CT Earthing (grounding) 1

[kvkev]

I have three protection CT's (1 in each phase) in the delta loop of a star-delta starter. The original installation had the CT secondary's (s2)connected together and grounded, and the s1's are connected to a motor protection relay (MPR)
However the protection relay manufacturer's generic data sheet recommends that for star-delta starting the 3 CT's must not be grounded. How does the operation differ between grounded and non-grounded CT secondary's

 

[slavag]

Hello Kvkev.
Grounding (earthing ) of Star point of 3xCT it's safety issue only. Grounded or not are not influence on the protection functionality.
But it's very dangerous current wiring w/o grounding, please check with mnf. what is a reason.
I think it's some missunderstanding. " Must not be grounding" is not mnf. issue.
Regards.
Slava

 

[sslobodan]

As Slava said it is only for a protection or to be precise by grounding CT you get the currents maximized at 1.7 of nominal. I can search over on my papers for detailed explanation of this but I believe that this satisfies as an answer to your question

 

[scottf]

The point of providing a ground reference to the secondary windings of CTs is to provide a voltage reference to ground of the windings in order that the winding does not "float up" towards the voltage of the primary winding via capacitive coupling.