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
I might be talking out of my hat here, but why not combine both systems? Why not export PME via the swa and 3rd core, tie both together at both ends, the tie both to a rod at the destination end too? Belt & braces?
That would still suffer all the disadvantages of PME earthing. Unless the rod had a ridiculously low resistance to Earth (<< 1 Ohm) you'd still risk a hazardous potential on the earthing system during an open PEN event and the rise in potential of the soil around the rod would likely be far too steep to provide anything like equipotentiality in the vicinity of the tub. A buried grid under the entire area would probably be the minimum if you were going for that sort of approach.
- Andy.
I might be talking out of my hat here, but why not combine both systems? Why not export PME via the swa and 3rd core, tie both together at both ends, the tie both to a rod at the destination end too? Belt & braces?
Otherwise, we need to be looking at siting the electrical apparatus, pump, water heater etc remotely from the tub and further isolating heated pumped water through the use of distilled water and the requisite length of plastic pipework.
Hi all,
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Lisa
That's sort of where my thinking lies Andy … sort of.
I think the real answer is that there are pro's and con's to each of the earthing arrangements, and sometimes mixing them up in an installation may well introduce con's from one arrangement to con's from the other, so perhaps you may end up increasing the risk over leaving everything on the original earthing arrangement.
AJJewsbury:
where the fault current exceeds the residual operating current, the touch voltage may also exceed 50 V until the device trips.
But the touch voltage won't persist - as the RCD will trip.
My point being, that the RCD trip is based on dry conditions … may be suitable for wet, but NOT submerged (so, consider steps metal into pool or sauna, and fault in heater element.
Remember I was comparing the situation with the problem of perceived shocks on PME systems - due to voltage drop along PEN conductor in normal, non-fault, conditions.
We say to avoid PME partly because perceived electric shocks from the PEN conductor which can potentially be up to (say) 18.4V above Earth, yet we recommend a TT system - possibly TT'd an entire installation - where there's no guarantee that voltages of anything up to to 50V might persist.
- Andy.
The PME diverted neutral currents may persist indefinitely … and local RoEP in an open-PEN situation may persist for a very long time (although it probably varies within that period).
But also, this is only the earth electrode resistance, not the effective resistance from the transformer earth to the point at which you are standing …
Which is probably just what you want if you want to calculate the voltage between the MET and the ground at the consumer's installation - it's less likely that the victim will have long enough arms to be touching part of the consumer's installation while stood next to the substation.
- Andy.
where the fault current exceeds the residual operating current, the touch voltage may also exceed 50 V until the device trips.
But the touch voltage won't persist - as the RCD will trip. Remember I was comparing the situation with the problem of perceived shocks on PME systems - due to voltage drop along PEN conductor in normal, non-fault, conditions.
We say to avoid PME partly because perceived electric shocks from the PEN conductor which can potentially be up to (say) 18.4V above Earth, yet we recommend a TT system - possibly TT'd an entire installation - where there's no guarantee that voltages of anything up to to 50V might persist.
- Andy.
Just to be clear regarding this, yes, you're unlikely to get a voltage greater than 50 V to “Earth" (well, the other end of the earth electrode resistance sphere of influence) … but I think that Supplementary Local Equipotential Bonding is far greater an issue.
Basically, your touch current can exceed Ra … for a number of reasons (as I explained in the previous post, this completely negates the earth electrode resistance itself from the loop) … and if you've selected a bonding resistance (permitted by BS 7671) in excess of 1K, you may have a touch current that would be OK in dry conditions, but not wet or submerged as we are discussing here.
Some of this is perhaps crossing out of BS 7671 into product standards for hot tubs themselves, but does need to be considered somewhere along the line.
AJJewsbury:
Does BS 7671 really require we keep the touch voltage below 50 V?
For TT systems that use RCDs, and for ‘persistent’ touch voltages , yes - 411.5.3 (ii).
The TT systems that have such low loop impedances that overcurrent devices can be used, there isn't a specified limit, but as the system then approximates to TN systems any reasonable leakage current isn't likely to do any harm.
- Andy.
Nope … this is effectively the same requirement as 415.2.2 …
… where the fault current exceeds the residual operating current, the touch voltage may also exceed 50 V until the device trips.
But also, this is only the earth electrode resistance, not the effective resistance from the transformer earth to the point at which you are standing …
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