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.
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).]
Re: C.T. as Terminal Block#62323 02/14/0609:07 PM02/14/0609:07 PM
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.
Re: C.T. as Terminal Block#62324 02/14/0610:20 PM02/14/0610:20 PM
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
Re: C.T. as Terminal Block#62325 02/14/0611:25 PM02/14/0611:25 PM
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.
Re: C.T. as Terminal Block#62326 02/15/0603:28 AM02/15/0603:28 AM
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.
Re: C.T. as Terminal Block#62328 02/15/0609:28 PM02/15/0609:28 PM
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.