Water pipe bonding

[333rocky333]

Sorry to confuse things, I was thinking just between hot and cold pipes, as you describe above, though I wasnt thinking straight as you did say it was all metal after your meter.
Some parts of the building im in has plastic for hot and cold then 2metre copper drops to the hot and cold taps, at the time the installer felt the need to crossbond between the 2 copper pipes.
At work im being told things about water creating a path from the extraneous bit through the insulated bit to the cold pipes and creating a risk of a voltage between the cold and the hot pipes, that in my case i assume are picking up an earth via the earthed electric water heater, though wouldnt that risk be negated by the main bond prior to the insulated bit on the cold anyway.
Then as you mention the risk of the Floating bits and the earthed via the heater bits.
To be honest im getting more confused, everyone at works telling me different things

 

[EFLImpudence]

at the time the installer felt the need to crossbond between the 2 copper pipes.

He probably didn't "feel the need"; he probably just did it as plumbers insist for below boilers - completely pointless.

At work im being told things about water creating a path from the extraneous bit through the insulated bit to the cold pipes and creating a risk of a voltage between the cold and the hot pipes,

Other than very short lengths you can ignore the water.

that in my case i assume are picking up an earth via the earthed electric water heater, though wouldnt that risk be negated by the main bond prior to the insulated bit on the cold anyway.

Yes.

Then as you mention the risk of the Floating bits and the earthed via the heater bits.

He hasn't replied yet but I am surprised at John saying that.
We do not have to consider pipes becoming live by contact with cables around the house, so the more floating bits there are, the better. That is the fewer negligible impedance paths to earth there are, the better.

Would you earth a sink (or the proverbial spoon) if it was not connected to any other earthed parts? If you're not sure, the answer is no.
Imagine touching a faulty live washing machine and sink (or spoon), would you rather the sink (or spoon) was earthed or floating?
That it may be earthed by an earthed pipe is just unfortunate and unavoidable to achieve ADS at the boiler or immersion. You don't want to make the path to earth easier by more earthing or bonding.
The best way would be to insert more plastic lengths of pipe.

To be honest im getting more confused, everyone at works telling me different things

I am not surprised.

 

[JohnW2]

He hasn't replied yet but I am surprised at John saying that. We do not have to consider pipes becoming live by contact with cables around the house, so the more floating bits there are, the better. That is the fewer negligible impedance paths to earth there are, the better.

I "hadn't replied yet", since, at the time I tried, the website seemed to be going through one it's iffy phases and wouldn't let me!

As you know, in general I totally agree with you, particularly when there are 'electric things around' - although, as I said, even with 'partially plastic' plumbing installations, it's very probable that most, if not all, of the bits of metal pipe will have some low impedance connection to earth (either 'incidental' or via main bonding). (electrically) 'floating' metal pipes are therefore probably pretty uncommon - so the issue is largely moot, with most metal pipework being earthed, even if one would prefer it not to be. Admittedly, that doesn't seem to be the case with what Rocky is describing - a primarily plastic installation with metal drops to the taps.

If, as per what you write, one's view is that "we do not [ever] have to consider [electrically floating] pipes becoming live ....", then it would be correct to say that it is preferable not to connect such a pipe to anything - since, as we are agreed, in general it's best to minimise the amount of touchable metal at earth potential ('if there are electrical things around'). However, whether your premise is always correct is something I'm not sure about.

Having said all that, it's very many years since I did any 'cross-bonding' of pipes - but, of course, with everything on my side of the meter metal, it's essentially inevitable that it's all already got a low impedance path to earth, even if I would be 'happier' if it didn't!

Kind Regards, John

 

[JohnW2]

Duplicate deleted

 

[JohnW2]

There's no such thing, electrically, as cross-bonding.

I've only just noticed this post (at the bottom of the previous page!). Yes, I know that isn't a usual electrical concept, which is why I put it in quotes; it was Rocky who introduced the term. However, I suppose one needs some terminology to refer to what plumbers seem to do all the time!

Quite frankly, I can't believe you wrote it. I thought I had dissuaded you from earthing isolated metal parts.

I never needed any dissuading (I'm not bernard!) - I'd love to live in an earth-free environment (even though I would thereby be denying myself ADS)!

However, 'bonding' pipes together (whatever you want to call it) does not necessarily mean earthing either of them - that's only the case if one (and only one) of them is already earthed. Although I admit that it would be a measure for protecting against the incredibly unlikely, I suppose there is a theoretical argument for 'electrically joining together' (since you don't like the b-word) two simultaneously-touchable metal parts which are both electrically floating - something which you certainly could not call 'earthing'.

