Earth wire for bath: BS6231 or BS6491 ?

[bernardgreen]

but can become live under fault conditions'

A metal bath meets those criteria for being conductive and not part of the electrical installation, but it cannot introduce any potential, as it's just a free standing object.

In the event of a fault could an un-bonded bath become Live ?

In some circumstances it could, spilt bath water around the bath legs that also reaches electrical equipment such as ceiling roses for the room under the bathroom is not considered as a fault in the electrical installation. It might not be a fault as such but the results cannot be ignored simply because that situation is not on the list of things that are "acceptable" as being a fault in the electrical installation.

Some people do stupid things, An example was a mobile phone on charge while being used by a person in the bath that was not bonded ( or earthed ) Charger fell into bath. Bath water and bath pulled up to about 115 volts ( mid way between Neutral and Live ). Everything in the bath and the bath itself is equipotential at 115 volts so the person is not at risk of shock while laying the water. Not until they are getting out of the bath and touching something that does have a path for current to ground.

DO NOT ATTEMPT TO VERIFY THIS

 

[JohnW2]

... An extraneous conductive part is 'a conductive part liable to introduce a potential, generally Earth potential, and not forming part of the electrical installation.

True.

Metal pipes are extraneous conductive parts, as they will typically enter the room from some other location

Whilst the concept of a conductive part which is extraneous to the building is straightforward enough, I have never been very clear about what I see/hear being said about a conductive part which is extraneous 'to a location' (usually a room) within a building. In practice, provided that any/all required main bonding is in place, the only potential that a part totally within the building could introduce would surely be that of the MET, which would be no problem. Nor do I see that that is altered by what you go on to say, namely ...

.... and often be in contact with earth or other conductive items, which themselves may be in contact with the earth, or are connected to other items such as the casing of a boiler, immersion heater and so on.

If the part is in contact with earth, it should be main bonded, and hence not liable to introduce any potential other than that of the MET. If it is indirectly connected to earth via one of the intermediaries you mention then, again, that intermediary should be main bonded (or in continuity with something that is main bonded) and hence, again, the part would not liable to introduce any potential other than that of the MET.

So, I suppose that what I'm saying/asking boils down to one simple question ... if everything that requires main bonding (i.e. every conductive part which is extraneous to the building) has main bonding, then under what circumstances can any conductive part travelling from one 'location' to another entirely within the building be liable to introduce any potential other than that of the MET (which, in a properly constructed installation, should be the same as the potential of any exposed-c-ps)?

Kind Regards, John

 

[flameport]

In the event of a fault could an un-bonded bath become Live ?

There is always some unbelievably unlikely and one in a trillion event which could cause what you want.

Bonding a steel bath might theoretically help in that one, individual event that occurs somewhere between once every 100 years and never.
For the rest of the time, connecting a steel bath to earth makes the situation far worse, and fatal accidents more likely.

 

[bernardgreen]

For the rest of the time, connecting a steel bath to earth makes the situation far worse, and fatal accidents more likely.

What about steel basin taps, if there is a conductive path from them to Earth via pipes then are they not the same hazard as a bath that is bonded ?

 

[JohnW2]

What about steel basin taps, if there is a conductive path from them to Earth via pipes then are they not the same hazard as a bath that is bonded ?

Touchable unnecessarily earthed metal is always a ('unnecessary') potential hazard. However, it often happens 'incidentally', due to mechanisms such as you suggest, but many/most people seem to accept the (very small) risk (or aren't even aware of it), rather than remove it (e.g. with insulating sections in pipework).

The larger amount of unnecessarily earthed metal (baths are bigger than basins), the greater the (very small) resultant hazard.

Kind Regards, John

 

[EFLImpudence]

What about steel basin taps, if there is a conductive path from them to Earth via pipes then are they not the same hazard as a bath that is bonded ?

Yes. What are you asking?

There are instances where it would have been better were an oven NOT earthed. Do you advocate that?

If you are so worried, fit a length of plastic pipe.

 

[bernardgreen]

If you are so worried, fit a length of plastic pipe

This cottage had a metal bath with a metal waste pipe to an external metal stack that went under ground. Hence the bath had a current path to ground. I never measured the impedance of this Earth Rod before the bath was ripped out.

Was the impedance low enough that current Live---Person---Bath---Waste----Stack---Ground would have tripped an RCD before the person was irreversibly harmed, or was it too high for 30 mA but low enough that 20 mA could flow un-interrupted through the person and thus result in possibly irreversible harm to the person. If that bath and waste were staying in the cottage then bonding the bath to the MET would reduce the risk of irreversible harm by making it far more likely that enough current would flow to trip the RCD on the circuit providing the Live

 

[JohnW2]

This cottage had a metal bath with a metal waste pipe to an external metal stack that went under ground. Hence the bath had a current path to ground.

That's a situation (which I mentioned above) in which not only is earthing of the bath unavoidable (unless you were prepared to get rid of that potential hazard by inserting some insulating section in the waste pipe), but in which the waste pipe would also require main bonding.

