C/T Ratio on 15kV Equipment

[dcheser]

I'm trying to determine the C/T Ration for a trip relay on a 15kV Breaker. The nominal voltage is 13.8kV, the breaker is rated for 1,200Amp. Am I wrong to assume that the C/T's are 1200:5?

This is vintage equipment installed in the mid 70's and I can't access the C/T's without opening the gear which of course is hot.

 

[SG-1]

The CT ratio would have been based on the original load & could be any ratio up to 1200A.

 

[rbalex]

1200/5 might be ok, but far more important is the actual load ampacity. Typically, depending on the application, a CT with a 5A secondary will have a primary 100 to 200% of the load ampacity. Maximum through fault may also adjust the ratio as would the general application, such as various differential schemes.

 

[dcheser]

The C/T is on a Westinghouse 50/50 trip unit for the breaker. I didn't know if there was a rule of thumb or not. On most of the equipment we have found that any manufacturers labeling is gone, corroded and so badly faded that it can't be read. Trying to build an electrical model for Arc Flash evaluation and information is proving difficult to obtain.

 

[zog]

I assume you mean 50/51. CO type? That should have a 5A secondary, but primary could be anything up to 1200A as SG-1 stated. This is a very important data point for an arc flash study, I would not make any assumptions. If you can't read the label you may need to do a ratio test to verify.

 

[rcwilson]

CT Ratio

CT Ratio

Is there an ammeter on the door of the switchgear? Most meters are 5 amp movements. If the meter is 0-1200A, it probably is a 1200:5 CT. Could be a 100:5, 150:5, 200:5, 250:5, 300:5, 400:5, 500:5 600:5 ..... or 1500:5.

Sometimes the CT ratio is listed on a test sticker attached to a relay.

If there is a way of estimating the primary amps on the circuit by looking at other metering, clamp on the CT lead on the relay and calculate the ratio. Not accurate but it will get you close.

 

[kingpb]

Could also be a 1200/5 MR as well. And no guarantee that it isn't higher than 1200A primary. Taking a look probably isn't going to do you any good either, may need to inject current and check ratio.

I would have thought their existing coordination study has it listed. What, you say no existing coordination curves.....yikes:weeping:, you have your work cut out for you........

 

[dcheser]

Yes it was a 50/51 CO-8 type unit. The best data I have so far is a breaker test report but nothing on the C/T ratio unless I am overlooking something. I will make a trip back down and check the relay unit itself and see if there is a Ammeter in the door. Don't remember one being there but never hurts to look again.

 

[rbalex]

The OP stated the gear is live. I don't see any tests or visual inspections that will resolve the issue short of a shutdown.

 

[dcheser]

Pretty much what I had figured. Was hoping soneone out there had a better solution because talk of shutdowns are taboo. Can't make product if the electricity is turned off.

Thanks for the input!!

 

[kingpb]

Unfortunately the inst. xfmr Ratio listed as 41-100 only leads you to an ABB document number about the CO relay. If you can come up with any type of model number, contact ABB. They bought the Westinghouse T&D business which included instrument transformers.

 

[rbalex]

Pretty much what I had figured. Was hoping soneone out there had a better solution because talk of shutdowns are taboo. Can't make product if the electricity is turned off.

Thanks for the input!!

Is this operation subject to Process Safety Management (PSM) If it is, it won't solve your problem this time, but it may help the next guy - and get you a little extra work now.

 

[zog]

Shame the ratio is not on the test report

 

[richxtlc]

Yes it was a 50/51 CO-8 type unit. The best data I have so far is a breaker test report but nothing on the C/T ratio unless I am overlooking something. I will make a trip back down and check the relay unit itself and see if there is a Ammeter in the door. Don't remember one being there but never hurts to look again.

What does an as left setting of >75 mean? 77,85, 125??????????? Unless this is the initial installation of this relay the as found setting on the instantaneous unit is a range not an as found. If this was a periodic test the as found should be what the actual pickup was when tested, not the range of the unit.

 

[SG-1]

Question: is the gear DH or DHP ? There should a nameplate on the gear.
The nameplate on the breaker will also indicate this. The breaker plate would say 150 DHP 500 or something simular.

Can you find a number on the gear that starts with S.O. 25Y or 56Y ?

What vertical structure are the CTs in, 1,2,3,or 4... ? Some people call these bays.

 

[rbalex]

Question: is the gear DH or DHP ? There should a nameplate on the gear.
The nameplate on the breaker will also indicate this. The breaker plate would say 150 DHP 500 or something simular.

Can you find a number on the gear that starts with S.O. 25Y or 56Y ?

What vertical structure are the CTs in, 1,2,3,or 4... ? Some people call these bays.

Getting the shop order is definitely a good idea; however, he indicated that most of the nameplates were missing.