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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?

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  • \$\begingroup\$ What warnings are given in the product documentation? Is it CE marked? The turns on the secondary are nothing to do with this as far as I can make out. \$\endgroup\$ – Andy aka Apr 18 '16 at 10:49
  • \$\begingroup\$ What is the mains? 120v? 220? 480? 600? 1200? \$\endgroup\$ – rdtsc Apr 18 '16 at 13:16
  • \$\begingroup\$ 240VAC. Edited the question to include that. Thanks \$\endgroup\$ – Jono Apr 18 '16 at 14:16
  • \$\begingroup\$ @Andy, the product documentation is probably quite full of warnings advising not to touch, and CE markings on every inch of available plastic. I am asking not for the specific item that I have, but of current transformers in general. \$\endgroup\$ – Jono Apr 18 '16 at 14:18
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    \$\begingroup\$ Note that CTs should always have a burden resistor on their secondaries when in a live circuit (i.e. when there is even a possibility of current flowing), otherwise it will (within practical limits) try to make current flow through fresh air by generating an extremely high voltage, generating an arc, damaging it's insulation, and probably damaging (ha ha) the person installing it as well :) \$\endgroup\$ – DaveBoltman Apr 18 '16 at 16:33
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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.

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  • \$\begingroup\$ I should like to think the CT remains isolated; that the core never becomes hot enough to melt the insulation; that the two circuits never physically touch. If such a fault occurred, then I'd hope that was the day I wore my thickest running shoes to prevent conducting live to ground. And I presume that I would need to touch both terminals of the open circuit to close it and get the shock. \$\endgroup\$ – Jono Apr 18 '16 at 14:29
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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.

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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.

    PCB mountable 5A current transformer

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

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    \$\begingroup\$ Never use the CT without a burden resistor. The low current is normally fine (still don't assume it can't hurt you.. because it can.. the usual current across the heart can kil you) but the open circuit voltage could arc and break the coil... If it has an internal resistor then that's fine. But do you know it has one? \$\endgroup\$ – Spoon Apr 18 '16 at 11:36
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    \$\begingroup\$ Just for the curious... I've used CT's where the output was 1A. The primary was in 1000 A. . context is important. \$\endgroup\$ – Spoon Apr 18 '16 at 11:40
  • \$\begingroup\$ Am I asking for trouble if both primary and secondary are simply fed through the same ferrite ring? It seems to me this would give 1:1 turns ratio - and therefore the highest number of amps - yet also be the easiest way to misconfigure a homemade device of this ilk. \$\endgroup\$ – Jono Apr 18 '16 at 14:32
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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.

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