In response to the query re our required earth electrode at each installation under our MEN earthing system, AS/NZS 3000 (Australia and NZ Wiring Rules) requires the driving of a single 12mm Dia copper clad rod 1.8 metres into the ground, Other electrodes are allowable but the rod is the most commonly used. There is no requirement to measure its resistance to the general mass of earth. There is a requirement to measure earth loop return impedance but, of course, for a TT-C-S system, the return circuit is via the PEN conductor and is always acceptably low. The driven rod is connected to the earth bar, which has a removable (for testing) link to the neutral bar and helps to hold down the voltage of the PEN conductor and the neutral conductor (also a PEN) of the supplying LV reticulation. Ideally, the reticulation PEN should also be connected to earth rods along its length but for the same purpose but I think this would be rare.

The problem we had with the proposed (and dropped) requirement for additional electrodes in consumer's installations on PME systems was the resistance to Earth it would need to achieve in order to be effective. For example if we had a 20Ω electrode, a broken PEN that affected just our one installation and the installation had loads connected that attempted to draw say 10A - then we'd have 10V * 20Ω = 200V developed between the installation's earthing system and true Earth - so hardly providing safety. OK you can refine the numbers - with some voltage lost across the electrode, so a reduced voltage appearing across loads, many loads (e.g. those having a fixed resistance) would reduce their current draw somewhat but then there's the possibility of larger loads happening to be connected. If the PEN break was further upstream then you might have a contribution from several consumer's electrodes in parallel, which should help - but on the flipside there would be several consumer's N currents to sink. In the UK even domestic installations can have up to a 100A supply - so to be completely sure of keeping things below the usual limit of 50V we'd have to be looking at ridiculously low values like 0.5Ω for each electrode. So we're interested in how it's approached elsewhere - or whether it's just a case of PEN conductors being more reliable where they're not built from rotting Edwardian paper & lead cables or corroding 1970s aluminium ones like much of ours seems to be.

 

By the way, there are devices in the process of patent approval that deal with the loss of neutral issue for EV application and they don’t require an electrode.

BS 7671 allows a couple of different 'voltage monitoring' approaches - some use a local electrode (or an artificial N star point for 3-phase systems) as a reference, but others (as mentioned) just monitor the L-N voltage. That second approach as a number of disadvantages - not least where supplied from a 3-phase distribution system that it's entirely possible for the L-N voltage to remain within limits (e.g. 230V+10%-6%) but the N-Earth voltage to greatly exceed 50V (or indeed 70V) - depending on the relative loading of the three phases. It also can't disconnect promptly without risking a lot of nuisance tripping since the L-N (or L-PE) voltage can be taken out of limits by "normal" faults elsewhere in the installation, distribution system or other installations, which can take anything up to 5s to clear (sometimes longer for some faults on the distribution system). Thus it's only really intended where there are no other practical alternatives - thus BS 7671 doesn't permit that approach in 3-phase installations where an artificial N point would provide a more reliable reference.


   - Andy.

In response to the query re our required earth electrode at each installation under our MEN earthing system, AS/NZS 3000 (Australia and NZ Wiring Rules) requires the driving of a single 12mm Dia copper clad rod 1.8 metres into the ground, Other electrodes are allowable but the rod is the most commonly used. There is no requirement to measure its resistance to the general mass of earth. There is a requirement to measure earth loop return impedance but, of course, for a TT-C-S system, the return circuit is via the PEN conductor and is always acceptably low. The driven rod is connected to the earth bar, which has a removable (for testing) link to the neutral bar and helps to hold down the voltage of the PEN conductor and the neutral conductor (also a PEN) of the supplying LV reticulation. Ideally, the reticulation PEN should also be connected to earth rods along its length but for the same purpose but I think this would be rare.

The problem we had with the proposed (and dropped) requirement for additional electrodes in consumer's installations on PME systems was the resistance to Earth it would need to achieve in order to be effective. For example if we had a 20Ω electrode, a broken PEN that affected just our one installation and the installation had loads connected that attempted to draw say 10A - then we'd have 10V * 20Ω = 200V developed between the installation's earthing system and true Earth - so hardly providing safety. OK you can refine the numbers - with some voltage lost across the electrode, so a reduced voltage appearing across loads, many loads (e.g. those having a fixed resistance) would reduce their current draw somewhat but then there's the possibility of larger loads happening to be connected. If the PEN break was further upstream then you might have a contribution from several consumer's electrodes in parallel, which should help - but on the flipside there would be several consumer's N currents to sink. In the UK even domestic installations can have up to a 100A supply - so to be completely sure of keeping things below the usual limit of 50V we'd have to be looking at ridiculously low values like 0.5Ω for each electrode. So we're interested in how it's approached elsewhere - or whether it's just a case of PEN conductors being more reliable where they're not built from rotting Edwardian paper & lead cables or corroding 1970s aluminium ones like much of ours seems to be.

 

By the way, there are devices in the process of patent approval that deal with the loss of neutral issue for EV application and they don’t require an electrode.

BS 7671 allows a couple of different 'voltage monitoring' approaches - some use a local electrode (or an artificial N star point for 3-phase systems) as a reference, but others (as mentioned) just monitor the L-N voltage. That second approach as a number of disadvantages - not least where supplied from a 3-phase distribution system that it's entirely possible for the L-N voltage to remain within limits (e.g. 230V+10%-6%) but the N-Earth voltage to greatly exceed 50V (or indeed 70V) - depending on the relative loading of the three phases. It also can't disconnect promptly without risking a lot of nuisance tripping since the L-N (or L-PE) voltage can be taken out of limits by "normal" faults elsewhere in the installation, distribution system or other installations, which can take anything up to 5s to clear (sometimes longer for some faults on the distribution system). Thus it's only really intended where there are no other practical alternatives - thus BS 7671 doesn't permit that approach in 3-phase installations where an artificial N point would provide a more reliable reference.


   - Andy.