Kind Regards, John

 

[bernardgreen]

I thought I had dissuaded you from earthing isolated metal parts.

Read more: http://www.diynot.com/diy/threads/water-pipe-bonding.458276/page-7#ixzz4KxvbiFUN

I never needed any dissuading (I'm not bernard!)

I do not advocate earthing isolated metal parts ( teaspoons etc etc ) when the risk of that item becoming non isolated is extremely small. My point it that some apparently isolated items ( metal baths, metal kitchen sinks etc etc ) can become non isolated and thus present a hazard. In most of these cases earthing ( to Ground ) is not appropriate but bonding to the installation's main Earthing terminal ( MET ) may be appropriate provided the item is inside the equipotential zone of the installation. Bonding an outside water tap to the MET is OK provided the MET remains close to Ground potential. In a TT installation that is 99% assured. In a PME system were the MET "earth" is the incoming Neutral ther is a significant risk that the MET is not at Ground potential and may is network fault situations be 230 volts above Ground potential. It that situation an outside water tap bonded to the MET presents a severe hazard of fatal electric shock. Other than nothing electric works the house owner will have no indication that the outside water tap is a severe hazard.

Some will discount the risk of the MET potential becoming a hazard as too small to worry about. If it really was too small to worry about then there would be no significant risk in exporting the MET along a cable to "Earth" the metal frame of garden tools. Or for the DNO to require my PME "earthed" MET and CPC cannot make contact with the TT Earth in the adjacent retail unit. ( One of my rooms is above the retail unit ) and that there is a non conductive section in the water supply from my cottage to the retail unit.

Each case needs to be considered individually and there is no fit all answer to the question of bonding , earthing and floating.

 

[333rocky333]

Its quite a large retail shop most of it is copper and the only insulating bits are plastic elbows, so the shortness of the insulating bits is my concern.
maybe the plumber could not solder.

1 or 2 sinks do have long lengths of plastic pipe then metal drops, but the majority is aprox 2 meter lengths of copper

 

[JohnW2]

Its quite a large retail shop most of it is copper and the only insulating bits are plastic elbows, so the shortness of the insulating bits is my concern. ... 1 or 2 sinks do have long lengths of plastic pipe then metal drops, but the majority is aprox 2 meter lengths of copper

To bring this discussion back to sanity, as EFLI has suggested, I think you're worrying unnecessarily. There certainly is no regulatory requirement to do <whatever you want to call it>, other than in a small proportion of bathrooms (and then you have to connect the pipes to CPCs, as well as to each other) and, for the reasons EFLI has stated, there's really not any other reason to do it, unless you are concerned about unbelievably unlikely (he doesn't like 'improbable') scenarios.

Simply 'joining together' (with a wire) two pipes which are both (electrically) 'floating' does not do any harm that I can think of (doesn't invoke the 'unnecessary/undesirable earthing' which EFLI seems to believe he has 'dissuaded' me from) - but it really does not achieve anything particularly useful, either.

Kind Regards, John

 

[JohnW2]

My point it that some apparently isolated items ( metal baths, metal kitchen sinks etc etc ) can become non isolated and thus present a hazard.

Indeed - and one has to undertake a 'risk assessment' to determine whether one believes that the probability of that happening is smaller or greater than the risk associated with 'unnecessarily' introducing a lot more earthed metal into the environment in question.

In a PME system ... It that situation an outside water tap bonded to the MET presents a severe hazard of fatal electric shock.

Although we're talking about an incredibly unlikely combination of events, we have agreed on a number of occasions that the safest thing for an outside tap is for it to be electrically 'floating' if the house has a TN-C-S electrical system.

Some will discount the risk of the MET potential becoming a hazard as too small to worry about. If it really was too small to worry about then there would be no significant risk in exporting the MET along a cable to "Earth" the metal frame of garden tools.

You seem to be stating the obvious - if someone believed that that risk was too small to worry about, they would, indeed, presumably agree that there was no significant risk in connecting the exposed metal of outdoor tools to the MET of their TN-C-S system. What was your point?

Each case needs to be considered individually and there is no fit all answer to the question of bonding , earthing and floating.

Exactly - as I said, a 'risk assessment' is necessary. However, it's not one-sided - as EFLI and I (although he doesn't seem to accept that!) frequently point out, if one connects to earth something which would otherwise be 'floating', one thereby increases some risks, so they have to be balanced against the risks associated with not connecting them to earth.

