AJJewsbury:

ADS should be the central focus IMO. Not earthing and bonding

I do have some sympathy for the idea that in a TN-S system, main bonding provides a limited and undefined benefit for faults in final circuits.


But what about mixed disconnection times?  A fault on a sub-main will likely take an extended time to disconnect (to ensure discrimination with downstream devices if nothing else) - perhaps up to 5s - the voltage on the fault is then usually imposed on the c.p.c.s of downstream circuits - including circuits that would themselves require 0.4s (or 0.2s) disconnection times. Without bonding at the local DB anyone in contact with exposed-conductive-parts would be exposed to the full fault voltage for a long duration compared with extraneous-conductive-parts or the general mass of the earth. Bonding certainly isn't a cure-all, but it would seem to weight the dice in our favour.


Also in TN-C-S/PME systems bonding helps to protect small c.p.c.s from carrying diverted N currents - so there are other risks to consider, not just shock.


Then in TT systems, bonding provides some backup should an RCD used for ADS fail.


  - Andy.

Higher rated circuits has a much lower R1+R2, producing voltage drop on the transformer and in turn a lower touch voltage whereby disconnection can be longer without worry of physiological harm. 

AJJewsbury:

ADS should be the central focus IMO. Not earthing and bonding

I do have some sympathy for the idea that in a TN-S system, main bonding provides a limited and undefined benefit for faults in final circuits.


But what about mixed disconnection times?  A fault on a sub-main will likely take an extended time to disconnect (to ensure discrimination with downstream devices if nothing else) - perhaps up to 5s - the voltage on the fault is then usually imposed on the c.p.c.s of downstream circuits - including circuits that would themselves require 0.4s (or 0.2s) disconnection times. Without bonding at the local DB anyone in contact with exposed-conductive-parts would be exposed to the full fault voltage for a long duration compared with extraneous-conductive-parts or the general mass of the earth. Bonding certainly isn't a cure-all, but it would seem to weight the dice in our favour.


Also in TN-C-S/PME systems bonding helps to protect small c.p.c.s from carrying diverted N currents - so there are other risks to consider, not just shock.


Then in TT systems, bonding provides some backup should an RCD used for ADS fail.


  - Andy.

Higher rated circuits has a much lower R1+R2, producing voltage drop on the transformer and in turn a lower touch voltage whereby disconnection can be longer without worry of physiological harm.