Earth bonding after boiler move

[JohnD]

The risk assessment therefore has to consider which is the more likely/common cause of electric shocks - touching two pipes simultaneously, or touching something else 'live' at the same time as touching a pipe/whatever. I don't know the answer, but suspect that the latter is probably more common.

Kind Regards, John

Except perhaps for boiler repairmen.

 

[JohnW2]

Except perhaps for boiler repairmen.

Indeed, as you said before, perhaps except for them.

However, a risk assessment has to look at the big picture (i.e. consider everyone, over all time). The boiler repairman will only be at risk of risk of a serious electric shock if an electrical fault exists at the very time he/she is working on the pipework, probably a few hours at most. In contrast, the risk to occupants who simultaneously touch something 'live' and an earthed pipe will be at risk of serious electric shock so long as the property is occupied (probably many decades), and that risk doesn't even necessarily require any fault to be present.

Kind Regards, John

 

[adaml]

OK so if I've understood correctly bearing in mind I'm just a paranoid homeowner.

Faulty earthed device with live metal case, I touch it and also touch unearthed CH loop, the electric flows to earth through the earthed faulty device
Faulty earthed device with live metal case, I touch it and also touch an earthed CH loop, the electric flows to earth through me and the earthed pipes

So isolated bits of metal are better to be not connected electrically with the earth, so theres less likelyhood of me completing a circuit between a faulty device and the earth if I touch them both at the same time?

 

[EFLImpudence]

Faulty earthed device with live metal case, I touch it and also touch unearthed CH loop, the electric flows to earth through the earthed faulty device

Well, it will only be live for the very short time until the MCB trips.

If you are touching it during this short time some current will flow through you IF there is any where for it to go. If you are touching another earthed part which has also become live there will only be the small potential difference between the two parts - none at all if they are bonded very close by.

So isolated bits of metal are better to be not connected electrically with the earth,

Yes.

so theres less likelyhood of me completing a circuit between a faulty device and the earth if I touch them both at the same time?

Not exactly - during a fault ALL earthed parts will become live. The only voltage between them (if you touch two) will be dependent on the voltage drop because of their impedances.

The problem is if you touch a live wire and an earthed part; all the current will flow through you. So if the part does not have to be earthed (as in bonded unnecessarily) it should not be so that there is no where for the current to go.

 

[EFLImpudence]

 

[JohnW2]

OK so if I've understood correctly bearing in mind I'm just a paranoid homeowner.

There's nothing wrong with being paranoid when one is talking about potential risks to life!

Faulty earthed device with live metal case, I touch it and also touch unearthed CH loop, the electric flows to earth through the earthed faulty device

As has been said, it it really is earthed, then it can only become 'live' for a fraction of a second, until the MCB trips. It's incredibly unlikely that someone would touch it during that 'window' of just a few milliseconds.

A far better example would be if someone touched a live conductor (e.g. due to a damaged vacuum cleaner lead) and also touched an 'unearthed CH loop'. Since the CH pipes are not earthed, there is no path through which current can flow, so no shock.

Faulty earthed device with live metal case, I touch it and also touch an earthed CH loop, the electric flows to earth through me and the earthed pipes

Indeed so.

So isolated bits of metal are better to be not connected electrically with the earth,...

Exactly, at least in relation to the sort of scenario we are considering.

As others have pointed out, there are other theoretical situations in which connecting bits of metal to earth (whether in the name of 'earthing' or 'bonding') can reduce risks. It's therefore a matter of 'swings and roundabouts', such that one has to make a 'risk assessment as regards which of the two types of situation is the more likely. I've expressed my personal view that, in general, the risk caused by 'unnecessarily' connecting bits of metal to earth outweighs the theoretical downside of doing that, but others may disagree.

.... so theres less likelyhood of me completing a circuit between a faulty device and the earth if I touch them both at the same time?

Essentially yes (EFLI has given a somewhat more detailed explanation).

Indeed, if one of the things you touch is not (electrically) connected to anything (earth or otherwise), it's not merely a case of "less likelihood of you completing a circuit" but, in fact, zero likelihood, since there can be no 'circuit'.

Hope that helps!

Kind Regards, John

 

[adaml]

Thanks for all the explanations folks. Very educational!