Testing if supplementary bonding is required.

Well, we are not theoretical physicists, so some tolerance is allowed. Even then, it's relative potential rather than absolute. +/- 25 V?

It's a bit more than a minor tolerance - with a TN system, low Ze and reduced c.p.c.s you can easily get a p.d. of half mains voltage even within the equipotential zone - and the effect of main bonding is at best "undefined" as there are no actual requirements to achieve (just c.s.a. for the bonding conductors - the length is uncontrolled and thus so is their impedance). It's all really a matter of best efforts than anything guaranteed.

   - Andy.

So I’m just going to quote “zones” just for the purpose of this topic. 


the zone with the increased extent of plastic service pipes leads to these zones being less than perfect from the equipotential side of things so am I right in thinking in within these zones we can still see in a TT system 240v to the ground within an installation it’s just a case of the RCD doing it’s job or if they fail it’s down to the resistance of the carpet and all the other things we stand on to limit the current?

Quite so. There are a few points to consider.

In a TT system, during an L-E fault, the cpc, and any wires to the electrodes and anything connected to that, normally  rise to a significant fraction of the full 230V, if you could measure it with a long lead, relative to the true terra-firma earth outside at a point  far away from any electrode.

This assumes that the other electrode in the loop, the one at the substation end, where the current comes back up again, is in good order, and most of the voltage drop is at the load end electrode. Sometimes it isn't, and that can make the N-E voltage move for other customers on the same substation.


Around each electrode is a 'fried egg shape' of approximate circles of increasing voltage as you approach the electrode, however, for an electrode of length l, by the time you are a distance of a few times 'l'  away you are far enough out that this gradient is not dangerous. However it is not unknown for cows and horses and other 'long wheelbase' animals to be shocked, if one set of hooves is much nearer the electrode than the other, so they straddle zones of  different voltage. A 'step voltage' shock is also possible for humans, but those who wear shoes or only take small steps do not normally notice.)

What happens indoors rather depends which "ground" - you will probably  not experience much of a shock, unless the floor has been removed and you are standing on bare earth, as all of the house radiators, heating pipes, earthed appliances will all rise with the TT electrode voltage, but this is where the few  truly earthed things (i.e connected to terra-firma, and far away) like phone lines and cable TV feeders, suddenly become dangerous.

The saving grace is indeed the RCD.

Mike.

it’s down to the resistance of the carpet and all the other things we stand on to limit the current?

Usually not the carpet - any vaguely modern building will either have a suspended floor or if solid have a damp proof membrane under it - which is conveniently insulating (electrically) and generally well protected from damage.


The is a greater risk for solid floors without a damp proof membrane - think the old victorian quarry tiled floor in sculleries and the like - or shower blocks at camp sites. Many DNOs will recommend against or plain refuse PME to such installations.


  - Andy.

Interesting stuff all this. So even in a broken pen situation the pipe work if metal will act as the electrode and should keep the installation at a similar potential. So say if an electrode connected to the MET of the same faulty installation and installed in the mud outside the electrode will be at near 240 but as we step a way the voltage should decrease?

yes, in some cases. The problem is that exactly which electrode (may be a deliberate one, or an accidental one like a security camera mounted on a metal fence post) carries the most current, and where exactly the volts pile up to a dangerous level is horribly variable with the exact local circumstances. The bulk of the voltage drop is almost never along the copper, as that is pretty low resistance.

Things are rarely as cut and dried as the C and G blackboard model (or whiteboard nowadays probably, or power point) in real life.

To illustrate what I mean -

An example of an incident from a few years ago - can you work out what happened?


I think it would be a brave designer who had anticipated this scenario, or anything like it.

Here they had (I assume) installed  metal doors and frames connected to the building metal work, and folk standing on the floor at not the same voltage.

Mike.