Cable capacitive leakage tripping RCB

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Following problems with un-explained tripping of an RCB I looked at the possibility that stray leakage currents were a significant part of the earth leakage un-balance. I looked at the inter conductor capacitance in cable and the earth leakage current that creates.

Test cable was a coil of 40 metres of 1.5 mm triple and earth on the test bench.

The cable conductor lay is Grey CPC Black Brown

An accurate milli-amp meter was connected between the CPC of the cable and the mains earth.

The current in the CPC was measured with different combinations of live and neutral on one or two of the conductors. Other conductors were left open circuit.

There was no current flow so the earth leakeage current induced by magnetic coupling was negligable.


brown wire live 2.6 mA

grey wire live 3 mA

grey wire live
brown wire neutral 0.9 mA

grey wire live
black wire neutral 0.1 mA

grey wire live
black wire live 3.4 mA

With a live and neutral either side of the CPC the leakage is very low. This is the condition in most cables in normal use.

With live both sides of the CPC the leakage is significant. This is the condition in a cable to a switch in the ON position.

The results suggest to me that the leakage on long switch cables will contribute significantly to the tripping of the RCB.

It would be interesting to find the capacitive leakage current from the live conductor into an earthed metal conduit.

The results suggest that 400 metres of switch cable would trip a 30mA RCB when all switches were ON ( assuming Live looped at ceiling roses )
 
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I'm concerned about this:

Test cable was a coil of 40 metres of 1.5 mm triple and earth on the test bench.

Any chance you could repeat the experiment with the cable taken off the reel and laid out in a snake shape, with the runs as far apart as you can get?
 
I guess you used mains voltage?
My calcs come out at 1mA of leakage per ~14nF at 230V 50Hz.
I am suprised you would get ~30nF from only 40m of wire.
With it being on a reel you are most likely may getting inductive/antenna effects.
The problem with this sort of thing is the positioning of the wire will also change the capacitance i.e. a cable surrounded by air will have less capacitance than one surrounded by metal/concrete/etc.
 
The prime circuit is the capacity of the dialectric ( the PVC insulation ) directly between the live conductor and the CPC.

There are other stray circuits.

I accept the cable being coiled will not give the same results as with the cable out straight. But with no current flowing ( other than the 3 mA leakage ( which is zero at the far end of the cable ) the inductive effects at 50 Hz ould tend to be minimal / insignificant.
 
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It has probably been too many years since I studied electronic theroy...

I thought there could be all sorts of things going on, a standing EM field, mutual conductance would be increased as the cable is coiled, there could be some phase shift.

The thing to do would be to attach the input and output to a scope so you can overlay and verify where exactly the difference is.

I wouldn't expect a net power input as, like all transformers with no load, The energy transfer is transient and not steady state, over the whole cycle the the standing magnetic field and back EMF balance themselves.

I have had voltage drop issues caused by cable runs >500m. The issue was inductance as resistance was less than 1 Ohm. Can't remmeber how much inductance there was but it was on a mH scale.
 

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