There is an article in PE and I have just received it via email.
I am disappointed that the article seems to suggest that it contains all you need to know yet no mention is made of earthing systems
If one ohm is reasonable rather depends on the other impedances in the rest of the PME system fed from the same substation. 20 ohms may be a book figure, but to expect that to do much equalization against all but the weediest PME supply is optimistic in the extreme.
The supply to this hot tub is not like a 5A cut out feeding one streetlamp. The load of the hot tub heaters will be a few thousand watts, so in the range of 15-30 ohms itself. If in lost neutral fault we found just the hot tub, and not even the rest of the house, trying to return to ground via a 20 ohm electrode, all the exposed CPCs for the house would be well over the 50V safe limit for dry conditions. A hot tub user is unlikely to be to dry.
To be safe, rather than merely regs compliant, while wet and with one foot earthed ,you may prefer 25 volts at a likely diverted neutral current of perhaps 15- 20A. Not so far off the one ohm figure really.
It would be far safer if the supply to the hot tub was 3 phase of course, as then the neutral current would only be the imbalance of the phases, but to suggest 3 phase in a normal UK house that would bring out most domestic electricians and some DNO employees out in a nervous rash.
Mike.
If one ohm is reasonable rather depends on the other impedances in the rest of the PME system fed from the same substation. 20 ohms may be a book figure, but to expect that to do much equalization against all but the weediest PME supply is optimistic in the extreme.
The supply to this hot tub is not like a 5A cut out feeding one streetlamp. The load of the hot tub heaters will be a few thousand watts, so in the range of 15-30 ohms itself. If in lost neutral fault we found just the hot tub, and not even the rest of the house, trying to return to ground via a 20 ohm electrode, all the exposed CPCs for the house would be well over the 50V safe limit for dry conditions. A hot tub user is unlikely to be to dry.
To be safe, rather than merely regs compliant, while wet and with one foot earthed ,you may prefer 25 volts at a likely diverted neutral current of perhaps 15- 20A. Not so far off the one ohm figure really.
It would be far safer if the supply to the hot tub was 3 phase of course, as then the neutral current would only be the imbalance of the phases, but to suggest 3 phase in a normal UK house that would bring out most domestic electricians and some DNO employees out in a nervous rash.
Mike.
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