Earthing a steel bath if fusebox has an RCD?

[EFLImpudence]

[assuming you mean 701.415.2(vi)]

Yes, sorry.

 

[JohnW2]

A bath is not an extraneous conductive part, and cannot introduce a potential into the bathroom.

It doesn't have to be earth potential.

It doesn't - but if it's only electrical contunity is a poor connection to something which is already bonded, how on earth can it introduce any potential other than that of the equipotential zone (which wouldn't qualify it as an extraneous-c-p).

What is the difference between an (external) extraneous-c-p with a resistance of <23kΩ (or >1666Ω) to the MET and a bath with a similar resistance to a bonded pipe with regard to R<50/Ia and/or RCD omission?

As I've just written, if the bath is not an extraneous-c-p (which I don't think it would be in the circumstances you describe), then those considerations do not apply.

Kind Regards, John

 

[EFLImpudence]

... But, by answering 'Yes', you've created a circular argument - 701.415.2(vi) only applies to extraneous-c-ps, so you can't use it to define whether or not something is an extraneous-c-p, can you?!

Well, yes I see your point but you could say a metal bath is not an extraneous-c-p unless it touches the ground.

It's the same as testing for supplementary bonding.
All the pipes may be correctly bonded but sill have a potential between themselves.

A (metal) bath can surely only be an extraneous-c-p ("liable to introduce a potential....") if it is in electrical continuity with something which enters the property from outside

But it's not earth potential that is the problem here.

and that 'something' is not itself already bonded. A poor connection to something bonded does not make it an extraneous-c-p.

So, if it has a resistance of >1667Ω (or <50/Ia) to an exposed-c-p, what would you do?

 

[JohnW2]

... But, by answering 'Yes', you've created a circular argument - 701.415.2(vi) only applies to extraneous-c-ps, so you can't use it to define whether or not something is an extraneous-c-p, can you?!

Well, yes I see your point but you could say a metal bath is not an extraneous-c-p unless it touches the ground.

Nothing is an extraneous-c-p unless it is connected to something outside of the equipotential zone (such as 'true earth').

It's the same as testing for supplementary bonding. All the pipes may be correctly bonded but sill have a potential between themselves.

I don't really understand what you are saying here. If all the pipes are "correctly bonded" (to the MET) how can there be significant pds between them?

A (metal) bath can surely only be an extraneous-c-p ("liable to introduce a potential....") if it is in electrical continuity with something which enters the property from outside

But it's not earth potential that is the problem here.

It doesn't matter what potential is "the problem" (and I'm not too sure what you mean by that, anyway). If it doesn't have electrical continuity with something outside of the equipotential zone which isn't already bonded, then it surely can't be an extraneous-c-p (cannot be liable to introduce any potential), can it?

and that 'something' is not itself already bonded. A poor connection to something bonded does not make it an extraneous-c-p.

So, if it has a resistance of >1667Ω (or <50/Ia) to an exposed-c-p, what would you do?

If it were not an extraneous-c-p, I probably wouldn't do anything. This is why I've never been too happy about your 'tests'.

Kind Regards, John

 

[Spark123]

If someone drops a double insulated appliance ( i.e. with no earth connection ) that is switched on into a plastic bath of water then the potential of the water will be about mid way between Neutral and Live ( circa 120 volts ). No protective device will be operated. The same lack of operation of protective devices would happen if the bath was metal but not earthed and had no conenction to metal pipes.

The risk of someone dropping a live double insulated appliance into a bath of water is a small but not insignificant risk.

With teh water at 120 volts any touching the water in the bath and and earthed ( or bonded ) pipe ( or nay other earhted or bonded item ) will get a shock. An RCD if fitted might operate quickly enough to reduce the duration of the shock to one that is short enough to not cause non reversible damage to the person.

My preference would be to earth or bond the bath so that the RCD operates when the appliance drops into the bath water.

We don't install sockets close to a bath for that reason, however if the person wants to qualify for a Darwin Award then there's little we can do about it.

