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# CT selection and amperage

## CT selection and amperage

(OP)
Hello
Can a Current Transformer with a ratio of 50/5 be effectively utilized on a 1200A bus or incoming cables?
What are the key considerations when using this CT on a higher ampacity application?
Also, any recommendations for CTs that can accurately detect 48 Amps?

### RE: CT selection and amperage

One CT per phase or one CT around all three phases? In the first case, multiply 50 by the CT TRF (thermal rating factor) and I've never seen that higher than 4, so the highest load current you could have would be 200A, perhaps less. But in the second case it would only be measuring unbalance and could be the proper size for the application.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

### RE: CT selection and amperage

budhiman (Electrical)(OP)25 Oct 23 21:30
"...#1. Can a Current Transformer with a ratio of 50/5 be effectively utilized on a 1200A bus or incoming cables? ....#2. What are the key considerations when using this CT on a higher ampacity application?...#3 any recommendations for CTs that can accurately detect 48 Amps?"
1. No. A 50/5 CT shall NOT be used on 1200 A bus/incoming cable. Not only the current rating but also the "window size" is unlikely to be able to accommodate 1200 A bar/cable.
2. The CT primary shall be = or > 1200 A. The secondary may be 1 or 5 A. Therefore, the CT shall be minimum 1200/5 or 1200/1, may use 1600/5 or 1600/1 ; which are the standard ratings.
3. Use any 1200/5 measuring CT would be able to detect 48 A, which 48A x 1200/5 = 0.2 A. Note: use "measuring" CT NOT "protection" CT.
3.1 Why you want to detect 48A accurately on a 1200A system? Do you mean 48A earth-fault current? If so, see 3.2.
3.2 To detect 48A for earth-fault protection, you may use a very big window zero-sequent CT (Z CT) with a corresponding protection relay by the OEM. In this case, detection accurately 48A zero sequency-current is possible without any difficulty.
Che Kuan Yau (Singapore)

### RE: CT selection and amperage

If your concern is how to measure 50A current accurately, you can do it provided the phase CTs (1200/1A or 1200/5A ratio) are metering class rated to 0.2S.
IEC6189-2 defines allowable error at 5% (of rated current) as 0.35% and at 1% (of rated current) as 0.75% for 0.2S class CTs.

R Raghunath

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