Intermediate Terminal Block For Current Transformer Secondary Wiring

[MMC2258]

I remember reading awhile back that it is acceptable to route your CT secondary wiring through an intermediate terminal block before getting to your shorting device (shorting terminal block, test block, etc) but I also have some people telling me that your first point of connection off the CTs should ALWAYS be to your shorting device.

Are there any codes, standards, recommendations that cover this?

 

[infinity]

Good question, we always go with the CT wiring to the shorting block first. Not sure if it's required.

 

[MMC2258]

found the source

found the source

Good question, we always go with the CT wiring to the shorting block first. Not sure if it's required.

This is where I saw it. Mind you it's a random power-point on the internet... Was just seeing if anyone had any more concrete evidence.

https://conferences.wsu.edu/forms/hrs/HRS14/2014HRS/Lectures/CTSafety.pdf (page 54 of 92)

"Terminal Blocks may have Shorting Capability, but CTCircuits can pass through regular, non-shorting terminalblocks."

 

[Smart $]

Don't know of any, but it stands to logic. A shorting block is to ground both sides of the CT wires so one can make modifications to meter wiring. If you have a terminal block on the CT side of the shorting block and want to modify wiring in between, you will lose the intentional short to ground while performing mod's.

 

[MMC2258]

Don't know of any, but it stands to logic. A shorting block is to ground both sides of the CT wires so one can make modifications to meter wiring. If you have a terminal block on the CT side of the shorting block and want to modify wiring in between, you will lose the intentional short to ground while performing mod's.

Thanks for the reply. In my application there should be no need to modify the wiring between the CT and the shorting device. The intermediate terminal block makes the installation between the CT and shorting device easier since they are in different sections.

 

[Smart $]

Thanks for the reply. In my application there should be no need to modify the wiring between the CT and the shorting device. The intermediate terminal block makes the installation between the CT and shorting device easier since they are in different sections.

Great... as long as you are the only one that will be doing work in the equipment. Then again, how good is your memory several years down the road.

 

[MMC2258]

Great... as long as you are the only one that will be doing work in the equipment. Then again, how good is your memory several years down the road.

It's the same concept for the wiring between the CT and shorting device regardless of the intermediate terminal block, right? Don't modify wiring before the shorting device without de-energizing the equipment or opening the respective circuit breaker.

I may not be fully understanding what you are saying. I do appreciate though you taking the time to discuss this with me.

 

[Jraef]

The issue is, opening the circuit under power creates dangerous voltages on the CT secondary that can arc, then that can trigger an arc flash event. Even though you say you will never work on it WHILE energized, having that TB leaves open the possibility that someone would remove a wire and forget to re-connect it, which could have the same effect. So although not expressly prohibited (AFAIK), it's nonetheless considered a bad idea.

 

[Smart $]

It's the same concept for the wiring between the CT and shorting device regardless of the intermediate terminal block, right? Don't modify wiring before the shorting device without de-energizing the equipment or opening the respective circuit breaker.

I may not be fully understanding what you are saying. I do appreciate though you taking the time to discuss this with me.

What I'm not understanding is, why not put the shorting block where you want to put the feed-through terminal block? A shorting block is essentially a feed-through terminal block when terminals are not shorted to the ground bus.

 

[dkidd]

That's what I would do too.

 

[MMC2258]

The issue is, opening the circuit under power creates dangerous voltages on the CT secondary that can arc, then that can trigger an arc flash event. Even though you say you will never work on it WHILE energized, having that TB leaves open the possibility that someone would remove a wire and forget to re-connect it, which could have the same effect. So although not expressly prohibited (AFAIK), it's nonetheless considered a bad idea.

You could make the same argument with the possibility of leaving a wire off of a shorting block though. It'd be pretty mindless to do either way IMO but an extra terminal point is an extra point of failure

 

[MMC2258]

What I'm not understanding is, why not put the shorting block where you want to put the feed-through terminal block? A shorting block is essentially a feed-through terminal block when terminals are not shorted to the ground bus.

the short story is that the intermediate TB is there to prevent a long wire run into adjacent sections to a low voltage compartment where the shorting block resides so you can short the CT without having to get into the high voltage compartment.

 

[Smart $]

the short story is that the intermediate TB is there to prevent a long wire run into adjacent sections to a low voltage compartment where the shorting block resides so you can short the CT without having to get into the high voltage compartment.

Your justification(s) will never outweigh the safety justification. Put a shorting block in both locations.

 

[MMC2258]

Thanks for the help everyone. I appreciate the honest conversation.

 

[Jraef]

You could make the same argument with the possibility of leaving a wire off of a shorting block though. It'd be pretty mindless to do either way IMO but an extra terminal point is an extra point of failure

True, but you don't NEED TO remove the wire from a shorting block... you just short it.