 

[JohnW2]

If someone drops a double insulated appliance ( i.e. with no earth connection ) that is switched on into a plastic bath of water then the potential of the water will be about mid way between Neutral and Live ( circa 120 volts ). No protective device will be operated. The same lack of operation of protective devices would happen if the bath was metal but not earthed and had no conenction to metal pipes. ... The risk of someone dropping a live double insulated appliance into a bath of water is a small but not insignificant risk.

We've been through this umpteen times before. I agree that it is a 'swings and roundabouts' situation but I personally think that the risks (such as you describe) associated with not earthing a metal bath are probably less than the risks of 'unnecessarily' creating a large amount of earthed metal within the room (which greatly increases the risk to, say, someone using a vacuum cleaner with a damaged lead in the room). Both risks are extremely small, but your and my views as to which is the greater appear to differ.

Kind Regards, John

 

[PBC_1966]

My preference would be to earth or bond the bath so that the RCD operates when the appliance drops into the bath water.

Ditto. There is no way you want to risk the possibility of a significant potential difference between the bathtub and the taps or other plumbing fixtures within reach, and the tub is of such a size and installed in such a way that to complete isolate it (along with everything else) from earth is impractical. Therefore it needs to be bonded.

I really don't understand why there now seems to be such a lot of debate about this given that the necessity was recognized many years ago. Here's what the 14th edition had to say about it:

D.14 The exposed metalwork of all apparatus which is required by these Regulations to be earthed, which might otherwise come into fortuitous contact with extraneous fixed metalwork shall be either effectually segregated therefrom or effectually bonded thereto so as to prevent appreciable voltage differences at such possible points of contact (see also Regulation B.53).

NOTE 1. - The extraneous fixed metalwork required to be bonded and earthed in these circumstances includes the following:

(i) Baths and exposed metal pipes, radiators, sinks and tanks, in the absence of metal-to-metal joints of negligible electrical resistance.

(ii) Where practicable, accessible structural metalwork.

(iii) Framework of mobile equipment on which electrical apparatus is mounted, such as cranes and lifts.

NOTE 2. - There are special requirements for bonding to metalwork of other services in P.M.E. installations (see Regulation D.34(iii) and Item 4(2) of Appendix 5).

And I don't have my copy handy to quote from, but I know the requiement existed in the 13th edition as well.

 

[bernardgreen]

Here's what the 14th edition had to say about it:

OTE 1. - The extraneous fixed metalwork required to be bonded and earthed in these circumstances includes the following:

(i) Baths and exposed metal pipes, radiators, sinks and tanks, in the absence of metal-to-metal joints of negligible electrical resistance

The problem of obtaining a definitive answers seems to arise from the need to "bond" items that might bring in "extraneous" potentials into the property and its "equipotential zone" and the need to "earth" items inside the zone whose exposed metal work might become Live ( kettles, electric fires etc etc ). And iti is further complicated by the fact that the potential of true ground if bought into the property via a water pipe or structural steel work can present a serious hazard in a PME system where the "Earth" CPC and MET ( Main Earth Terminal ) are at the potential of the incoming Neutral. Most of the time the potential difference between "Earth" ( MET ) and Ground is a few volts so bonding has no purpose while that is true. But the MET can rise well above Ground potential when there is a netork fault or un-even loading on the three phase supply to the area. The risk of this difference become high enough to be a hazard is very small but is considered to be significant as that is the reason to bond service pipes to be at the same potential as the MET and "Earth" inside the property.

 

[JohnW2]

My preference would be to earth or bond the bath so that the RCD operates when the appliance drops into the bath water.

Ditto. There is no way you want to risk the possibility of a significant potential difference between the bathtub and the taps or other plumbing fixtures within reach, and the tub is of such a size and installed in such a way that to complete isolate it (along with everything else) from earth is impractical. Therefore it needs to be bonded.

