Regulations Myths

[plugwash]

Indeed, but provided one can do it, unplugging is the ultimate in "completely disconnected", isn't it?

Absolutely.

Personally I would never rely on a socket switch for safety isolation and similarly when using a FCU for isolation, I'm both switching it off AND removing the fuse.

But I would also not assume that other people would take the same approach as me.

Quite - and I would say that is definitely an "UNsafety feature" (albeit I imagine very rare). !

Which does leave the question, which is more likely, that the neutral pole on a socket switch will fail un-noticed, or that the supply neutral to the socket will become live.

 

[ericmark]

Do we bother ?
The days when the DNOs came round disconnecting local earth rods when applying PME (I recall it happening to a friend) are long gone - I assume. Having a local earth connected with the DNO earth no longer seem to the the taboo it once was, and it seems to be actively encouraged now - it was rumoured that Ed18 rev 2 would make it mandatory.

I did not know they ever did that, I have seen boxes the size of a small van connected between the PME earth and site earth, so there was a limit on the current which could flow. And also seen the wire between the PME earth and someone earth mat melt.

My house has TN-C-S and an earth rod, both connected together, with a loss of the DNO supply the inverter connects the neutral to the earth, but it does not disconnect the DNO earth.

I know sometimes diodes are put in the earth, so there is around 3 volts before any current will flow through the different earths, however TN-C-S is banned for boats, and hard to ensure a TN-S supply is not changed to a TN-C-S, so those boats nearly always have a TT supply, and with my son, I know shore supply went through an isolation transformer in the battery charger so shore earth was never connected to the narrow boat.

Caravans and boats in use have to be a set distance from the building due to fire regulations, so using TT not a problem, but in storage with a supply to keep batteries topped up, that is something else. And two buildings with supplies of different type close to each other, be it a tin shed, or another house, the worry is can anyone touch the two earthing systems at the same time?

I look at where I work, we have steel construction, and think 4 or 5 supplies, impossible for the earths not to connect together, and the earthed metal runs for around 8 miles, (it's a railway) so likely the buildings are earthed far better that the DNO's supply, how much current can flow through the earths?

 

[JohnW2]

Personally I would never rely on a socket switch for safety isolation and similarly when using a FCU for isolation, I'm both switching it off AND removing the fuse.

Same here (and hoping that the fuse was in the L )

But I would also not assume that other people would take the same approach as me.

Indeed - never 'assume', particularly when one is talking about the behaviour of people!

Which does leave the question, which is more likely, that the neutral pole on a socket switch will fail un-noticed, or that the supply neutral to the socket will become live.

As I've already suggested, I think the former in incredibly UNlikely. As for the latter, what are you thinking about - the infamous 'lost PEN', perhaps - if so, I again think it's incredibly unlikely that such would happen at the very time that one needed isolation - so I don't think I would lose any sleep over any of that (as I always point out, people DO get struck by lightning!).

 

[ebee]

I have seen "more than just a very few" instances where the L & N of a socket were transposed and have probably been like that for quite some time too.
Not completely unknown for the supply from a (sealed) meter to have the L & N transposed either, I reported one such finding to the local electric board a few years back. Their attitude was sort of thanks for telling us we will correct it as and when it is convenient for us. I retorted that if old mrs smith switched her light off and went to replace her bulb in the bulb holder and got killed because of it then they would be asked, whilst gripping the rail, by a solemn bloke in a wig why they had not treated it urgently. I asked for the direct landline number for their inspection department and guess what? It got dealt with within a few hours without me actually having to contact the inspection department.

Crabtree and a few more makes had been DP switching on the 13A sockets for years whilst MK Logic were still only SP switching for years.

DP switching also offers protection from the "Borrowed N" scenario. So, if you use a decent working socket both switching off and unplugging isolate all Live conductors. The other alternative is to use unswitched sockets to encourage unplugging .

