Light reading

MrJack96:

So locally inside the installation the touch voltages should be small for a broken PEN conductor with installed main bonding only when in contact with the mass of earth can we see the touch voltages rising? 

This is a very interesting question. How do we form what used to be called an "equipotential zone"  ... depending on what type of building we have?


In many modern homes, internal water pipes are likely to be plastic, and external water, and possibly gas, pipes, will also be plastic ... so that's the reason we don't use the term "equipotential zone" any more in BS 7671, although the concept is VERY important when we come to considering SPDs, EMC, and lightning protection.

MrJack96:

So if 20amps would flow for a normal load in the system would the same flow in a broken PEN conductor? 

The broken part of the PEN conductor can't carry any current at all of course, because it's broken. Bits upstream or downstream of the break might carry some current - e.g. flowing to or from the ground or extraneous-conductive-parts - but the extra resistance will often limit the current to something less than normal - it all depends on the fortuitous parallel paths (if any). On one extreme where you had say premises both up-stream and down-stream of the break both bonded to the same extraneous-conductive-part (say and old iron water main) then there would be hardly any extra resistance and current flows might be close to normal. On the other extreme with say no extraneous-conductive-parts at all (say all plastic water/gas pipes) you might not have any parallel paths at all, and a break in the PEN would result in no current flow whatsoever - all the appliances in the installation would appear to be dead, but have 230V (above Earth) on both L and N ... likewise all exposed-conductive-parts.


   - Andy.

MrJack96:

Yeah thank you I thought that would be the case the extra resistance the earth would limit current flow meaning the parts extraneous conductive parts within the installation would raise to near 230v with respect to the earth outside. 

I think it's better to say that "extra resistance of the earth might limit current flow". As Andy says, the current may well divert down a very low impedance cast iron gas or water pipes !


In fact, things are slightly more complicated especially where the broken PEN conductor is in a three-phase cable. In this case, the L-N voltage may not always drop - it might well increase above Uo, in theory as much as about 350 V (which is a little counter-intuitive, but entirely possible with the right power factor loads on each phase).


And regardless of this, it takes only a few 10s of mA to kill someone, so even of the loads appear not to be working, because not enough current may flow, there may well still easily be enough to cause serious harm, or at worst kill someone.

MrJack96:

So the reason for that diagram in GN 5 is to limit touch voltage by having low resistance earth electrodes which would be difficult to achieve wouldn’t it? 

Yes, that particular section, 14.5, is to provide guidance for designers choosing to use an additional consumer's earth electrode to limit touch voltage to try and mitigate against PEN faults in the PME distribution network.


Is it difficult to achieve? Well, this depends on the usage of power in the installation.


As Andy says, for any load above 3 kW (10 A) it is almost impossible to achieve a touch voltage of 50 V or less for a single installation, even with foundation earth electrodes (which at best achieve about 3 ohms). This also ignores any diverted neutral currents from other installations that may connect to your installation via any shared extraneous-conductive-parts. You may, however, achieve 100 V.


Bear in mind, though, that, before the advent of EV charging, unless you had storage heating, most larger loads will not be connected for long periods. So, going back a few years, or with an installation that doesn't have storage heating or EV charging, the average power usage for a dwelling was a little above 2 kW. So when this guidance was first established, it was very different.


When we bring in long-term large power loads like EV, definitely as Andy says, even achieving 70 V is not reasonable, and this is what the IET Code of Practice for EV Charging Equipment Installation says.


In a three-phase installation, the calculation is a little more complicated (see Annex A722), taking into account the worst-case unbalance of loads. However, for most three-phase installations that are supplied by PME, there are potential issues with the balance of load on the network supplying the installation.



Overall, I guess if most PME installations in an area had foundation earth electrodes, the situation would be very different, and the measure would be far more effective and easier to achieve.