Electronic – How safe is a current transformer

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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?

Best Answer

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.