Testing if supplementary bonding is required.

AJJewsbury:

MrJack96:

Am I right in saying in both cases (broken PEN)the installation should be safe only considering all bonding requirements have been met? The only issue is outside the Equipitential zone? That’s why in some cases it’s not a good idea to bond outside taps due to the fact if the Met was at 230v within the installation the tap would then be at the same potential meaning the folk who ends up touching this will be in a dangerous scenario?

Indeed - the risk from an outside tap with a TT system is small as disconnection should occur reasonably rapidly - certainly within 1s under current regs - but for a PME system where a broken PEN condition might exist for days or weeks, an insulating insert in the pipework feeding an outside tap is often recommended (or better still these days, entirely plastic pipework). The same can apply for Class I (earthed) appliances outdoors - e.g. outside lights and for the same reason the vast majority of electric garden tools are Class II (double insulated).


   - Andy.

I would add that we no longer refer to an equipotential zone for electrical safety. This is chiefly because in some installations, i.e. those with plastic service installation pipes, and external plastic service pipes, there may well simply be no such thing - a house say with no gas service, plastic external water service, plastic internal piping, and no structural steelwork, would certainly find it difficult to have a useful equipotential zone.


It's also true that in very large installations, even if an equipotential zone is formed, there will be voltage differences between points on the earthing system, which can be transferred from one place to another on screens of communications and control cables, for example - there are ways of managing that very easily, but it's just not good practice to consider the term "equipotential" for the two distinct ends of a screen or armour of a cable over 1 km long, for example !

AJJewsbury:

MrJack96:

Am I right in saying in both cases (broken PEN)the installation should be safe only considering all bonding requirements have been met? The only issue is outside the Equipitential zone? That’s why in some cases it’s not a good idea to bond outside taps due to the fact if the Met was at 230v within the installation the tap would then be at the same potential meaning the folk who ends up touching this will be in a dangerous scenario?

Indeed - the risk from an outside tap with a TT system is small as disconnection should occur reasonably rapidly - certainly within 1s under current regs - but for a PME system where a broken PEN condition might exist for days or weeks, an insulating insert in the pipework feeding an outside tap is often recommended (or better still these days, entirely plastic pipework). The same can apply for Class I (earthed) appliances outdoors - e.g. outside lights and for the same reason the vast majority of electric garden tools are Class II (double insulated).


   - Andy.

I would add that we no longer refer to an equipotential zone for electrical safety. This is chiefly because in some installations, i.e. those with plastic service installation pipes, and external plastic service pipes, there may well simply be no such thing - a house say with no gas service, plastic external water service, plastic internal piping, and no structural steelwork, would certainly find it difficult to have a useful equipotential zone.


It's also true that in very large installations, even if an equipotential zone is formed, there will be voltage differences between points on the earthing system, which can be transferred from one place to another on screens of communications and control cables, for example - there are ways of managing that very easily, but it's just not good practice to consider the term "equipotential" for the two distinct ends of a screen or armour of a cable over 1 km long, for example !