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#62321 - 02/14/06 06:55 PM C.T. as Terminal Block  
master66  Offline
Member
Joined: Apr 2002
Posts: 257
Masontown, PA, USA
I did something today that I probably should not have done.

We installed a new service at a store (120/208 3-phase 400A). There was an existing service located at the front center of the store that included a C.T. cabinet and remote meter.

We installed a new meter socket with a 400A disconnect (breaker) on the side of the building and ran a subfeed (120/208 400A) to the existing C.T. cabinet. The power company disconnected the old service and abandoned the old metering equipment and C.T.s.

I thought "Why not use the old lugs on the old C.T.s as a splice block. That way I don't have to buy any." So, thats what I did. I terminated the black, red and just as I cut the blue to length, the lead lineman came in a asked what I was doing. I explained my idea. He told me that it wasn't a good idea because if the C.T.s were not being used for metering, they would literally burn up.

Well, at this point it was too late because the wires were cut and were too short to connect to the other wires eliminating the c.t.s. So, after some checking, I learned that if I disconnected the metering wires from the C.T.s and shorted the secondary terminals on the c.t.s everything would be OK.

I would like to know from you guys if this is all true because right now the store is operating through the old c.t.s and I did short the secondaries as instructed.

Just so you know, I won't be doing this again. Ever.

thanks
Brian


Work Gear for Electricians and the Trades

#62322 - 02/14/06 08:08 PM Re: C.T. as Terminal Block  
Redsy  Offline
Member
Joined: Mar 2001
Posts: 2,056
Bucks County PA
True, without a load on the CT secondaries they may even explode. This was discussed in detail in a previous thread. Shorting out the secodaries eliminates the problem, unless someone removes the jumpers. If you are that concerned, make up crimp splices to "piece together" the lines.

BTW,
I've had CTs for motor load monitoring that were supposed to go to a recording device, shorted out for years with no problem.

[This message has been edited by Redsy (edited 02-14-2006).]


#62323 - 02/14/06 09:07 PM Re: C.T. as Terminal Block  
WFO  Offline
Member
Joined: Sep 2005
Posts: 202
Cat Spring, TX
Most CT's are shipped from the factory new with shorting bars installed across its terminals for safety. It is the meterman's responsibility to realign these, which is usually done by loosening a clamping nut, turning the shorting bar sideways, (where it doesn't touch either terminal) and re-tightening the clamping nut so it doesn't move.
If the bar was still there, you could have reversed the above procedure and left the CT's shorted directly at the CT. Then you would have been free to remove the old wire and use the terminal strip.

In the long run, this probably would have been safer anyway, since the CT would be shorted regardless of what was done externally. I'm surprised the POCO abandoned it.


#62324 - 02/14/06 10:20 PM Re: C.T. as Terminal Block  
master66  Offline
Member
Joined: Apr 2002
Posts: 257
Masontown, PA, USA
Thanks for the replies. I did do a search on current transformers and did some reading. I never knew there was so much to these things. Had I known, I would never considered doing what I did.

As I stated in my original post, I shorted the CT. I used a piece of #12 THHN about 2" long on each CT. Some of the reading that I did mentioned keeping one terminal grounded. Neither of the terminals on my CTs are grounded. I removed any existing metering wiring and installed the jumper as described.

Is this going to be OK or do I need to do something more?

MAN, do I love this forum.

Thanks again
Brian


#62325 - 02/14/06 11:25 PM Re: C.T. as Terminal Block  
WFO  Offline
Member
Joined: Sep 2005
Posts: 202
Cat Spring, TX
You never said what ratio the CT's were. Like any electrical device, they have a thermal capacity. If they are sufficiently large to handle the existing load....no problem.
If they were abandoned as being too small for the load, they could overheat.

CT's will have a rating factor on them (normally written as RF on the nameplate). They also have a temperature rating with this factor.

For example, a 200/5 CT might have a RF of 2.0 @30 degrees C. This would mean that the CT can handle up to two times its rating as long as it did not exceed 30 degrees. So it would handle 400 amps.


#62326 - 02/15/06 03:28 AM Re: C.T. as Terminal Block  
Trumpy  Offline


Member
Joined: Jul 2002
Posts: 8,222
SI,New Zealand
Brian,
This doesn't sound too good at all.
I personally would have used the terminal block.
There was a discussion about CT's in the Occupational Safety Area.
It can be found here .

{Message edited to add link}

[This message has been edited by Trumpy (edited 02-15-2006).]


Let's face it, these days if you're not young, you're old - Red Green grin

#62327 - 02/15/06 04:41 AM Re: C.T. as Terminal Block  
RODALCO  Offline
Member
Joined: Dec 2005
Posts: 854
Titirangi, Akld, New Zealand
as Trumpy says, read the link re CT's.

It is fine as long the CT's are shorted out across their secondary terminals. e.g. the terminals which used to go to the meter or testblock. ( already mentioned )

It is important that the new load does not exceed the maximum continuous rating of the CT's, in your case the CT has to be 400 Amps or bigger.
In case of multitap CT's the common or S1 tap and the >400 Amps tap may be used to short the CT out with.
It wouldn't do any harm to leave a warning note in the CT chamber DO NOT REMOVE JUMPERS !! for anyone else who sticks it's nose in the CT cabinet.

How hard was it actually to remove the CT's from the busbar or cable, this would have eliminated the whole problem, or were these CT's with a wound primary turn(s) ??

edited for typo's

[This message has been edited by RODALCO (edited 02-15-2006).]


The product of rotation, excitation and flux produces electricty.

#62328 - 02/15/06 09:28 PM Re: C.T. as Terminal Block  
Larry Fine  Offline
Member
Joined: Feb 2005
Posts: 693
Richmond, VA
66, just so you understand what is happening here, the secondary of a CT is a coil of wire that surrounds the line conductor. The line current induces a current in the secondary coil.

A typical CT rating is 400:5, which is the same as 80:1. This means that, for every 80 amps on the line wire, 1 amp flows from the coil to the meter, as a representation of the actual line current.

As long as the secondary circuit is a low impedance, there is almost no voltage developed across the coil, but if the circuit is opened, the current transformer becomes a voltage transformer.

Now, the 80:1 CT has become a 1:80 step-up voltage transformer, with a (80 x 120) 9600-volt output. Most often, this overvoltage causes insulation damage and arcing in the CT.


Larry Fine
Fine Electric Co.
fineelectricco.com

#62329 - 02/15/06 09:34 PM Re: C.T. as Terminal Block  
master66  Offline
Member
Joined: Apr 2002
Posts: 257
Masontown, PA, USA
Thanks again and I appreciate your concern.

Yes, they are sized properly and were double checked today by the power company's meter tech.

I did read that post yesterday and found it very informative. I also talked to everyone that I came across today that works for our company to tell them about my new found respect for CT's. None that I talked to knew anything about CT's. I like to make sure that I and everyone else around me that could ever make the same mistake learns from my mistake.

It is nice to know that you guys are always here.

Brian



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