The apartment complex I’m in is only a year old, so we’re still in the Defect period with the builders. I’m on the Committee of the Owners Corporation, so I get to see the utility areas. The builders put in private metering; those meters always show 243V (expected) but 0W. I just assumed they didn’t install the CTs.

I was there when an electrician popped the panel on a switchboard with a private meter for something else, and was astonished to see that the three-phase feed did indeed have a CT installed. They just hadn’t done the final step of connecting the CT to the meter. There’s no sign of a shorting load across the contacts either.

Some quick Googling confused me, because the CT was 125:0.1, so 1250x on a 240V circuit, meaning 300kV across these open contacts at peak load. Only 30kV could arc in air over 1cm, but the contacts are maybe 1mm apart in an unplugged RJ12. (Datasheet for both meter and CT: http://www.crompton-instruments.com/downloads/2015/EPP-2238-0315_Integra_DL1.pdf)

OK, so it isn’t arcing. Good. That’s not actually my question. More Googling implies that the CT is now fully saturated magnetically, so useless anyway. Is that true? Are the CTs now so hopelessly compromised that simply plugging them in won’t work now? Even worse: is the existing installation dangerous, going to arc at the first dust particle the settles across the terminals?

The installation instructions for the CT states “Don’t open circuit the CT while the primary is energised.” That’s ambiguous. One way to read that is that it’s the act of disconnecting the CT that’s dangerous - which I can fully imagine since it would draw an arc. But that might mean leaving it open is fine? There might be circuitry in the CT to detect open circuit?

I’m going to also assume that connecting the CT to the meter while energised is also a bad idea, but perhaps slightly less so. One factor stopping the committee from finishing the private metering has always been the fact that we’d have to drop the power while CTs were being installed. A couple of minutes of no power is a lot more palatable than many hours’ worth! Although, if we’d have to replace the CTs anyway, it wouldn’t be a saving.

If the CTs hadn’t been installed, then it isn’t a defect - they installed V&I meters but only wired them up for volts. But because they installed the CTs, it is a defect that they didn’t wire them up: especially since the CTs even come with the wiring. Getting the builders back to wire them up will be easy; but if the CTs don’t work anymore, getting the builders to replace them will be much harder.


1 Answer 1


Current transformers don't work on voltage, they work on current - they are called current transformers after all.

If they have a rating of 125:0.1, or simplified, 1250:1, it means that 125A of current in building wiring will produce only 0.1A of current in the measurement coil. But only when properly loaded, not when left unconnected.

To think this in terms of voltage, the primary coil is only the building wire that goes once inside the measurement coil. So it will have an extremely small voltage over the primary coil. So for all practical purposes, the primary voltage will be much less than 1 volts.

If it were as large as 1 volts, it would be stepped up to 1250 volts in the secondary coil with no load. As dry air has breakdown voltage of about 3kV/mm, it should not spark even if wires were just 1mm apart.

  • \$\begingroup\$ Yes, the CT has no idea what the voltage is in the wiring it is measuring. \$\endgroup\$ Mar 27, 2022 at 2:08
  • \$\begingroup\$ @Justme OK, good! No arcing; that means it’s not dangerous. But what about the CTs themselves? Are they saturated magnetically? One article I read stated that the CT was now useless, implying it had to be replaced. \$\endgroup\$ Mar 27, 2022 at 2:30
  • \$\begingroup\$ There’s a bit more to it than that - the transformer is in series with the load and so the voltage across it depends on the impedance that it presents. Thus would normally be 1/1250 of the value of the burden resistor, although that’s open circuit and so the CT will present a high impedance until it saturates, and during that time it will generate a huge voltage on the secondary. Operating CTs without a load is a very bad idea; it’s quite possible that they have arced until the insulation broke down and are now shorted. \$\endgroup\$
    – Frog
    Mar 27, 2022 at 2:34
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    \$\begingroup\$ Magnetic saturation is an entirely reversible process and is commonly used with some inductor circuits, it won’t harm the magnetic core \$\endgroup\$
    – Frog
    Mar 27, 2022 at 2:36
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    \$\begingroup\$ @Frog Ah! So the problem is that the CTs might now be internally fried. If I connect one and the meter still reads 0W, then it’s dead. Thanks! \$\endgroup\$ Mar 27, 2022 at 2:39

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