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.
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).
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).