Things do happen that seem unlikely, even almost impossible, but when you have seen enough of them you kind have learn to lookout for such "Unlikely events", I bet that is true not just of issues found by Electricians but by Plumbing and Heating Engineers (I have seen and heard quite a few of those too) and Car Mechanics, Joiners, Builders etc etc etc.
Yes, things do happen at a rate that you might not imagine, human nature is not perfect, carelessness and incompetence is lurking all around us. Folks do make mistakes I`m afraid!

Never underestimate the sheer determination of a complete and utter idiot!

 

[Harry Bloomfield]

Things do happen that seem unlikely, even almost impossible, but when you have seen enough of them you kind have learn to lookout for such "Unlikely events", I bet that is true not just of issues found by Electricians but by Plumbing and Heating Engineers (I have seen and heard quite a few of those too) and Car Mechanics, Joiners, Builders etc etc etc.
Yes, things do happen at a rate that you might not imagine, human nature is not perfect, carelessness and incompetence is lurking all around us. Folks do make mistakes I`m afraid!

I spent most of my working life, working with big, expensive infrastructure - where mistakes can cost thousands. A final double check, always, and then a check for any tools left behind, before shutting the doors.

 

[JohnW2]

DP switching also offers protection from the "Borrowed N" scenario.

Could you perhaps explain that? I don't think I've come across, or even heard of, a 'borrowed N' in a sockets circuit, and nor can I really think of how that would arise.

 

[plugwash]

As I've already suggested, I think the former in incredibly UNlikely.

I tend to agree, socket switches do fail, but the design makes it unlikely for them to fail without it being mechanically noticed.

As for the latter, what are you thinking about - the infamous 'lost PEN', perhaps

Well in a 'lost PEN' everything is live including the "earth". I was thinking a lost neutral further downstream than that. Still pretty unlikely in typical fixed wiring. Gets rather more likely in portable/mobile power scenarios.

 

[ebee]

Could you perhaps explain that? I don't think I've come across, or even heard of, a 'borrowed N' in a sockets circuit, and nor can I really think of how that would arise.

Well I think you may be aware of the borrowed N scenario in circuits (commonly lighting circuits are the ones to watch for) but any circuit that we isolate could be be suffering from borrowed N unti we are sure we have properly isolated our bit.
The number of permutations that people will do wrong used to amaze me sometimes but nothing amazes me for many years.
Usually it’s the result of someone committing DIY but not always , it’s sometimes the result of “
Electricians” and even sometimes Electricians too .

If there is double pole switching in the circuit immediately before the portion you are working on then it is pretty sure to be safe to touch providing the switches are working and there are no other connections further downstream.

The classic example of the simple two circuits two L and two N and how a bridge between circuits might occur an give rise to the borrowed N problem is pretty widely considered and I am sure that many folk on here will be aware of it.
What I am not sure of is that everybody will consider the same scenario presenting in a power circuit.

Unrelated but equally as surprising to some might be the double fused ring final scenario (someone probably has a better name for it).
I sure all of our trades folk on here will be aware of it even if they have not yet come across it themselves yet in real life - two ring finals , each with two L connections back at the consumer unit.
One L leg of each ring terminated in one fuseway and the other L leg of each ring connected in the other fuseway.
Result, two supposedly separate circuits each become a radial circuit still energised if one fuse removed or one MCB tripped to off position . Also if both fuses intact/ MCBs on then what kind of current curve characteristic on the conductors? Probably not even merely a simple doubling,

Conclusion - if say a boiler circuit has a simple plug running it and providing nothing else is connected upstream we can be reasonably sure that pulling the plug will always isolate it. Merely switching the socket off may or may not protect us from transposed L/N or a borrowed N in the socket circuit. A healthy DP switched socket would achieve that isolation a SP switched socket would not be any gurantee .

It is a very long time since I would ever exclaim “well I would have never expected anyone to have done that”

 

[JohnW2]

Well I think you may be aware of the borrowed N scenario in circuits (commonly lighting circuits are the ones to watch for) but any circuit that we isolate could be be suffering from borrowed N unti we are sure we have properly isolated our bit.

Sure, we very often discuss the issue of a 'borrowed neutral' on a lighting circuit (usually in relation to tripping RCDs!), which is particularly common with 2-way switching and/or the hall/landing lights situation. It can also occur if someone 'borrows a neutral' from some other lighting circuit to power some new-fangled replacement for a light switch.

