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.
If we took the DNO earth resistance RB as 20 ohms then the consumers earth rod could have a maximum resistance of 5.26 ohms.
I get 72Ω for a DNO resistance of 20Ω and 3.6Ω for 1Ω - and I reckon they're minimum acceptable values if the condition is to be met. I.e. for safety the customer's extraneous-conductive-parts must have a value of that or more (but someone please check my maths!). The lower the resistance of the consumer's "electrode" the lower the voltage difference around it - and hence the greater the voltage difference around the DNO's electrode(s).
What puzzles me though is what on earth you're meant to do with this information. If the water or gas pipe happened to have lower resistance to Earth ('cos it's buried under half the town) what are you supposed to do about it? You can hardly leave it unbonded, or increase its resistance to Earth. I presume they don't want you to start introducing deliberate impedances into bonding conductors (which would be defeated by c.p.c.s to boilers etc anyway). You can hardly change the DNO's Earthing either.
If we took the DNO earth resistance RB as 20 ohms then the consumers earth rod could have a maximum resistance of 5.26 ohms.
I get 72Ω for a DNO resistance of 20Ω and 3.6Ω for 1Ω - and I reckon they're minimum acceptable values if the condition is to be met. I.e. for safety the customer's extraneous-conductive-parts must have a value of that or more (but someone please check my maths!). The lower the resistance of the consumer's "electrode" the lower the voltage difference around it - and hence the greater the voltage difference around the DNO's electrode(s).
What puzzles me though is what on earth you're meant to do with this information. If the water or gas pipe happened to have lower resistance to Earth ('cos it's buried under half the town) what are you supposed to do about it? You can hardly leave it unbonded, or increase its resistance to Earth. I presume they don't want you to start introducing deliberate impedances into bonding conductors (which would be defeated by c.p.c.s to boilers etc anyway). You can hardly change the DNO's Earthing either.
