Commissioning Engineer- Arjun Athiyaman.

[arjun_athi]

Dear All,

Can anyone explain me why the cable shield installed in cable side is passing through the CT before it is grounded through the link box,
What happen if I am grounding the Cable shield without passing through the CTS.

 

[jim dungar]

It has to do with monitoring the current flowing on the shield.

Many CT manufacturers recommend that the ground shield be passed 'backwards' through the CT so that current flowing on the shield does not impact the measurement of the phase current. Also if the shield is actually a concentric neutral and it is grounded on both ends, the effect would be a equal to having a shorted turn in the CT.

If the CT is mounted just on the shield, it may be desirable to measure the current flowing only on the shield (i.e. ground fault current) or concentric neutral.

 

[rcwilson]

If the CT is used for ground fault (zero sequence) protection, and the cable shields are effectively grounded at both ends, current flowing in the shields will give false trips. Running the shield ground wire back through the CT subtracts the shield current so the CT only measures the actual cable current.

We tripped all feeders in a large facility (150 MW) when the utilitiy substation next door had a lightning strike on a 500 kV line. Some fault current from the flashover out on the transmission line went back to the substation via our cable shield ground wires that were routed outside the shields. Ground fault relays for all major feeders in the facility tripped.

 

[mbeatty]

If the CT is used for ground fault (zero sequence) protection, and the cable shields are effectively grounded at both ends, current flowing in the shields will give false trips. Running the shield ground wire back through the CT subtracts the shield current so the CT only measures the actual cable current.

We tripped all feeders in a large facility (150 MW) when the utilitiy substation next door had a lightning strike on a 500 kV line. Some fault current from the flashover out on the transmission line went back to the substation via our cable shield ground wires that were routed outside the shields. Ground fault relays for all major feeders in the facility tripped.

I have also seen tripped feeders due to shield ground wires being routed in this manner.