Shielded Wire For CT's?

[JoeStillman]

For a relay class CT on an outdoor substation, it has been suggested that we use shielded wire for the secondary. The circuits are in an underground, dedicated, schedule 40 steel conduit.

What is the shielding for?

 

[jim dungar]

For a relay class CT on an outdoor substation, it has been suggested that we use shielded wire for the secondary. The circuits are in an underground, dedicated, schedule 40 steel conduit.

What is the shielding for?

It serves no purpose.

Current transformer output is normally .5AAC and above, only in extreme situations might 'noise coupling' be a problem. CT conductors are frequently twisted for wire management reasons not for cross talk problems

Current transducer output is normally 0-10VDC or 4-20mA, as with any process signal, shielding and twisting are often used to mitigate 'noise coupling'.

 

[Jraef]

Yeah, I'm thinking too that someone is calling a Current Transducer a "CT" because the acronym would look the same, but for most people the term CT is used to denote a Current Transformer and a transducer is just called a transducer to avoid confusion. You would want shielding for the signals from a transducer, but on the secondary of a current transformer, it does nothing useful.

 

[Bugman1400]

Not so fast! What voltage level is this? Some utilities have a standard to shield CT cables for 230kV and above. Currents can be induced onto CT cables for EHV subs or during high current faults at lower voltages. These induced currents can cause nuisance tripping for differential and ground overcurrent schemes.

 

[mbrooke]

Not so fast! What voltage level is this? Some utilities have a standard to shield CT cables for 230kV and above. Currents can be induced onto CT cables for EHV subs or during high current faults at lower voltages. These induced currents can cause nuisance tripping for differential and ground overcurrent schemes.

But doesn't single end grounding prevent this? The way I see it is a dual end grounded sheath will carry fault current and in turn induce that into the shielded CT circuit?

 

[jim dungar]

Some utilities have a standard to shield CT cables for 230kV and above.

The OP said the shielding was suggested, what you are describing is a requirement.

Not every question needs an answer that is good for 99.99% of possible installations, sometimes just covering 99.98% of probable installations is sufficient to help the OP. After all, we aren't usually provided 100% of the installation details.

 

[Bugman1400]

The OP said the shielding was suggested, what you are describing is a requirement.

Not every question needs an answer that is good for 99.99% of possible installations, sometimes just covering 99.98% of probable installations is sufficient to help the OP. After all, we aren't usually provided 100% of the installation details.

I think the OP was asking what the shield is for. I gave an answer that has practical and technical merit. The technical response is contrary to your response that it served no purpose.

 

[jim dungar]

The technical response is contrary to your response that it served no purpose.

If the application is only for a CT lead between<35kV cubicle style outdoor equipment - what purpose does the suggested shielding serve?
My initial post acknowledged that there would be some situations where shielding should be considered. Though, in almost all of these cases shielding would be covered in the project specs rather than simply being 'suggested'.