Is it safe to use a metal light switch faceplate on a non-RCD protected circuit?

[opps]

I guess there may still be a few about. I was taught at tech college that the speed of current cut by an MCB was such that it would not be able to kill a two year old.

Yeah... but what if the 2 year old was standing in a padding pool wired to an earth rod...

 

[Harry Bloomfield]

Only twice in the past 20 years have I seen old style units protected by fuse wire although I guess there may still be a few about. I was taught at tech college that the speed of current cut by an MCB was such that it would not be able to kill a two year old. Think about it, if metal light switches were at all dodgy, they would not sell very many would they?

Then I would suggest you go back to college. The current flowing through a two year old, or in fact a thirty year old would not trip an MCB - think maybe 30mA or so. The smallest MCB in common use is 6000mA. Protection of people from electric shock is not the purpose of MCB's, rather it is what RCBO's and RCD's are designed to help prevent, even then, only in limited circumstances.

 

[EFLImpudence]

I find this all very strange, lots of folk use metal faced light switches, they are everywhere these days and perfectly safe unless seriously interfered, which the average child would be incapable of unless he were to set about using a hammer with all of his might.

Quite.

It's true that "only half" of modern consumer units are protected with an RCD - the power and cooker circuits which can carry a heavy load due to the thickness of the cables used

Not sure whether 'half' is correct.
It is the cooker which draws high current which means a thick cable is required.

However, the RCD is to protect people, by switching off the power, when they touch a live part causing the current to flow through them which an MCB would not do.
It doesn't really matter about the load current or size of the cable.

A live faceplate or cooker carcass is the same

but all circuits (including those for lighting) are never the less protected by MCBs these days which switch off instantly if there is a problem and are far safer than relying on just an earth wire.

They would not switch off, when because of a fault the faceplate becomes live, if there were not an earth wire - that is why there is one.

Only twice in the past 20 years have I seen old style units protected by fuse wire although I guess there may still be a few about.

I am sure there are still many that have fuse wire but fuse wire works the same as an MCB.

I was taught at tech college that the speed of current cut by an MCB was such that it would not be able to kill a two year old. Think about it, if metal light switches were at all dodgy, they would not sell very many would they?

The point is that the power is switched off, in the event of a fault causing the faceplate to become live, before anyone touches the live faceplate.

 

[Spetznaaz]

Just finished the job. Not sure how well i have done.. I need a breather then i will respond to all the replies and post some pics.

So much dust.

 

[Spetznaaz]

Right, so first off, apologies for the mammoth post this will probably be.

I have a few concerns i was wondering if you guys could help with (probably should have made another thread but bit late now).

I flipped the switch on the consumer unit for the downstairs lights, my contactless voltage pen no longer beeped when close. I then put the red probe for my multimeter on the live and black on the earth... voltage was detected. Same with black probe on the L1 wire. I was scared to work on it like this so i switched the main breaker off and there was then no voltage detected. Pain in the arse every time i wanted to drill or hoover. Is this normal?

I did not run an earth wire from the faceplate earth to the backbox. My reasoning behind this was that one of the screws that goes through the faceplate, makes contact with the earth and secures to a fixed part of the backbox. I got a 1.5mm2 earth wire which is quite thick - my thinking was, if i try and squeeze two wires into the faceplate earth terminal it could compromise the holding power of the terminal and if the earth ended up detaching.. not good. While i know it is best practice to connect them, am i right that it's still safe?

My non-contact voltage pen buzzed like crazy with the plastic wall mounted unit. