Light reading

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.

 

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.