The voltage capability of the CT has to be high enough to overcome the voltage drop across its own winding plus the voltage drop across the connected burden (pilot wiring, relay, etc). Your 5P5 CTs should have a rated burden associated with them, normally expressed in VA (5, 10, 20, 30 VA are...
As stated by Randyman and prc, BCT cores are not typically connected to ground at least in the US. Secondary winding resistance/Megger/hipot testing is a pretty std test for the breaker mfrs and during commissioning. It provides assurance that there is adequate insulation between the secondary...
A bit late to the discussion, but Runsor is right on the accuracy limits of 5P and 10P classes at Ip. Also, instrument security factor is an optional specification for IEC measuring CTs so it shouldn't technically be a problem to get a dual rated protection/measuring CT (although it may be a...
