 5P20 is suffiecient because up to 20 times the magnitude of fault current it does not saturate & Numerical relay is very very sensitive & accurate ?

You cannot - because, even when you use two 5P20 CTs, it is not guaranteed that both the CTs will exactly saturate at 20 times the rated primary current. For, the definition for 5P20 is that the CT will be able to maintain its accuracy (i.e.) a composite error of +/- 5%, AT LEAST up to 20 time its rated primary current. Which also means that the CT may saturate anywhere after 20 times. And, how much after 20 times, will definitely differ between two identical CTs. Now, unde such a condition, there will be differential current due to the differential saturation of the CTs and the Diffl. Relay would operate even for external faults.

May be, if your fault level is low, then you can use a high ALF 5P CT. But, would it not be uneconomical & bulky?