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Anonymous Poster #1

# Current Transformer Question

08/26/2017 9:30 AM

Hi,

In a substation, there is LA, PT, CT etc. But when looking at it, I am not able to figure out if it is a protection CT or metering CT.

How can we determine if it is a protection CT or metering CT just looking at site and not referring to the diagram?

Regards,

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#1

### Re: current transformer:

08/26/2017 9:46 AM

If you can't see what it's connected to, I suspect you can't.

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#2

### Re: Current Transformer Question

08/26/2017 2:11 PM

You could probably figure it out from the ID plates on the equipment....

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#3

### Re: Current Transformer Question

08/26/2017 2:32 PM

Can you see any markings on the CTs? If so, and you are in an area adhereing to ANSI standards, there will be a set of alpha-numeric characters that look like "0.3 B 0.2", or "2.5 C 20" that depict the accuracy, class and minimum burden ( in that order). The CTs with the lower value in the first set, the accuracy, will be the metering CTs. So of those two examples, the one that says "0.3" is the metering CT, meaning it is accurate to 0.3%, the one that is 2.5% accurate is going to be used for relaying.

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#4

### Re: Current Transformer Question

08/26/2017 10:20 PM

If you can not see the whole picture you should leave well enough alone.

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#5

### Re: Current Transformer Question

08/26/2017 11:21 PM

Unless that guy is an electrician.

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#6

### Re: Current Transformer Question

08/26/2017 11:22 PM

If you are not an engineer...you cant.

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#7

### Re: Current Transformer Question

08/27/2017 2:02 AM

Check the CT rating label and accuracy class, usually CL-1 for metering and 5P10, 10P10 and CL-X etc.. for protection

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#8

### Re: Current Transformer Question

08/27/2017 10:54 AM

It depends on the wire numbering system, but if the wire numbers at the CT and the meter amps phase switch are same then that is obviously metering. Also, protection CT terminal wire numbers and those at one protection relay [or protection panel input terminals] will be same.

But some wire marking systems mark wire ends with code for what they connect onto, rather than wires connected together having unique number.

If the system is dead, you could check continuity on the wires or inject a few volts AC on CT terminals with a transformer from wall socket and see what gets volts at meter or protection points.

As in other replies, the CTs must be marked with type codes, which will be recognised if you put them in a reply.

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#9

### Re: Current Transformer Question

08/27/2017 10:46 PM

Hi,...as some others have said rating plate is best way to check..I. e....10P10 class would be protection .and 5 or 10CM would be metering...In some cases metering only needs Two c.t,s on R and B..so two ct,s would be metering. .Note clue could also be if each ct has more than two terminals..but be careful sometimes two types of protection could be used per phase..but then it would be on all three phases..carefully make sure power is off.. could be ct,s on outgoing..or !! On incoming cable ..check before opening up ct chamber..Note type of termination and Insulation, it needs to be replaced as it was..have assumed this is a high voltage switchboard. .Note also there should be NO ct terminals which have no wires connected..at least unused terminals should be shorted out..Best is to have experienced technician helping you..regards

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#10

### Re: Current Transformer Question

08/29/2017 4:36 AM

If one is not using the <...diagram...> then it matters not one jot.

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