As I've just written to bernard, it really is 'swings and roundabouts' - and, as I said, FWIW my personal view is that the risks you and bernard want to address by earthing a bath are probably less than those created by 'unnecessarily' creating a lot of earthed metal within the room. Opinions will undoubtedly always differ!

Kind Regards, John

 

[DIYedboy]

Blimey lads..............eeerrrr that's cleared that up then

I also thought that supplementary bonding wasn't necessary due to having that lovely RCD, it seems you are saying that I must sup bond in the bathroom too?

 

[EFLImpudence]

...

... But, by answering 'Yes', you've created a circular argument - 701.415.2(vi) only applies to extraneous-c-ps, so you can't use it to define whether or not something is an extraneous-c-p, can you?!

Well, yes I see your point but you could say a metal bath is not an extraneous-c-p unless it touches the ground.

Nothing is an extraneous-c-p unless it is connected to something outside of the equipotential zone (such as 'true earth').

In the OP's question, the equipotential zone is only the bathroom.
Bonded parts can be extraneous-c-ps to the bathroom.

It's the same as testing for supplementary bonding. All the pipes may be correctly bonded but sill have a potential between themselves.

I don't really understand what you are saying here. If all the pipes are "correctly bonded" (to the MET) how can there be significant pds between them?

Poor wording - between themselves and exposed-c-ps.

A (metal) bath can surely only be an extraneous-c-p ("liable to introduce a potential....") if it is in electrical continuity with something which enters the property from outside

But it's not earth potential that is the problem here.

It doesn't matter what potential is "the problem" (and I'm not too sure what you mean by that, anyway)

Yes, it does.

If it doesn't have electrical continuity with something outside of the equipotential zone which isn't already bonded, then it surely can't be an extraneous-c-p (cannot be liable to introduce any potential), can it?

We are talking about parts with relatively high impedance to other parts so - yes.

and that 'something' is not itself already bonded. A poor connection to something bonded does not make it an extraneous-c-p.

So, if it has a resistance of >1667Ω (or <50/Ia) to an exposed-c-p, what would you do?

If it were not an extraneous-c-p, I probably wouldn't do anything. This is why I've never been too happy about your 'tests'.

Then in respect of 701.415.2(vi) what does "effectively connected to the PEB" mean and how do you determine this?

 

[EFLImpudence]

I also thought that supplementary bonding wasn't necessary due to having that lovely RCD,

It is not if the above conditions (quoted by Taylor) are met.

it seems you are saying that I must sup bond in the bathroom too?

Not in the bathroom generally, but you may have to supplementary bond the bath to the pipe if it is not "effectively connected to the PEB" (main bonding).

 

[EFLImpudence]

What is the difference between an (external) extraneous-c-p with a resistance of <23kΩ (or >1666Ω) to the MET and a bath with a similar resistance to a bonded pipe with regard to R<50/Ia and/or RCD omission?

 

[DIYedboy]

Many thanks all of you for your brain melting answers.............I was told about a window cleaner that was killed because the metal window he was cleaning was supplementary bonded, have you heard that one?

 

[JohnW2]

In the OP's question, the equipotential zone is only the bathroom.

If one takes that view, it's surely even more clear that the bath can't be an extraneous-c-p - since it is entirely contained within the bathroom?

Then in respect of 701.415.2(vi) what does "effectively connected to the PEB" mean and how do you determine this?

As I said before, 701.415.2(v) only applies IF the bath is a extraneous-c-p. As I also said before, you create a circular argument if you attempt to turn that on its head and say that if it is not "effectively connected to the PEB", then in must be an extraneous-c-p.

As I also said, this is why I have concerns about your 'test' method of 'determining' whether something is an extraneous-c-p. I presume you would agree that if a bath, totally contained within a bathroom, were totally floating ('infinite' resistance to MET/PEB), it would not qualify as an extraneous-c-p (just as with doorknobs and spoons/forks etc.)? Do you believe that it would suddenly turn into an extraneous-c-p if I connected it via a 15kΩ resistor to something in continuity with the PEB/MET - and, if so, why?

Kind Regards, John