Current Transformer

[mjc1060]

I have to remove and replace a protective relay for a 4160V circuit breaker. When I remove the protective relay is the CT secondary considered open? Do I need to short the CT before the relay is removed?

 

[petersonra]

I have to remove and replace a protective relay for a 4160V circuit breaker. When I remove the protective relay is the CT secondary considered open? Do I need to short the CT before the relay is removed?

if the primary side is still energized you definitely want to short out the secondary side or you will get an infinite voltage there. the CT should be wired to a shorting block anyway. Just short it out before you mess with it any.

 

[Hv&Lv]

If the CT wires are ran through a test switch it should shunt when you open them.

 

[Jraef]

Hopefully you are not having to do this live, and that the equipment vendor didn't cheap out and fail to install shorting blocks. Worth checking for.

 

[GoldDigger]

if the primary side is still energized you definitely want to short out the secondary side or you will get an infinite voltage there. the CT should be wired to a shorting block anyway. Just short it out before you mess with it any.

The voltage may seem infinite, but it is finite and predicted by the turns ratio of the CT and the load impedance on the primary side, as well as the saturation characteristics of the CT core.

 

[synchro]

if the primary side is still energized you definitely want to short out the secondary side or you will get an infinite voltage there. the CT should be wired to a shorting block anyway. Just short it out before you mess with it any.

The voltage may seem infinite, but it is finite and predicted by the turns ratio of the CT and the load impedance on the primary side, as well as the saturation characteristics of the CT core.

And also possibly limited by whatever is the weakest link in the insulation breakdown voltage on the secondary side.

 

[Jraef]

And also possibly limited by whatever is the weakest link in the insulation breakdown voltage on the secondary side.

I've witness that one first hand... that was not a fun day.

 

[petersonra]

The voltage may seem infinite, but it is finite and predicted by the turns ratio of the CT and the load impedance on the primary side, as well as the saturation characteristics of the CT core.

Ok, so I exaggerated

 

[xformer]

I've witness that one first hand... that was not a fun day.

Can you share any details? I was explaining to a group of people this same issue of short circuiting CT secondary if primary is energized.

 

[hillbilly1]

Here’s what happens when one goes open! LOL! The meter failed on this one, mucho dinero to fix!

 

[xformer]

Here’s what happens when one goes open! LOL! The meter failed on this one, mucho dinero to fix!

How long did that take to occur? Looks like it burned up.

 

[hillbilly1]

How long did that take to occur? Looks like it burned up.

Don’t know how long it took, customer owned meter/transformer/high line from street. I just walked into the store about 15 minutes after it blew a pole fuse. Melted the secondary feeds to the store. The poco went ahead and upgraded their metering at the street. Store was on generator for about two weeks.