Lots LV faults can raise the N voltage - not just broken CNEs, but L-N & L-PE faults, line conductors coming into contact with true earth (either damaged/downed cable or an uncleared earth fault in a TT consumer's installation) - some of which can persist indefinitely, but as we're excluding fault conditions I'll skip them (not to mention HV faults). My thoughts then drift towards conditions that might affect the voltage of the ground around the LV electrode(s) - transferred voltages from say electric trains, or probably more likely these days, tram systems (as they will often run parallel with LV mains) - traction currents returning along the rails will surely alter the local ground potential. Hopefully substation earthing design minimises such effects, but perhaps not always, especially when new tram systems are built along existing streets.


I guess thunder storms aren't "faults" - so I could perhaps also offer the effects of lightning as another possibility - especially as SPDs make it more likely that CNE conductors will be pulled up to half the surge voltage. very short duration of course.


   - Andy.

Andy


Thanks. I am thinking that this is a very rare circumstance under fault free conditions. Maybe possible but not probable.


I may put my PQA on my overhead PME supply in Norfolk where I have an Earth electrode to see what voltage I can record over a few days.


For the old school forum members can you recall any nuisance tripping problems with VOELCBs? If you still have a VOELCB installed any nuisance tripping problems?

It is assumed that the general mass of earth is always 0V, but is this always true?  How about a poorly conductive soil where somewhere in the vicinity there is a large earth leakage raising the voltage of the soil for an appreciable distance, taking the potential of your correctly installed earth rod away from "the general mass of earth"?

Some yers ago when inspecting & testing a show cave open to the public, I tried to explain this to the HSE Inspector for Mines & Quarries who had requested the I&T.

He couldn't get his head around the fact that the earth return path to PME was via the SWA arnour, and that due to it's resistance, a potential difference could result between it and the 'earthiness' of the installation in the cave itself. At that time, the electrical installation at the destination-end was situated in all metalic enclosures. I suggested that a supplementary rod should be installed at the cave/mine-end but he couldn't grasp the potential difference between the two!

The Crabtree E60 double pole 60 Amp V.O.E.L.C.B. was a very popular solid well made device. I still come across them and they still work well. They are sold on Flebay as well. The earth electrode is ESSENTIAL to the correct operation of the V.O.E.L.C.B., so if the earth  electrode has rotted away or the insulated electrical cable from the "E" terminal of the device to the earth electrode in the ground is not intact because somebody has pruned through it at a flower bed the V.O.E.L.C.B. will NOT function at all. This originally may have only been a 2.5mm2 cable. The test button confirmed the existence of a good earthing lead to the earth electrode, and if it was not correct the device would not trip off when the test button was pressed, unlike a modern R.C.D. An R.C.D. just tests the internal workings of the device and not the condition of the installation earthing. The test button of the V.O.E.L.C.B. confirms the correct operation and condition of the whole device and earth electrode.


A Cratree E60 that I have here boasts an internal detection trip coil of 500 Ohms. resistance. The minimum tripping current is 35 mA. The maximum earth resistance is to be no more than 500 Ohms. Some of these devices have an internal overload device like an M.C.B.


I would have no hesitation in re-using a Crabtree E.60 for my personal use where the installation's metalwork was independent of any neighbouring house. I have refurbished some for fun. The are just so well made.


They work like a Voltmeter looking for a Voltage of about 25 to 50 Volts whether that Voltage originates from inside your own installation or originates from an outside source. It can not tell the difference.


This is a video about it by John Ward.....https://www.youtube.com/watch?v=0s7Wqivg0lw

Z.

It is assumed that the general mass of earth is always 0V, but is this always true?

The general mass of the earth (i.e. the planet in general) is indeed 0V, but local patches of soil can easily be very much different - even under ideal soil conditions the rise of earth potential around an electrode can be into the hundreds sometimes even thousands of volts during a fault (take a look at BS 7671 table 44.2 for some "interesting" numbers as a starting point).

   - Andy.

A note on the John Ward video. The overload device on the E.60 V.O.E.L.C.B. did not appear to work correctly. It probably did not operate correctly with the cover of the device removed, as the operating lever overshoots and is not in the correct position when the O/L device is pressed. With the device's cover replaced I am sure that the O/L mechanism will operate correctly and switch off the main switch via the lever.


Z..

Interesting question - the largest steady state voltage I'd expect between a CPC and the terra-firma ground in a non-fault state would be about half the possible  L-N loop voltage drop, and that assumes no cancellation of current from other phases,  and that the earth electrodes and so on  at the substation end have a lower impedance than any intermediate electrodes or bonded services.

I have on occasion used small 12V lamps between L and E in  a TT system to indicate trouble, and normally at most they light dimly, say half a dozen volts, and the only time they light brightly or blow, is when there have been serious faults.

Short transients, like brief under-voltages, I'd associate with faults and fuses blowing, or inrush of heavy machinery, but again, I'd expect a PEN to rise no more than the coressponding  live voltage falls, and probably rather less  in the majority of credible faults.

A 70V figure  is sometimes quoted for things like supplies to traffic lights, as the rise of earth permitted if the supply fails and in effect becomes a 230v version of single wire earth return supply- this is important, as no  current ADS can detect such a condition - and this figure drives a specification as to how good the local earthing must be.


If we want an absolute figure for an upper bound, assume the statutory limits of mains supply, 207 to 253 volts (230 =/- 10%) and assume 253 at the end with the supply transformer and the good earth, and 207 at the load, and assume a single phase line pair - so 46v drop, 23 volts dropping down the live, and 23V dropping 'up' the neutral, as towards the load, the voltage gets closer to the live.


This would be a really bad system but maybe meeting the rules just about.  Personally I have never seen a 23V offset, and probably not really seen more than 12v, in any system not being abused or suffering with a supply fault.


There are significant gradients in voltage across the earth, but around an electrode, the voltage is only really distorted a lot for about  a rod radius.  - in terms of step voltage danger, banning bare feet for an electrode length around the electrode is normally enough. There are tricks like gravel that can reduce the step voltage risk,, in 2 ways, firstly because it is not a good conductor, and will not support puddles,  and secondly because it us unpleasant to stand on without thick soled footwear, so there is less likely to be any one there.

Assuming the ground is uniform, and has no buried conductive material, the voltage contours are more or less independent of the soil resistance, the current varies, but the distance to half voltage is largely unaffected.