Sorry if this question is too basic for this forum... I'm an interested amateur but far from a professional. I'd really appreciate information from someone more informed than myself.

Hypothetical challenge: Assume I have several UK mains electricity (~240v ~50Hz AC) insulated cables and have access only to the insulated exterior of the cable.

Can I (hopefully accurately) determine the direction and magnitude of power flow? Please assume that it is unacceptable to strip the insulation, or break the circuit, to insert traditional test equipment.

Added: How accurate an answer can I expect to achieve, given the constraints?

  • 2
    \$\begingroup\$ Google current clamp. Not so easy for voltage though. \$\endgroup\$ – Eugene Sh. Dec 17 '18 at 21:47
  • \$\begingroup\$ AC flows in both directions. Accuracy will depend upon accuracy of measuring device. Usually 0.5% or lower , <1.2V on 240V. \$\endgroup\$ – StainlessSteelRat Dec 17 '18 at 22:09
  • \$\begingroup\$ Do you have access to each cable in the pairs or they're both inside a common protective insulation? \$\endgroup\$ – Dorian Dec 18 '18 at 10:26
  • \$\begingroup\$ For this question, I'm assuming I only have access to 'normal' mains cable - with 3 inner insulated cables inside an outer insulation. \$\endgroup\$ – aSteve Dec 19 '18 at 20:53

have access only to the insulated exterior of the cable.

If all you got is the cable, then no.

You can use a current clamp meter around a wire. It measures the magnetic field created by the current inside the wire. But you can't use this around a cable, as the cable contains two wires with current going in opposite directions, which means their magnetic fields cancel. Whatever magnetic field there is to measure around the cable will depend more on its geometric properties than on the actual current.

Can I (hopefully accurately) determine the direction of current flow

I'm going to pretend not to notice that this is AC and therefore current reverses direction 100/120 times per second. I think you meant the direction of power flow, which is from the utility to the load. Unfortunately, your cable contains two live wires (and maybe one Earth) and the current in both live wires goes in opposite directions since current always travels in a loop...

You can measure current by stripping the cable and getting to the wires. You don't need to strip the wires' insulation or cut them. Snap a current clamp around one wire and you can measure the current.

Measuring voltage without breaking the insulation is another challenge... What I usually do is to simply puncture the wire insulation with the tip of the multimeter probe. Of course after doing that the wire will no longer be waterproof, so it is only OK to use indoors.

  • \$\begingroup\$ Yes - my lack of experience shows. I meant direction of power flow. I am really only interested in power. \$\endgroup\$ – aSteve Dec 17 '18 at 22:18
  • \$\begingroup\$ I'm not blaming you ;) if you want power flow on AC you got to measure voltage and current, and you can't do that without stripping the cable a bit... \$\endgroup\$ – peufeu Dec 17 '18 at 22:53
  • \$\begingroup\$ I'm blaming me... I should have known better - I did physics at high school. ;) Current clamps are interesting. I guess I need to think 'further outside the box' if I am to find a solution. As an ultimate goal, I am looking for a way to verify the performance of a grid-connected solar system. Perhaps I need to take a radically different approach. \$\endgroup\$ – aSteve Dec 18 '18 at 7:19

For current, a current clamp is a complete answer.

For voltage, you will need to know the dimension of the inner conductor underneath the insulation, and the dielectric constant of the insulation, but you can make a good guess for both of those.

Wind a thin metal foil over the insulation. Wind some insulation over that, and another foil, which you connect to earth. Connect a length of coax, inner to the first foil, outer to the second. Take this to your oscilloscope.

From the dimensions and plastic properties, estimate the capacitance you have cable to foil, and between foils, using the standard capacitance formula for coaxial cylinders. These, together with the capacitance of the cable, and the capacitance and resistance of the scope input, form a mostly capacitive voltage divider. You could probably estimate the gain of this to within a few 10s of percent, or you could calibrate it for higher accuracy.

If you only need a voltage waveform reference to use with the current clamp to determine power flow direction, then the absolute gain of the voltage divider is not too important. To minimise the phase shift due to the scope's input resistance, parallel a large enough capacitor with it. For instance, 100nF//1Mohm gives you 100mS, but would give you a huge voltage pot-down ratio with your few pFs pickup capacitance.

You may want to precede the scope with a FET follower, or a high impedance opamp to buffer the signal. Putting this right at your foil-wound pickoff capacitor would minimise the signal loading and improve your output signal no end.


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