Arcing on neutral when isolated

[Gricey12]

This one is baffling us.
A supply and DB have been installed .
A section board feeds the DB via a 95mm 4 core swa with no separate earth, only the armouring as CPC.
Outgoing circuits are connected at the DB and SOME in the field
A 4mm neutral wire came into contact with some totally un earthed medical trunking and a slight arcing was seen but registered a potential difference of about 1 volt when metered.
However the DB was isolated via a 4 pole isolator, so the mains end of all neutrals were not connected to anything whatsoever .
So where is the voltage coming from if the neutral is effectively disconnected at both ends?
Answers please

 

[pc1966]

Can you current clamp the whole feed SWA to see if it is acting as a good earth rod for the local PME network, and if so how many amps?

Also have you checked the N really is isolated from true Earth, etc?

 

[Gricey12]

Can you current clamp the whole feed SWA to see if it is acting as a good earth rod for the local PME network, and if so how many amps?

Also have you checked the N really is isolated from true Earth, etc?

Thanks for your input. I'll try those suggestions. I'm sure the neutral is fully isolated though.

 

[Lucien Nunes]

There are two scenarios that could possibly produce an arc yet show only a low voltage when tested: High voltage via high source impedance (e.g. capacitive coupling from energised conductors at line voltage) or low voltage via low source impedance (e.g. a small potential difference between local real earth and a MET connected to a loaded CNE.) Note that I am using 'high' and 'low' voltage comparitively, not with their official definitions. The first scenario reads low because the high source impedance of the stray current is loaded down by the measuring instrument's input resistance; the second reads low because the voltage is in fact low.

You mention that one side of the arc is to a floating piece of trunking. Regardless of what is going on with the neutral bar in the DB, that suggests that this is a high voltage via high source impedance scenario. Do any of the installed circuits share containment our long routes bundled with energised cables? If so, is there any kind of electrostatic screening between them (e.g. FP foils) or are they free to couple capacitively? How big is the piece of trunking that was able to form the arc and have you actually proven that it is floating?

Whilst the description you have given does in theory support the high impedance option, I am sceptical that any 'ghost voltage' with high enough source impedance to read just 1V on any instrument will deliver enough current to form a visible arc. You didn't mention what measuring instrument was used so we don't know the input resistance and can't yet make inferences about the actual magnitude of the current.

 

[marconi]

What you may have experienced is a result of the difference in potentials between where the armouring of the 95mm2 is connected to ground and the local ground near the medical trunking. I have attached some quickly sketched electrical art which attempts to illustrate what test voltages would be measured by a voltmeter connected on side to the supply transformer combined HV/LV earth and a test rod driven into the ground some distance away from it. When an electrode is driven into the ground and a current flows into it, the potential of the ground phi(V) decays away with distance something like this. The great mass of the Earth is at zero potential but locally to an electrode the earth rises in potential; how much depends on the electrode resistance Ra which in turn depends on the soil and electrode physical arrangement eg: length matters more than thickness. So you can see that close to the electrode the potential is high but further away it is low. How does this apply to your situation.

What I think is happening is that the armouring of the 95mm2 is connected to the combined HV/LV earth at the substation which makes it at the potential of the electrode terminal - E1 in my electrical art. At the site where you are working everything is at a local potential. Specifically for the medical trunking it is at some potential V4 with respect to the great mass of Earth. Its potential depends on whether it is actually connected to the ground neat to it via an electrode or more nefariously via stray capacitance and resistance. Irrespetive of that, being a largish metal structure it has the electrical property of capacitance which means to become charged or discharged a current must flow to or from it. With ac of course this happens at the mains frequency.

At you site, the substation potential E1 is transferred to it via the armouring. When the TPN isolator is open as you said, the potential of the neutral conductor is determined by the stray resistance of capacitance between it and conductors amd metal work connected to the armouring of the supply 95mm2 cable. I have assumed in my diagram an outgoing circuit wired in swa which means that the neutral is surrounded by an armoured conductor at potential E1 with respect to local ground. The neutral I have drawn will assume a potential similar to E1. before being touched by the neutral, the medical trunking is at a potential of E2

I think you can now see for yourself that when this neutral touches the medical trunking originally at local potential E2 a charge/discharge current will flow to/from the trunking - the trunking will rise to a potential V4 with respect to local earth, where V4 depends on the impedance between the Neutral and the armouring, E1 at the substation and E2 locally.

Or something like this. Got to go to church now.

 

[marconi]

Thus, if you temporarily/permanently connect the armouring/cpc to the medical trunking the two will become equipotential at E1 Volts with respect to the mass of the earth. If you now touch the neutral to the medical trunking no sparks should appear.

PS: I should have said 'charged discharged at twice mains frequency'.