However, as I said, I've not come across or heard of anything similar happening with a sockets circuits - and, as I said, struggle to think how/why that would arise.

Kind Regards, John

 

[ebee]

Sure, we very often discuss the issue of a 'borrowed neutral' on a lighting circuit (usually in relation to tripping RCDs!), which is particularly common with 2-way switching and/or the hall/landing lights situation. It can also occur if someone 'borrows a neutral' from some other lighting circuit to power some new-fangled replacement for a light switch.

However, as I said, I've not come across or heard of anything similar happening with a sockets circuits - and, as I said, struggle to think how/why that would arise.

Kind Regards, John

Yep it does, I once discovered a timed overun Perm L ona bathroom extractor fan running unfused from a socket circuit yep the Sw L an the N were on the lighting circuit, I had a recent EICR fro an approved contractor and this was not mentioned, neither was the complete omission of an earthing conductor on the one way consumer unit feeding the shower (the only reading that could give an Zdb fir that unit was the back feed from the E in the T & E in the shower deriving it from the shower pipework. Yet a comment than in a basement (cellar) enclosed fluorescent fittings were metal and could be touched ( do not know what reg they thought that breached by being in touch reach in a cellar with a sink present though (the fmetal fittings could conceiveably be touched by a person standing on the floor but not by a person touching the pot sink or the metal tap, and a comment about a repurposed 6.0 T & E 32Amp circuit feeding one immersion heater then piggybacked in 2.5 T & E to a second immersion heater (the comment on this fault was "Not Permitted".
Yes I have seen some crappy certificates and reports from those who should be expected to produce better.

PS - the extractor Perm L was not fused down from the 32A MCB that was running the power circuit and it was of course on a different circuit to the SW & N of the 6A extractor circuit - just to clarify.

 

[SimonH2]

I suppose so - but, in most cases, if one can switch it off, one could probably also unplug it.

We've got a number of sockets where that isn't the case. Worst case we have to move furniture to unplug, but can reach the switch.

 

[JohnW2]

We've got a number of sockets where that isn't the case. Worst case we have to move furniture to unplug, but can reach the switch.

Fair enough. I did say "in most cases" and I still think such a situation is pretty unusual. I've certainly come across a good few situations in which a plug could not be removed without moving furniture, but in 'most' of them one would also have had to move furniture to get to the switch.

 

[ericmark]

Yes, I have sockets behind furniture. But that does not stop me checking the sockets on the extension lead which I can reach. To plug in tester and press button is easy. I do think this

is not as clear as it could be to show correct polarity, if reversed the curved line with arrow is shown below L and N with arrow reversed. But most socket testers show Line - Neutral reversed even the cheapest models, but with a FCU to test you need to dismantle something, so it could easily be left that way for years.

 

[SUNRAY]

Could you perhaps explain that? I don't think I've come across, or even heard of, a 'borrowed N' in a sockets circuit, and nor can I really think of how that would arise.

Seems this one had both lighting circuits linked by the horrible 'converting 2 lights to 2 way method' as well as the electric shower to operate the bathroom fan.

help needed with bathroom light

Hi I have been abroad working and left the builders doing a loft conversion, part of the regs was to fit a bathroom fan. The builders have gone bust. When I switch the bathroom light off it blows BOTH lighting 6A trips in the fusebox. I thought I'd be clever and disconnect the new fan wires...

www.diynot.com

I used to find wall lights picking a neutral from the nearest circuit be it lighting or 30A power/cooker.



and this elderly installation with a single loop of neutral for a number of different circuits: https://www.diynot.com/diy/threads/borrowed-neutrals.566620

 

[ericmark]

I have found borrowed neutrals on sockets, and never did sort it out, of all places, in a bank. Also, many other cases, mainly where there was a three-phase supply with no neutral and someone in error has got a contactor with 230 volts instead of 400 volts coils.

But the most common has to be landing lights. But that is not the only borrowed neutral one finds.