Securing Electricity Meter Cupboard

[JohnW2]

Seems like a malicious person with access to your meter cupboard (in a TN-C-S or TN-S system at least) could disconnect your earth (making your installation unsafe) or, worse, connect it to a live supply.

If adequate bonding were in place within the building, connecting the earthing conductor to the L would probably be less dangerous than you might imagine.

If any of the bonded extraneous-conductive-parts had a low impedance to earth, it would probably blow the cutout fuse, anyway.

Kind Regards, John

 

[penguin16]

If adequate bonding were in place within the building, connecting the earthing conductor to the L would probably be less dangerous than you might imagine.

If any of the bonded extraneous-conductive-parts had a low impedance to earth, it would probably blow the cutout fuse, anyway.

Kind Regards, John

I suppose you could get "to earth" through a water or gas pipe which would hopefully trip something.

Certainly wouldn't really fancy a Home Alone esque electrocution when turning a tap on!

 

[JohnW2]

I suppose you could get "to earth" through a water or gas pipe which would hopefully trip something.

That's what I said in my second sentence/paragraph.

Certainly wouldn't really fancy a Home Alone esque electrocution when turning a tap on!

As I said/implied, if adequate bonding was in place (and remained in place) in the building, then you would not get a shock, let alone be electrocuted, if you touched a tap. You may want to read up about "equipotential zones".

Kind Regards, John

 

[ban-all-sheds]

Seems like a malicious person with access to your meter cupboard (in a TN-C-S or TN-S system at least) could disconnect your earth (making your installation unsafe) or, worse, connect it to a live supply.

They could do what they like.

The real question is what kind of "they could do {whatever}" scenarios are worth putting a lot of effort into counteracting when you can only theorise about the sort of "they" and what they could do?

FYI - I could do just what you suggest to my neighbour, should I choose, as his (TN-S) supply loops through my house.

 

[penguin16]

You may want to read up about "equipotential zones".

I will indeed!

 

[JohnW2]

I will indeed!

You may find it interesting.

The crucial point is that you will only get a shock if you touch two things that are at appreciably different potentials (voltages). Bonding within a house seeks to ensure that all touchable metal is at the same potential, so that no matter what two things you touch, you won't get a shock. It doesn't matter if all the metal in the house is at earth potential, 230V above earth potential or 1,000V above earth potential. If everything is at the same potential, you won't get a shock.

There are, needless to say, some complications in practice - but that's the general message.

Kind Regards, John

 

[ban-all-sheds]

Bonding within a house seeks to ensure that all touchable metal is at the same potential, so that no matter what two things you touch, you won't get a shock.

It absolutely does not.

It only equalises the potential of extraneous-conductive-parts.

Unearthed exposed-conductive-parts could still present a lethal shock hazard in the event of a fault.

This is not pedantry, or your oft-invoked "this is all about what words actually mean so I'm not interested" - it is fundamental to understanding bonding vis-a-vis earthing, and how an equipotential zone might not be as nice as an earthed equipotential zone.

 

[JohnW2]

It absolutely does not. It only equalises the potential of extraneous-conductive-parts. Unearthed exposed-conductive-parts could still present a lethal shock hazard in the event of a fault.

It seeks to equalise the potential of extraneous-c-ps with the potential of exposed-c-ps - that's why it is required that the main bonding of extraneous-c-ps is connected ('bonded') to the MET and also that all exposed-c-ps are connected ('earthed') to the MET (via CPCs).

This is not pedantry ...

I see. I was trying to simplify the situation for someone who clearly had limited understanding of the relevance of potential differences. Would you have been happier if I had written:

"Bonding and earthing within a house seeks to ensure that all touchable metal is at the same potential ...."

Kind Regards, John

 

[ban-all-sheds]

Not entirely.

"Earthed equipotential bonding and earthing" would be better.

And "touchable metal" includes cutlery, door handles, spectacle frames....

 

[JohnW2]

Not entirely. "Earthed equipotential bonding and earthing" would be better. And "touchable metal" includes cutlery, door handles, spectacle frames....

As I said, I was just trying to impart some generalised concepts to someone who didn't even seem to understand much about potential differences. Your contribution will, in my opinion, be doing anything but helping him.

That's all I have to say.

Kind Regards, John

 

[ban-all-sheds]

[EDIT]
We need to work up to what makes Thing A have a potential difference wrt Thing B, and how/when/why it matters if you can touch one of them. Or both.

Any 'complexities' need to be worked up to and clarified, not treated as if they don't exist, IMO.

Ideas to follow.
[/EDIT]

 

[penguin16]

Thanks John.

I was rather thinking of a, rather unlikely, situation where you had live on earth but no other path to earth on the installation except that would be created through a person themselves. I assume you could quite a shock before the circuit tripped.

 

[bernardgreen]

"Bonding and earthing within a house seeks to ensure that all touchable metal is at the same potential ...."

Everything in a house has a potential. Everything you touch will result in current flowing between your finger and the object until you and the object are at the same potential.

How much current ( Amp-Hours ) flows will depend on the capacities of you and the object and the initial potential difference between you and the object. The rate of flow ( Amps ) will be determined by the decaying potential difference and the impedance of finger and the object.

Highest potential difference are found in static electrical event. Shuffling across a carpet can raise the persons body a few hundred if not thousands of volts above carpet potential and hence nuch higher than most objects in the room and ground.
Touch any earthed item and a spark will be seen and heard and possible felt as a pamg of pain. Maybe a few amps for less than a milli-second by which time the potential on the body has been discharged.

Touch a spoon on a wooden table and current will flow to charge the spoon to the same potential as the body. The capacty of the spoon is extremely small so very little current flows before the potentials are equalised.

Touch a person ( who has not been shuffling ) and current will flow until both bodies are at the same potential, this flow can create a spark and maybe a pang of pain.

 

[EFLImpudence]

I was rather thinking of a, rather unlikely, situation where you had live on earth but no other path to earth on the installation except that would be created through a person themselves. I assume you could quite a shock before the circuit tripped.

Depending on which conductor was actually cut, you could indeed.

THE Earthing Conductor - supply to MET
or the installation CPC - MET to CU.

I think John's dismissal of the occurrence quite strange.

 

[JohnW2]

I was rather thinking of a, rather unlikely, situation where you had live on earth but no other path to earth on the installation except that would be created through a person themselves. I assume you could quite a shock before the circuit tripped.

Well, yes, IF there were a path to earth through the person, then that would be the case.

However, there would only be a path to (true) earth through the person if they were in contact with something that itself had a path to (true) earth. If the incoming 'earth wire' (hence the entire 'earth system' of the installation) has been connected to the live supply, and if anything else entering the house from outside (like water/gas pipes) is effectively connected to that 'earth system', then, in simplistic terms, there should not be anything at true earth potential within the building that could provide a path through a person to true earth. If there were no floors and they were standing on damp soil with bare feet, that would be a different matter.

I say "in simplistic terms" since, as I wrote before, in practice there are complications. In particular, if high currents start flowing in the conductors involved in earthing and bonding, then not all the potentials in the house will be exactly the same.

Kind Regards, John