Securing Electricity Meter Cupboard

[ban-all-sheds]

Sorry for my earlier, slightly intemperate, reply. I don't think the position I stated was wrong, but I could have stated it better.

 

[JohnW2]

Depending on which conductor was actually cut, you could indeed. ... THE Earthing Conductor - supply to MET ... or the installation CPC - MET to CU. ...

Maybe I misunderstood, but I thought we were talking a hypothetical scenario in which the Earthing Conductor was disconnected from the provided 'earth (TN or TT) and, instead, connected to the incoming L feed. ... i.e. the MET and everything connected to it (including all CPCs, hence exposed-c-ps, and all bonded extraneopus-c-ps) would be connected to L, and nothing else in the building would have a path to earth (other than through the structure of the building)

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

If my above-mentioned understanding of the situation is/were correct, would you still regard what I have said as "strange".

Kind Regards, John

 

[JohnW2]

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. ... Touch a spoon on a wooden table and current will flow to charge the spoon to the same potential as the body.

Everything you say is correct, but you are talking about something very different (and much more 'subtle') than what is being discussed here at an elementary level.

You are talking essentially about electrostatic phenomena, which can result in the very brief flow currents between charged objects in the absence of conventional electrical circuits. That is very different from the situation which exists when one has complete circuits consisting of things connected by low-impedance conductors.

Kind Regards, John

 

[bernardgreen]

You are talking essentially about electrostatic phenomena,

I was making the point that every metal object will create a current flow if touched by a person at a different potential. It is only a noticable effect when the charge on the item ( volts time cpacitance ) is large enough for a detectable current to flow.

 

[SimonH2]

I'm assuming the main scenario under consideration is where water and gas pipes are plastic (the norm these days I believe) and so switching the main earth connection to something live is not generally going to trip/blow anything.
So what's the typical conductivity of a damp concrete floor - say in a bathroom ? Is that enough to cause a dangerous current to flow through someone stepping out of the bath/shower with wet bare feet and touching a tap ? In this case I can see an argument that electric U/F heating will increase safety since it will raise the floor potential to the internal false earth level.
Then we also have to consider things like an outside tap which will now be live, and the person may be wearing damp shoes and stood on wet ground. Ditto for using electrical equipment plugged into the house (or a shed/garage fed from it).

 

[EFLImpudence]

Maybe I misunderstood, but I thought we were talking a hypothetical scenario in which the Earthing Conductor was disconnected from the provided 'earth (TN or TT) and, instead, connected to the incoming L feed. ...

Yes. As I said, it depends how strictly it meant THE earthing conductor and whether the vandals know what they are doing.
The short bit from supply to MET could do what you say - namely:

the MET and everything connected to it (including all CPCs, hence exposed-c-ps, and all bonded extraneopus-c-ps) would be connected to L, and nothing else in the building would have a path to earth (other than through the structure of the building)

However, the CPC from MET to CU disconnected from the MET and connected to live could leave all exposed-c-ps live and all extraneous-c-ps still connected to earth.

If my above-mentioned understanding of the situation is/were correct, would you still regard what I have said as "strange".

I just thought it strange you dismissed Penguin's suggestion so casually. It could be very dangerous.

 

[penguin16]

Yes. As I said, it depends how strictly it meant THE earthing conductor and whether the vandals know what they are doing.
The short bit from supply to MET could do what you say - namely:


However, the CPC from MET to CU disconnected from the MET and connected to live could leave all exposed-c-ps live and all extraneous-c-ps still connected to earth.


I just thought it strange you dismissed Penguin's suggestion so casually. It could be very dangerous.

In my scenario, the person doing this probably wouldn't have access to the MET (unless it was in the meter cupboard) so would likely just be connecting the supply -> MET to live.

Thanks for all the insights into this though chaps!

 

[penguin16]

I was making the point that every metal object will create a current flow if touched by a person at a different potential. It is only a noticable effect when the charge on the item ( volts time cpacitance ) is large enough for a detectable current to flow.

I thought it was quite a good way to explain potential difference, so thanks!

 

[JohnW2]

Yes. As I said, it depends how strictly it meant THE earthing conductor and whether the vandals know what they are doing. The short bit from supply to MET could do what you say - namely: ... However, the CPC from MET to CU disconnected from the MET and connected to live could leave all exposed-c-ps live and all extraneous-c-ps still connected to earth.

