Water main earth

[JohnW2]

Why do you feel that internal pipework needs bonding? Were it not for the almost inevitable 'incidental' connections from pipework to earth (CH components, immersions etc.) it would arguably be safer not to bond/earth internal pipework, when the incoming supply was in plastic, wouldn't it?

Not when you have metallic pipework extending throughout the house between different locations. Would you really want, say, a damaged kettle flex draped over a kitchen tap to leave all the pipework and taps in the bathroom live?

We've often discussed this at length, and it's a case of swings and roundabouts. I could equally ask you whether you would want 'unnecessarily earthed' metal pipes/taps/baths/whatever around for you to touch at the same time as you were touching a damaged kettle (or whatever) flex.

If it were achievable (which, in practice, it isn't), the ideal would be to have no exposed earthed metalwork in a house. It would then not matter if you touched something which was at 230V above earth potential.

Kind Regards, John

 

[DetlefSchmitz]

I don't get this. John has just agreed that figures like 3k through tap water (in a plastic pipe) is what he expects, but still doesn't consider this as a contender for an extraneous conductive part, where 10mA is often considered to be the criterion. Sums don't add up.

[changed does to doesn't]

 

[PBC_1966]

If it were achievable (which, in practice, it isn't), the ideal would be to have no exposed earthed metalwork in a house. It would then not matter if you touched something which was at 230V above earth potential.

You'd have to go a little further than just no exposed metalwork. Concrete floors, for example, can be quite conductive enough to earth to present a shock hazard to somebody touching a point at 230V to earth.

Remember the old rule from the 13th edition about metallic light fittings being allowed to go without an earth so long as mounted out of reach from somebody standing on the floor, out of reach of earthed metalwork, and in a room with a non-conductive floor.

 

[EFLImpudence]

Why do you feel that internal pipework needs bonding? Were it not for the almost inevitable 'incidental' connections from pipework to earth (CH components, immersions etc.) it would arguably be safer not to bond/earth internal pipework, when the incoming supply was in plastic, wouldn't it?

Not when you have metallic pipework extending throughout the house between different locations. Would you really want, say, a damaged kettle flex draped over a kitchen tap to leave all the pipework and taps in the bathroom live?

That would be earthing - which, as said, probably already exists; if not, best left that way.

It depends which you consider the greater risk- the pipe becoming live by contact with a conductor (not normally considered) or a path of negligible impedance aiding current flow through a body.

Besides, as you say there are the inevitable "incidental" paths to earth any with copper piping running all around a house (not to mention probably direct connections via an immersion heater feed etc.), which would require an enormous amount of care and effort to prevent.

A short length of plastic pipe, at each location, to isolate the pipe would be better.

 

[JohnW2]

I don't get this. John has just agreed that figures like 3k through tap water (in a plastic pipe) is what he expects, ...

... not quite. I said that the figure was much as I had expected in a bath or basin full of water. I would expect far higher resistances through water in even a fairly short (say, 'a few feet') length of plastic pipe.

but still doesn't consider this as a contender for an extraneous conductive part, where 10mA is often considered to be the criterion. Sums don't add up.

See above. When a water supply enters a property in plastic, it will usually remain in plastic for a pretty (or very) long way after it leaves the property, so the resistance to earth would usually be very high. However, if (I would think unusually) one had reason to believe that the plastic pipe which entered the house was very short, turning into (underground) metal very soon after leaving the property, then one might have to consider it as a potential 'extraneous-c-p'.

Kind Regards, John

 

[JohnW2]

You'd have to go a little further than just no exposed metalwork. Concrete floors, for example, can be quite conductive enough to earth to present a shock hazard to somebody touching a point at 230V to earth.

Sure, that's always a possibility, although I think the 'wet floors and walls' scenario is overplayed, other than a potential cause of 'tingles' (and, even then, probably only for people standing on wet concrete floors with bare feet!). However, even if there are some 'unavoidable' (usually fairly high impedance) paths to earth, that's not a reason for 'making things worse' by filling the building with 'unnecessarily earthed metalwork'. The less things there are around which have a path to earth, the less the chances of someone getting a serious shock, even if they touch 230V.

