Why is one connection of a CT secondary earthed.

[abbppm]

A few simple questions .
1.Why is one terminal connection of an instrument current transformer grounded/earthed?
2.What effect would it have if it were removed under load conditions?
3.Does it matter which connection is earthed?

 

[scottf]

Typically, both leads from the CT secondary are run back to the meter/relay and one leg is grounded there. This is to keep the voltage level on the secondary from floating. it is possible to ground one leg of the secondary at the CT and then the corresponding leg at the meter/relay, but this is generally not done.

You ask about removing the ground while in-service...it depends on how the secondary is connected to the associated equipment. If both leads are run back to the connected equipement and then one leg is also grounded, then removing the ground should not cause an immediate problem. However, during transients, you could see relative voltage on the secondary of the CT get very high.

Generally, the non-polarity terminal is the one that is grounded. To be honest, I can't think of a good reason why it would matter...I've never really thought about it.



If the CT secondary is grounded at the CT, then removing the ground would be very dangerous, as you would essentially open-circuit the CT secondary. Depending on the CT core/coil configuration, very high voltages may appear across the CT secondary. If the CT has a relay accuracy rating, the open-circuit voltage could be in excess of 50 kV or more.

 

[busbar]

Aside from scottf's mention of open-circuiting CTs, omitting or lifting a CT-secondary ground can cause electrostatically induced high voltage from winding to ground.

See IEEE C57.13.3
thread238-18531 thread238-12608

http://www.powellelectric.com/ptb_pdf/ptb87.pdf

http://www.geindustrial.com/products/applications/GET-97D.pdf