I based what I said on context. Penguin was talking about the possibility of malicious meddling in an external meter cabinet (see title of this thread). I was assuming that the MET, complete with its bonding connections to extraneous-c-ps, would be within the house (not the external box), in which case the only cable available (in the external box) for interference would be the true "Earthing Conductor" from incoming earth to the MET - and so I assumed that he was talking about the possibility of transfring that coinductor from the 'incoming earth' to teh incoming supply live/line.

I agree that to disconnect the CU from the MET and, instead, connect it to L (leaving extraneous-c-ps connected to MET) would create a dangerous situation, but I did not think that was what was being suggested, not the least because it would nearly always have to be done within the house, not in the external meter cabinet.

I just thought it strange you dismissed Penguin's suggestion so casually. It could be very dangerous.

As above, perhaps I misunderstood what was being suggested (although I can't really see how). If my understanding was correct, then I think everything I said was reasonable, and not dangerous.

Kind Regards, John

 

[JohnW2]

I thought it was quite a good way to explain potential difference, so thanks!

I'm glad his comments helped you. However, bear in mind that he was talking about potential differences due to things (people, spoons or whatever) having somehow become 'charged' with electricity, hence creating potential differences, but without their being electrical connections to anything. In that situation, if two things (one of which may be a person) touch, then a current will flow between them, for a tiny fraction of a second, until the potentials are equalised. It is the (usually) very brief duration of those currents due to electrostatic charging which means that they are usually not harmful (lightning is an obvious exception!)

The situations we've been discussing here are very different. We have been talking of things, at least some of which are connected (to earth, to supply live or whatever) via copper conductors. In that situation, if potential differences exist between two objects (one of which may be a person), if those two objects touch then current will flow indefinitely (unless/until something stops it) - which is why (if a person is involved) such currents/shocks can be dangerous or lethal.

Kind Regards, John

 

[penguin16]

Yes, I get that.

So, in a properly installed and functioning installation, what is the most likely cause for someone to be seriously electrocuted at home?

 

[JohnW2]

Yes, I get that. So, in a properly installed and functioning installation, what is the most likely cause for someone to be seriously electrocuted at home?

Someone is probably going to telly you that "electrocuted" means 'killed by electric shock' - so that one can't really have "seriously electrocuted" (a bit like "slightly pregnant"!).

Having got that out of the way, if it is a "properly installed and functioning installation", with modern protective devices, and with no faults (or design deficiencies) in the installation or anything connected to it, then it ought to be next-to-impossible to get an electric shock.

Once one starts 'tampering', the surest way to get a serious (potentially fatal) shock is to open up something (switch, socket, light fitting, appliance etc.) whist it's live and touch both Neutral and Live/Line conductors simultaneously. That's particularly dangerous since no protective device (like an RCD) can help in that situation.

As for what, in practice, is the most common cause of domestic electric shocks, I don't know. It might be due to foolish 'tampering' with the installation ("DIY") without taking adequate (and basic) safety precautions. Damaged (or even cut) appliance leads might be another cause. There are incredibly (surprisingly?) few fatal domestic shocks in the UK, and there is very little information available about non-fatal ones, particularly more minor ones that do not require medical treatment. I'd be interested to hear what other people think.

For what it's worth, over the past 60 or so years, the only electric shocks which I have experienced, or which have been suffered by any members of my family, have been due to someone "doing something silly" nor, at least, not being careful/cautious enough about what they were doing.

Kind Regards, John

 

[John D v2.0]

Id guess the most common would be damaged equipment eg broken plugs or flexes, or maybe light fittings with no bulb fitted. But on an RCD protected circuit it would be much less dangerous assuming you're not up a ladder.
Or some prankster connecting your door handle to the incoming L conductor. Or just hitting you over the head with a 10mm SWA cable.

 

[John D v2.0]

PS if you search YouTube some guy actually set up a system to give himself shocks, it seemed even at around 20mA it was very painful, so an RCD wont save you from falling off a ladder.

 

[Jadele]

so an RCD wont save you from falling off a ladder.

Damn I have been putting a RCD at the bottom of the ladder for years! Are you saying its not going to save me if I fall?