Kind Regards, John

 

[DetlefSchmitz]

When a water supply enters a property in plastic, it will usually remain in plastic for a pretty (or very) long way after it leaves the property, so the resistance to earth would usually be very high.

Rather than just dismissing this, I would expect a responsible answer to be that you should check the resistance to ground, and decide on that basis whether to bond.

 

[PBC_1966]

The less things there are around which have a path to earth, the less the chances of someone getting a serious shock, even if they touch 230V.

Which could also be an argument for not earthing the metal casings of appliances......

 

[JohnW2]

When a water supply enters a property in plastic, it will usually remain in plastic for a pretty (or very) long way after it leaves the property, so the resistance to earth would usually be very high.

Rather than just dismissing this, I would expect a responsible answer to be that you should check the resistance to ground, and decide on that basis whether to bond.

I didn't "dismiss" it. Indeed, if you'd carried on with my quote, it would have read "....However, if ..... then one might have to consider it as a potential 'extraneous-c-p'.".

In practice, a problem with trying to "check the resistance to ground" in the situation we're talking about is that one would have to partially dismantle the plumbing system and/or disconnect every possible path to ground (including the main earth and any other bonded items) from the MET in order to do that 'checking' - and one would then have to decide what one was going to use as the "ground" to measure to.

I may be wrong, but I suspect that a high proportion of electricians would probable be "irresponsible" and take the view that if a water supply entered a property in plastic that it did not need main bonding, particularly given that, as I've said, internal metal pipework will inevitably be earthed, anyway. It certainly would, IMO, be plain silly to even consider installing a 'usual CSA' main bonding conductor in that situation - even if the incoming plastic pipe were short, there's no way that you'd get as much as an amp through the water in it, even if you 'connected the water' to 230V!

Kind Regards, John

 

[JohnW2]

The less things there are around which have a path to earth, the less the chances of someone getting a serious shock, even if they touch 230V.

Which could also be an argument for not earthing the metal casings of appliances......

It's called Class II

Do you, perchance, subscribe to the "bernard view" that one should earth an otherwise electrically isolated metal bath?

Kind Regards, John

 

[PBC_1966]

Do you, perchance, subscribe to the "bernard view" that one should earth an otherwise electrically isolated metal bath?

http://www.diynot.com/diy/threads/e...fusebox-has-an-rcd.445955/page-5#post-3501391

 

[EFLImpudence]

The less things there are around which have a path to earth, the less the chances of someone getting a serious shock, even if they touch 230V.

Which could also be an argument for not earthing the metal casings of appliances......

It could indeed.


If you know in which order any faults will occur you can then earth and unearth as appropriate.

As this is not known it is considered wiser to earth metal casings to prevent the more likely hazard.

 

[securespark]

The question I would ask regarding the conductive qualities of water is:
Has anyone dangled a couple of lengths of copper cable in to a bath, then taken continuity test between them?

Make it more interesting and introduce a pd of 230V between those cables and add a human being to the water!

 

[securespark]

if someone with a 'sweaty body' has been in the bath, the amount of salt thereby added to the water would probably have a marked effect on conductivity

I had a salt bath the other day.

I was going to put half a kilo in but dropped the bag (I am a clumsy burger these days...) and (I know this is odd but I was interested) tasted the water before I got in to see how salty it was. It hardly tasted salty at all.

I wonder how much the conductivity of my bath was raised?
Would a person really sweat enough to raise it markedly?

I'm off to the bathroom to experiment now!!

 

[JohnW2]

I wonder how much the conductivity of my bath was raised? Would a person really sweat enough to raise it markedly?

I suppose that depends upon what you mean by 'markedly'. If you took a bowl of totally pure water and dropped one or two grains of salt into it, I suspect that you would lower the resistance from "near infinite" to "measurable".

Kind Regards, John