Bonding a metal bath

For older installations you'd have to look at the worst case which would depend on the size of the protective device protecting the bathroom circuits.

But what about situations like my old Grandmother's house. Hot water cylinder in alcove cupboard in 2nd bedroom - immersion heater on 15A BS 3036 and plastic water cistern in the loft.

Lets say the bathroom is being refurbished and all its circuits (but not the rest of the house) will be on 30mA RCDs. Without supplementary bonding the hot water pipework is in contact with the immersion's c.p.c. but isolated from the main bonding on the cold water by the plastic cistern. A fault on the immersion would likely hold the hot pipework at a hazardous voltage for possibly several seconds - generating a serious shock hazard between the hot and cold taps in the bathroom.


If you consider only the protective devices of the bathroom sockets you'd think that an acceptable resistance would be 50V/30mA = 1667 Ohms - which certainly be met between the hot and cold taps (cold via main bonding to MET, hot via immersion c.p.c. to MET) and so conclude no extra supplementary bonding is required - yet the situation is clearly unsatisfactory. So in short, I argue that you need to consider all circuits that could impose a hazardous voltage on extraneous-conductive-parts that are accessible within the bathroom - not just circuits within the bathroom. Without very detailed knowledge that probably means considering every circuit in the entire installation - including any submains - which likely makes the whole thing much more onerous.

 

from what I remember it could be as high as 500A. Guidance suggests 0.05 Ohms between extraneous metalwork

I've not got the latest guidance, but I thought it has been revised along the lines of expect up to 0.05 Ohms for the connection between bonding conductor and extraneous-conductive-part (i.e. the resistance of the clamp) - plus the expected resistance of the bonding conductors themselves. After all you don't need that many metres of 4mm² to get to 50mΩ - and BS 7671's actual requirement is 50V/Ia for supplementary bonding - which could be considerably higher. For sure if you happen to measure <0.05Ohms then you can pass it without having to bother with any calculations, but that's not quite the same as an acceptable limit.


  - Andy.

For older installations you'd have to look at the worst case which would depend on the size of the protective device protecting the bathroom circuits.

But what about situations like my old Grandmother's house. Hot water cylinder in alcove cupboard in 2nd bedroom - immersion heater on 15A BS 3036 and plastic water cistern in the loft.

Lets say the bathroom is being refurbished and all its circuits (but not the rest of the house) will be on 30mA RCDs. Without supplementary bonding the hot water pipework is in contact with the immersion's c.p.c. but isolated from the main bonding on the cold water by the plastic cistern. A fault on the immersion would likely hold the hot pipework at a hazardous voltage for possibly several seconds - generating a serious shock hazard between the hot and cold taps in the bathroom.


If you consider only the protective devices of the bathroom sockets you'd think that an acceptable resistance would be 50V/30mA = 1667 Ohms - which certainly be met between the hot and cold taps (cold via main bonding to MET, hot via immersion c.p.c. to MET) and so conclude no extra supplementary bonding is required - yet the situation is clearly unsatisfactory. So in short, I argue that you need to consider all circuits that could impose a hazardous voltage on extraneous-conductive-parts that are accessible within the bathroom - not just circuits within the bathroom. Without very detailed knowledge that probably means considering every circuit in the entire installation - including any submains - which likely makes the whole thing much more onerous.

 

from what I remember it could be as high as 500A. Guidance suggests 0.05 Ohms between extraneous metalwork

I've not got the latest guidance, but I thought it has been revised along the lines of expect up to 0.05 Ohms for the connection between bonding conductor and extraneous-conductive-part (i.e. the resistance of the clamp) - plus the expected resistance of the bonding conductors themselves. After all you don't need that many metres of 4mm² to get to 50mΩ - and BS 7671's actual requirement is 50V/Ia for supplementary bonding - which could be considerably higher. For sure if you happen to measure <0.05Ohms then you can pass it without having to bother with any calculations, but that's not quite the same as an acceptable limit.


  - Andy.