Main bond to bath?

[Ben Blaukopf]

I had a strange earth wire going from the old CU somewhere upstairs. I've finally tracked it down. Thought originally it might be an optimistic main bond for the cast iron waste, but not , it terminated at an earth clamp on the metal leg of the plastic bath. There was no supplementary bonding (everything is now RCDed, so still no supplementary)

So - a 6mm main bond to the bath, which was presumably a cast iron bath when the bond was actually put it. But why? Was it ever practice to bond baths? As far as dating goes, house was built in about 1935, there is evidence of some old VIR cable, and the current setup is largely imperial stranded T+E with Wylex rewireable fuses (now replaced with a new split load).

[John Rumm]

It was never actually a requirement, but alas it was a not uncommon practice. This basically seems to be a result of people not understanding the purposes or function of equipotential bonding.

If you treat a bathroom as an equipotential zone, then you would typically bond anything that is capable of introducing a potential from outside of that zone into it. So metallic water pipes entering from elsewhere, the CPC of any circuits that are accessible in the room, and any extraneous metalwork (CH pipes, metal stud work perhaps).

One thing that you can pretty much guarantee is not going to introduce a potential from outside of the zone into the room is a bath, that is entirely contained in the one room, and in this case, made out of plastic!

:-)

[Harry Bloomfield, Esq.]

John Rumm explained on 21/06/2021 :

We were rewired in the mid 1980's, in a full refurb. Metal bath, is bonded to hot and cold, plus radiator. All H&C are cross bonded throughout the house to sinks as are all radiators.

[John Rumm]

Yup, hot and cold bonded, and CH pipework (some people bond the rad itself - it will do the job but is not actually required)[1], is what you would expect. The bonding does not need to go back to the CU or be connected (directly) to that of other equipotential zones.

[1] CH pipework can be bonded where it enters the room - it does not need to be on show with earth clamps hanging off the rad tails.

[Andrew]

My 1976-built house had a 4mm bonding connecting the bath to the CPC of the cooker outlet in the kitchen immediately below with the cable buried in the plaster.