How safe is a current transformer

Question

I acknowledge that the subject line indicates a "how long is a piece of string" style question, but I hope to make it more specific below.

I have a "current transformer" (CT) purchased as part of an Efergy energy monitor unit. It appears to be a split core ferrite that closes around the primary (live) with a large number of turns on the secondary. The secondary has a 2.5mm audio jack attached for sampling. I assume there is a resistor inside the plastic case of the mini-CT to enable a voltage drop across it (facilitating sampling).

No wires have been cut to attach the mini-CT, it is simply coupled to one live wire. The live wire is at 240VAC RMS and carries a current of several tens of amps to a household.

Is it safe to touch all the connectors of the audio jack, with my feet touching the ground and the mini-CT attached around an AC live wire that's currently drawing a current?

Would the advice of the previous sentence change if I reduced the ratio of secondary turns from 1:2000 (I don't know if this is the true number, I'm just guessing) to 1:10 or 1:1?

Answer 1

There are two ways to get a shock from such a setup:

1. Connecting to the live circuit. This is unlikely as there would have to be insulation breakdown of the live conductor, the CT and the CT wire insulation.
2. A shock from high voltage generated by the CT during measurement. Theoretically this is possible on an open-circuit CT when significant current is flowing in the primary. In practice it doesn't seem to be much of a problem in small CTs. To get a shock you would need to touch both terminals simultaneously. The shock current would flow from terminal to terminal through your body and not to ground (through your feet mentioned in the question).

Is it safe to touch all the connectors of the audio jack, with my feet touching the ground and the mini-CT attached around an AC live wire that's currently drawing a current?

Bare feet? No, not safe for any work near mains. You should be wearing decent work shoes.

Answer 2

It is NOT safe to assume that the current transformer has an INTERNAL burden resistor.

It is not a matter of a voltage between the current transformer and ground. That is very unlikely unless there is some major breakdown of insulation of both the cable your are monitoring AND the current transformer.

The RISK is from the voltage generated by an unloaded current transformer sensing some significant current. And "tens of amps" very much qualifies as "significant current".

Unless proved otherwise, it is ALWAYS safe to assume that the bare output of a current transformer is DANGEROUS.

Answer 3

You need to consider:

1. Assuming that mains current (from say a wall outlet) is being sensed, the insulation / casing of the CT will have to be rated to block well over the line voltage. (230V or 110V RMS). If this is the case, the secondary of the CT should be quite safe to touch since it will be well isolated from the line voltage.

2. Assuming your turns ratio is large enough on the secondary side, the current in the secondary will be small and so will the voltage across the burden resistor.

3. In most "readymade" CTs (see below) you cannot change the number of secondary turns, so your output volage range is fixed for a given input current range, and is typically <5V.

   

If you are making your own transformer, then you need to design the ratio suitably.

Answer 4

A CT can become unsafe if the burden resistor on the secondary becomes open circuit. The output volts will try to go sky high but transformer core saturation will limit that. The extra core heating could cause dangerous temperatures despite the load current being below the rating of the CT. In fact you could overheat the CT without blowing the fuse. Precations should be taken to avoid this potential safety hazard.