CT Sensor sensitivity

I have seen on here some people saying that the CT isn't sensitive at low currents, or rather that the 0-1V scale is barely used.

Isn't a CT just a fancy transformer? Say we have a 0-1V scale, and a 100A CT measuring a 16A circuit, can we not just put the input cable through the CT 4 (or preferably 5) times?

Apart from space in the CT, ?possible heating issues? what else is the problem?

Robert Wall's picture

Re: CT Sensor sensitivity

If you'd searched the forums, you'd have found exactly that mentioned. If you'd read the test report on the YHDC CT, you'd have seen that is exactly how it was tested.

coldpenguin's picture

Re: CT Sensor sensitivity

Thank you for your informative answer.

Yes, I had read that test report, then found at least one thread where someone asks whether there are more sensitive CTs, but no-one suggested it as a work-around.

One thread (from 2011) did suggest that a properly designed CT would be more sensitive.

My here assumption being that the tester had used specific equipment to generate the current for the tests, which might (would had) have thinner core wires than you would find in a 16A/32A ring main, thus the question about physical size, or that there was some other reason, such as increased heating effect/derating.

Robert Wall's picture

Re: CT Sensor sensitivity

In the absence of specific information about your installation conditions, it is very hard to give any sort of answer. Having now mentioned "32 A" and "ring main", that gives me a few clues. If your circuit protection is a 32 A MCB, then you must use a cable appropriately rated. That would mean, if your cable is in free air, 4.0 mm2 would be needed, and I think you'd probably get 5 turns through a standard 100 A YHCD ct. If the circuit is a ring main in 2.5 mm2, you'd have 2 cores per turn and it's looking very very doubtful. 5 turns would probably be OK provided the cables are not tightly bunched except where they pass through the ct, and you're in a cool location. If they are bunched, then they have to be derated to around 80% (from memory) and then the 4.0 mm2 is not good enough.

You can also increase the burden value, provided you stay within the (unpublished) VA rating of the CT, but that will increase the errors due to the core losses whereas more primary turns reduces them. You might want to consider both methods.

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