Touch Voltage Calculation

yes, but it is non-text book, being harder ?

If we take your figures for the indoor wiring then the main bonding alone does not on its own do much for you, and supplementary is more use.

If = Uo/Zs

Assume Zs is 0.75 ohms from (Ze - 0.1) (R1 is 0.3) (R2a is 0.3) (R2b is 0.05)


With the fault current of  300 A flowing, the MET rises by  0.05 *300 15V

but the remaining 215V are shared between R1 and R2, so the point of fault is 107v  above the MET and any main bonded pipework, and 115 above any unbonded plumbing so not so useful on its own.

Think of the metal services as in parallel with R2B.


Redo that with  a  10m shower circuit in 10mmsq so R1 * R2A are lower (about 18 milliohms each), or on TNS with R2B that could be 0.5 ohm and then  main bonding helps more , but  for now lets stick with your figures. You need an RCD to get the power off fast if the exposed voltage is this high, as the idea is to be off in half a heartbeat, and avoid the onset of fibrillation.


Or consider a fault not  at the load end, but near the consumer unit, so the main drop is accross R2B, and main bonding helps.



In your case, with a long thin final circuit ( 300milliohms is 15-20m of 1mmsq, or  more like 40-50m of 2.5mmsq, or the far point of a very long ring   etc ) if we did not have the fast ADS, then we would really benefit from the old style bathroom bonding that went out when the 17th came in, where the CPC of the shower, the bathroom light etc all bonded to each other and to  the bathroom radiators and to the bath taps. Now we are adding several lengths of 15mm copper tube in parallel with the R2a - and as 15mm tube is about 30mm2 cross-section equivalent, so 16-18milliohms per 30m length we are now winning quite a bit.

yes, but it is non-text book, being harder ?

If we take your figures for the indoor wiring then the main bonding alone does not on its own do much for you, and supplementary is more use.

If = Uo/Zs

Assume Zs is 0.75 ohms from (Ze - 0.1) (R1 is 0.3) (R2a is 0.3) (R2b is 0.05)


With the fault current of  300 A flowing, the MET rises by  0.05 *300 15V

but the remaining 215V are shared between R1 and R2, so the point of fault is 107v  above the MET and any main bonded pipework, and 115 above any unbonded plumbing so not so useful on its own.

Think of the metal services as in parallel with R2B.


Redo that with  a  10m shower circuit in 10mmsq so R1 * R2A are lower (about 18 milliohms each), or on TNS with R2B that could be 0.5 ohm and then  main bonding helps more , but  for now lets stick with your figures. You need an RCD to get the power off fast if the exposed voltage is this high, as the idea is to be off in half a heartbeat, and avoid the onset of fibrillation.


Or consider a fault not  at the load end, but near the consumer unit, so the main drop is accross R2B, and main bonding helps.



In your case, with a long thin final circuit ( 300milliohms is 15-20m of 1mmsq, or  more like 40-50m of 2.5mmsq, or the far point of a very long ring   etc ) if we did not have the fast ADS, then we would really benefit from the old style bathroom bonding that went out when the 17th came in, where the CPC of the shower, the bathroom light etc all bonded to each other and to  the bathroom radiators and to the bath taps. Now we are adding several lengths of 15mm copper tube in parallel with the R2a - and as 15mm tube is about 30mm2 cross-section equivalent, so 16-18milliohms per 30m length we are now winning quite a bit.