Details of BS7671:2018 Amendment 1 are here.

When there is a broken CNE situation in a three-phase main, the rise of local earth (neutral) potential relative to Earth (the general mass of Earth) depends on the phase unbalance in the system downstream of the break.


If you have a single-phase supply from a three-phase main, unless you are unfortunate enough to be the last property on the main downstream of the break, your MET voltage to Earth will shift up and down, as loads are turned off and on ... or in some cases, loads become open circuit due to damage by overvoltage. It's therefore likely the voltage will shift every few minutes. Therefore, whilst not an exact science, statistics not being available to determine an "absolute risk", it is highly improbable that a broken CNE conductor event will go unnoticed indefinitely by the device described in 722.411.4.1 (iv). Devices that offer equivalent, or improved, safety to the measures described in 722.411.4.1 (iv), or, where applicable, 722.411.4.1 (iii), are of course permitted by 722.411.4.1 (v).


Luckily, if you happen to be the last property on the main downstream of the break, the device described in 722.411.4.1 (iv) works just fine.



A caveat for some of the protective devices, is that they may cease to function correctly in prosumers' installations operating in island mode (where they remain connected to the PME earthing system).

When there is a broken CNE situation in a three-phase main, the rise of local earth (neutral) potential relative to Earth (the general mass of Earth) depends on the phase unbalance in the system downstream of the break.


If you have a single-phase supply from a three-phase main, unless you are unfortunate enough to be the last property on the main downstream of the break, your MET voltage to Earth will shift up and down, as loads are turned off and on ... or in some cases, loads become open circuit due to damage by overvoltage. It's therefore likely the voltage will shift every few minutes. Therefore, whilst not an exact science, statistics not being available to determine an "absolute risk", it is highly improbable that a broken CNE conductor event will go unnoticed indefinitely by the device described in 722.411.4.1 (iv). Devices that offer equivalent, or improved, safety to the measures described in 722.411.4.1 (iv), or, where applicable, 722.411.4.1 (iii), are of course permitted by 722.411.4.1 (v).


Luckily, if you happen to be the last property on the main downstream of the break, the device described in 722.411.4.1 (iv) works just fine.



A caveat for some of the protective devices, is that they may cease to function correctly in prosumers' installations operating in island mode (where they remain connected to the PME earthing system).