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.
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.
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.
