Functioning of a CT

(New to electricity.) If I had to clamp a CT on a cable, should I trim it and attach it to a single wire or rather put the CT on the cable itself. I was peeping through some community posts(that I no longer can find) in which it was said that if it were to be on the cable it would neutralize and have an output of zero. So I am a bit confused.
and if I were to trim it and then clamp on a single wire in a cable, would it be safe as the mains cable is a bit exposed.

Look in the ‘Learn’ Section - there are FULL details there.

So, since in 3 phase systems multi-core cables are used, the only solution would be to trim the wire and look for the live to clip it and read the power output. Is this the only way to read it? And if so yes, is it safe to trim the twin core cable because wouldn’t it increase the probability of a person getting electrocuted.


Let me explain the make-up of a typical cable. Starting from the inside, you have the copper (usually, sometimes aluminium) conductor. This is surrounded by a layer of PVC (usually, but other plastic materials are used) insulation. The insulation is good as insulation, but it is not very strong mechanically. If there is another form of mechanical protection present, plastic or steel conduit or trunking, then no more protection is needed. The thickness of the insulation is determined by the voltage rating.

If there will not be anything around the wire to protect it, then a second layer of much stronger and tougher plastic is added for protection. This is the sheath, it’s not insulation. You can have, as you do, two, three or four insulated wires twisted together inside the same sheath to form a multicore cable.

If the cable will be buried, or in a place where serious damage is probable, you will then have a layer of steel wire armour and a further layer of very tough plastic to keep moisture out and prevent the armour going rusty.

So the direct answer is, yes you can with a lot of care strip back the sheath and separate the cores so that you can put a c.t. on each of the three phases. I do not recommend doing it this way. You must be very careful to not damage the insulation on the cores, and you must protect the whole assembly where the cable cores are exposed, because you have no mechanical protection for the insulation on the individual wires, which as I’ve said is easily damaged.

A much better solution is to find a place where the wires are already separated - such as inside a terminal box or switchboard.

If you can’t do that, I think a safer and probably easier way is to cut the cable, put each end through a gland into a terminal box, then you can strip back the sheath, separate the cores, thread them through the c.t. or c.t’s as necessary, and join the wires in a terminal block. This way, everything is protected and safe.

Note: You never need a c.t. on the neutral wire. If you have a split-phase system or a 3-phase system, the neutral carries the imbalance of the current in the phase conductors, so it tells you (almost) nothing.

Hi Robert thanks again for the descriptive answer. Do you have any links or any blogs which guide you through the procedure that you just mentioned in the second last paragraph. It would make things a lot easier for me.

How to buy a box, drill holes for glands, strip the cable and connect the wires in a screw terminal connector block? Sorry, no.

I sense a bit of frustration in your tone and that is completely understandable for me. I went a bit far and thought of being spoon fed everything. You have no idea how much you have helped me for my uni project. Anyways one day I hope to be a net contributor to the community rather than a taker. Gratitude.