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We would like to know if a shielded high voltage cable 8000V AC, 3 phases, is "live". Whether there is current flowing or not.

Ideally we design a little device that's strapped around the cable (non contact with conductors) and a light lights up when live. We are also fantasizing of having this sitting on the cable, with no batteries, using somehow a bit of power from the cable.

I must admit that I'm a bit rusty on the electricity fundamentals.

  • I do understand that there should be no magnetic field measurable (no current, and all conductors in one cable).
  • I presume there is also no electric field measurable outside the shielded cable.
  • I was looking up different devices, and was even trying a simple capacitive sensing detector AC voltage testing tool (Klein NCVT-2) (was only rated to sensing 1000V). To no avail.

I wonder if I am overlooking something or if there are other means of sensing the presence of voltage.

ps as a last resort, it would be possible to open up the connector, so we would have access to individual conductors.

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  • \$\begingroup\$ Theres no net magnetic field far from the cable, but if your probe is very close to one of the individual conductors inside, the fields from the others don't cancel. At 50 Hz the shielding won't attenuate them much. Try holding a small coil of wire against the cable. Small is <1/4 the diameter of the whole cable. Measure the voltage induced with a scope or millivoltmeter and see if you can find an orientation and position that picks it up. \$\endgroup\$ – tomnexus Jan 24 at 20:52
  • \$\begingroup\$ @tomnexus: good idea! That would only be for current carrying conductors though, right? \$\endgroup\$ – lode Jan 24 at 21:04
  • \$\begingroup\$ Yes. If there's no load, this won't detect any current \$\endgroup\$ – tomnexus Jan 25 at 4:57
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Assuming the cable is unshielded, if you are close enough to it neither the magnetic field nor the electric field would cancel. The fields of the closest conductor would dominate.

A small (~1cm long) conductive surface comparable in size to the conductor cross-section a few millimeters away from one of the conductors should have a capacitance of more or less 1pf to it. At 50Hz this is an impedance of \$\approx 3 G\Omega\$ quite high but not a problem.

On a \$3M\Omega\$ input impedance (lower than most multimeters and high-impedance op-amps) you should still have a very measurable ~1V signal from your 1000V conductor.

Place a short jacket with three conductive surfaces that are located 120º from each other, and you should be able to pickup each individual phase just by rotating the jacket until it is aligned with the conductors in the cable.

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