Perhaps you can shed some light on this condition for TNCS systems in the Irish Rules. I can only see that as RE decreases RB becomes more onerous but to be honest I am not really getting the essence.
So this stops you having a installation earth electrode with a resistance equal to or less than the resistance of the distribution network electrodes, is that to stop the consumer installation earth rod becoming the primary network earth with the possibility of associated diverted currents.
20/0.2631 = 76 ohms minimum consumer earth rod resistance.
Thank you AJ, that makes sense. There must be some sort of guidance for the DSO in order to establish RE or perhaps a deemed to satisfy provision. If RE was one ohm, for example, then RB would need to be no more than 50/160 ohms when Uo is 230v.
Oops!! That should be 50/180 ohms when Uo is 230v. This is a DSO condition so I don’t think RE should be made the subject of the inequation.
So if the supply is TNCS-PNB the relationship between the network and consumer earthing may not satisfy the requirements, so does that affect the installation methods in Ireland?
If we were now installing earthing to installations supplied by a TNCS system and achieving 4.07 ohms or less as was the proposal to reduce touch voltages then the suppliers earth rods resistance would have to be 1 or less, would that be achievable if you combined the two separate requirements?
Your puzzlement is very understandable, and comes from formatting: a long dash is apparently missing in the Irish standard. See the attached image, which has an excerpt from the international standard (IEC 60364[-4-41] 2005) that BS7671 and the Irish regulations are based on.
The missing dash made it look as if the formula concerns a broken-neutral situation. But the broken neutral is a separate matter, handled only vaguely (no formula) as the first of two example conditions. The formula you wondered about is the second condition. It is simply voltage division between the overall earthing resistance of the neutral and the resistance of the 'best' separate contact with earth that a line conductor might accidentally make. The formula is obscured a bit by its unconventional arrangement. It means (as several have already suggested here - well done for getting around the formatting red-herring!) that the touch voltage on the neutral should not exceed 50 V in the event of a line fault to this separate electrode.
In Germany they seem to like to have all customers on a LV net being either TN-C-S or TT. Sometimes they make all the customers change, through a change of policy by the local company. I suppose part of the rationale is that a TT customer (with perhaps an RCD blocked by dc currents?) could raise the potential of the neutral to an uncomfortable level for TN* customers. Some countries go for only one or the other direction. The UK is pretty flexible.
Ah, thank-you Nathaniel - that makes a lot more sense!
I guess for situations like a private supply or local generator it could be the sort of thing you could control.
I'm still a little puzzled by the supply network operator is responsible for compliance with that formula in Germany - how would they know a value for a Re (which is presumably on consumer premises). Do they just think of a number (based on their preference for Rb) and tell consumers they must comply with it?
I'm purely speculating about how they "ensure" it, but I'll try suggesting a rationale.
1. No LV net has both TT and TN* customers. I'm not sure that's always the case in Germany, but I infer it's typical given that I've seen pages about a "network" (i.e. its customers!) all changing from one form to another: e.g. TT-TN or TN-TT). Imagine a DNO in the UK suddenly requiring that!
2. If all customers are TT, there would be no need to fulfil the <= 50 V condition for the neutral, as it's not a "PEN".
3. If all customers are TN-C-S, they are all supposed to have extraneous parts in their installations bonded.
The DNOs couldn't reasonably keep checking that no new unbonded electrode has appeared within a customer's land, able to contact a line conductor in some vaguely credible fault. So I suppose they just rely on the customers' responsibilities to have correct installations. Companies often seem content to point to their contracts with other parties to confirm that a condition is fulfilled. As long as points 1,2,3 above are true, then customer installations shouldn't be a problem. I suspect it's other objects in the network that are more of an issue, like conductors falling on fences, railways or whatever. But, as I warned .. speculation only!
The IEC version of this installation standard is split into many parts, and would cost a fortune to buy in full. It's amusing how many national exceptions litter its pages, for disconnection times, RCD provision, and much more. Perhaps the Irish standard meant to delete that condition, but only marked the dash before pressing delete. (I admit it's more likely it's intended to be there.)
It's interesting how the condition is mentioned as "TN-C-S" rather than "TN*". In fact, there's no reason why it should apply less to a TN-S system. It's clearly based on the assumption that the network's source and conductors have negligible impedance compared to the earthing of the conductors.
The subject of voltage division between earth electrodes on the neutral and line reminds me of one of the comments on a 1937 paper (IEE) from the early days of "PME" investigation in the UK. "Ordinary earthing" here means TT, which typically meant earthing installations to a water pipe, which in turn was typically metallicly connected to the whole water pipe system of the village. So the voltage-division in the case of an installation earth fault was a very unfair one, but in this case it was unfair on the supply transformer's electrode and any cows nearby. If not using PME, it was proposed that the supply earthing should also connect to the water system (a sort of TN-S).