Kind Regards, John

 

[EFLImpudence]

This cottage had a metal bath with a metal waste pipe to an external metal stack that went under ground. Hence the bath had a current path to ground. I never measured the impedance of this Earth Rod before the bath was ripped out.

Was the impedance low enough that current Live---Person---Bath---Waste----Stack---Ground would have tripped an RCD before the person was irreversibly harmed, or was it too high for 30 mA but low enough that 20 mA could flow un-interrupted through the person and thus result in possibly irreversible harm to the person. If that bath and waste were staying in the cottage then bonding the bath to the MET would reduce the risk of irreversible harm by making it far more likely that enough current would flow to trip the RCD on the circuit providing the Live

If you are so worried, then insulate the bath from the metal waste pipe or fit a plastic bath.
Raising blue moon scenarios is pointless when there are simple methods to alleviate your fears.

[Not serious]What about connecting the bath to the supply neutral? [/Not serious]

 

[securespark]

Only in very exceptional circumstances - such as when the bath has a metal waste pipe which disappears underground somewhere.

Or where the bath is metal and connected to a metal building structure. I had one where an old cast bath was standing on an RSJ.

 

[JohnW2]

Or where the bath is metal and connected to a metal building structure. I had one where an old cast bath was standing on an RSJ.

Fair enough - but that must be even more exceptional than a metal waste pipe which enters the ground. However, I'm sure that it would not be beyond the wit of man (or yourself) to engineer a method of electrically isolating the bath from the RSJ, just as an 'insulating section' could be inserted into a metal waste pipe.

Kind Regards, John

 

[flameport]

Even in the case of metal wastes and RJSs, the actual bath still doesn't need bonding - the metal waste pipe or RSJ is where the bonding should be connected.
The bath may end up connected to those items due to it being in contact with them, but the bath itself does not require any wire connecting to it.

Same situation as those ridiculous radiator bonding clamps that were touted about a few years ago - entirely pointless as it's the metallic pipes connected to the radiator that needed bonding, not the radiator itself.

Unless the radiator was installed partially in the room and continued through the wall into the next room because someone had constructed the wall over the radiator. (I have seen such a thing on one occasion, although it wasn't a bathroom)

 

[JohnW2]

In view of the rapid flurry of messages which followed, it could well be that some people did not notice the post above from me which asked this question ...

Whilst the concept of a conductive part which is extraneous to the building is straightforward enough, I have never been very clear about what I see/hear being said about a conductive part which is extraneous 'to a location' (usually a room) within a building. ...........

........ So, I suppose that what I'm saying/asking boils down to one simple question ... if everything that requires main bonding (i.e. every conductive part which is extraneous to the building) has main bonding, then under what circumstances can any conductive part travelling from one 'location' to another entirely within the building be liable to introduce any potential other than that of the MET (which, in a properly constructed installation, should be the same as the potential of any exposed-c-ps)?

Can anyone help me with an answer that question?

Kind Regards, John

 

[flameport]

For a bathroom, bonding is done either in the room, or immediately outside of it.
While items may well be connected to the MET by themselves or via other connections, that isn't the point - it's to ensure items within the bathroom remain at the same potential regardless of what happens elsewhere.

Although the resistance (actually impedance) of pipes and wires is small, it certainly isn't zero. In the event of a fault, current flow may be substantial and any small differences in impedance over long wires/pipes/other to the MET, consumer unit or whatever else will result in the voltage at various points being different. Bonding is done at the location to ensure the impedance of those connections is as low as possible, and any voltage differences are also as small as possible.

Another factor is that pipes and other services may be altered elsewhere in the building, often by those with no knowledge or care about bonding or any other electrical issues.
What may be connected today can easily change next week due to unrelated repairs or modifications.

 

[JohnW2]

For a bathroom, bonding is done either in the room, or immediately outside of it. .... While items may well be connected to the MET by themselves or via other connections, that isn't the point - it's to ensure items within the bathroom remain at the same potential regardless of what happens elsewhere. .... Although the resistance (actually impedance) of pipes and wires is small, it certainly isn't zero. In the event of a fault, current flow may be substantial and any small differences in impedance over long wires/pipes/other to the MET, consumer unit or whatever else will result in the voltage at various points being different. Bonding is done at the location to ensure the impedance of those connections is as low as possible, and any voltage differences are also as small as possible. .... Another factor is that pipes and other services may be altered elsewhere in the building, often by those with no knowledge or care about bonding or any other electrical issues. What may be connected today can easily change next week due to unrelated repairs or modifications.

Yes, I understand all that. Are you therefore saying that, for those reasons, you feel that any conductive part which enters a bathroom (from somewhere outside the bathroom, but within the building) has to be considered to be an extraneous-c-p with respect to that location (and hence subjected to supplementary bonding, if conditions for its omission are not satisfied)?

As you will be aware, EFLIs view appears to be that deciding whether a conductor entering a location needs to be considered to be an extraneous-c-p for that location can be based on measurement.

Kind Regards, John