Current Transformer Question

[EECougar]

I have a bushing current transformer on a transmission circuit breaker. If I short the current transformer and ground it, does current still flow through the closed loop?

 

[davidbeach]

Yes.

 

[scottf]

Yes

 

[jraef]

Even if you can't see it on a meter?... kind of like the tree falling in the forest.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[EECougar]

We actually have a CT going to a relay but the relay is reading zero amps and zero degrees so we are thinking that a shorting screw was left in place somewhere.

What we were wondering was if we took a ammeter if we could test the current near the CT to see if this was the case. The ammeter would be a wrap-around type.

 

[davidbeach]

Yep, between the CT and the shorting terminal there will be current in the loop.

 

[dpc]

We once found a relay that had been "operating" like that for about 20 years. Relay had been through several bench tests and was in great shape - except for the shorting jumpers out at the circuit breaker.

Make sure you don't accidentally open circuit the CT secondary as you are getting things fixed up.



David Castor

http://www.cvoes.com

 

[electricpete]

I dunno. Tree in the forest got me thinking. Good one.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[marks1080]

If a CT pops in a yard and no one is around to hear it...

 

[waross]

CT's are made safe by shorting them. Many have shorting hardware built into the terminals of the CT. Be very careful opening the shorting links if current is flowing in the primary conductor.
When the CT is shorted, the resistance is very low so the I²R is very low.
I lost a good meter on an open CT. Someone cut the leads in a wire bundle behind the control board. I was trying to locate a particular CT so I took a voltage reading across the relay and was behind the panel checking voltages across CTs looking for a match. (Yes, I admit it, I was trying to get lucky.)
I measured one CT and my meter made a buzzing sound and never worked again. After safeing the CT I found the leads cut in a bundle. I thanked God that I hadn't been running my hands near the cut ends following a conductor.
I was sure glad to go home that day.
The joys of working in the third world.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

"I lost a good meter on an open CT."

I lost some good underwear in similar circumstances. A bad crimp open circuited a massive 18000/5 Class X CT on a generator diff scheme while we were doing final verification tests with the unit running - the arc looked like it was about 2" long although I imagine it was much smaller. I vaulted clear over a guard rail on the platform just to get away from it and landed at grade about 10' below then ran to trip the unit. Ordinarily I might not have reacted quite like that but the transformer in question was the replacement for one which blew up a year before so there was an element of voodoo hoojoo when the crimp broke and struck the arc.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[86ed]

20 years "operating"!! That's pretty funny.

 

[ANGIBEW25]

Hi EECougar:
You stated,
"We actually have a CT going to a relay but the relay is reading zero amps and zero degrees so we are thinking that a shorting screw was left in place somewhere."

Depending on the type of relay, don't forget to check the case. Over the course of time I have found several mismatched relay/case combinations.

Of course if you are using a clamp around type Amprobe to measure current then a defective or mismatched case will not be your problem.

Also check for a stray ground in the wiring. If the CT polarity side wiring (X1) is inadvertently grounded, it could shunt the current to the relay.

 

[westex]

Take an outage on the CB and shoot current to prove your current path.