Kind Regards, John

 

[EFLImpudence]

Simply 'joining together' (with a wire) two pipes which are both (electrically) 'floating' does not do any harm that I can think of (doesn't invoke the 'unnecessary/undesirable earthing' which EFLI seems to believe he has 'dissuaded' me from) - but it really does not achieve anything particularly useful, either.

I thought you were referring to a situation where one of the pipes WAS earthed by, e.g., the boiler.
Wasn't that what rocky was talking about?

Sorry if I misunderstood.

 

[JohnW2]

I thought you were referring to a situation where one of the pipes WAS earthed by, e.g., the boiler. Wasn't that what rocky was talking about?

I don't think so. He appeared to be talking about relatively short metal drops to the sink (presumably for taps) both downstream of plastic interruptions, wasn't he?

Kind Regards, John

 

[bernardgreen]

if someone believed that that risk was too small to worry about, they would, indeed, agree that there was no significant risk in connecting the exposed metal of outdoor tools to the MET of their TN-C-S system. What was your point?

My point is that "earthing" the metal case of a power tool in the garden is not recommended and because of this the cable supplied with many garden tools is two core.

otherwise be 'floating', one thereby increases some risks,

The risks are
1) a person already touching or holding a faulty item / appliance at 230 volts touches the sink with their other hand.
~~~~ Not earthed then no severe shock.
~~~~ Earthed then severe shock possible

2) the sink becomes live by fault contact with a live wire or by a defective appliance being placed on the sink .
~~~~ Not earthed then the sink is now a live item, touching it and something that is earthed will be a severe shock hazard
~~~~ Earth then a protective device will (should ) operate and remove the Live supply.

Which is the most likely and which has the worst outcome.

 

[333rocky333]

Im getting you all confused now

The hot pipes are via a electric water heater and 70% of entire system is via metal pipes with just plastic elbows.

There are 4 sinks, only recently I confused you by mentioning the plastic pipe, with the metal drops, thats only done on the warehouse sink.

regarding the EAWR i found this,not sure if its relevant.

http://www.legislation.gov.uk/uksi/1989/635/regulation/8/made

 

[JohnW2]

My point is that "earthing" the metal case of a power tool in the garden is not recommended and because of this the cable supplied with many garden tools is two core.

It might not be 'recommended', and most (if not all) outdoor tools are Class II these days, anyway, but I rather doubt that the incredibly unlikely (I would say 'incredibly improbable' if someone was not looking over my shoulder!) combination of events you referred to is the reason for that recommendation.

The risks are: 1) a person already touching or holding a faulty item / appliance at 230 volts touches the sink with their other hand. 2) the sink becomes live by fault contact with a live wire or by a defective appliance being placed on the sink . Which is the most likely and which has the worst outcome.

That's the judgement ('risk assessment') that one has to make. Opinions will obviously vary, between individuals and between situations, but I suspect that your opinion might well often differ from that of many others. What you say about the two scenarios is a little misleading, in that you fail to mention that an RCD, if present, will hopefully limit the effects of the shock in your first scenario.

FWIW, I would personally say that your second scenario is the less likely. Contact between 'a live wire' and a sink is almost never going to happen. I doubt that many people put electrical items on the sink in the first place but, even if they did, many/most of the more movable ones are Class II and/or without exposed metallic parts - and, in any event, are very rarely faulty. In fact, I would say that the more likely scenario with a 'movable' kitchen electrical item would, if it were faulty, be your scenario (1), in which case earthing the sink would increase the hazard.

Kind Regards, John

 

[EFLImpudence]

My point is that "earthing" the metal case of a power tool in the garden is not recommended and because of this the cable supplied with many garden tools is two core.

That is not the reason.
Double insulated is deemed safer than earthed exposed-conductive-parts.

Earthing is NOT a good thing; it is a necessary evil.

The risks are
1) a person already touching or holding a faulty item / appliance at 230 volts touches the sink with their other hand.
~~~~ Not earthed then no severe shock.
~~~~ Earthed then severe shock possible

2) the sink becomes live by fault contact with a live wire or by a defective appliance being placed on the sink .
~~~~ Not earthed then the sink is now a live item, touching it and something that is earthed will be a severe shock hazard
~~~~ Earth then a protective device will (should ) operate and remove the Live supply.

Which is the most likely and which has the worst outcome.

No.1 is the most likely and therefore judged to be the hazard against which we should be protected.
It's not a question of the worst outcome but the greater protection.

 

[EFLImpudence]

http://www.legislation.gov.uk/uksi/1989/635/regulation/8/made

That is exactly the reason for NOT earthing/bonding isolated parts.

They will not become live during a fault - but if you earth them, they will.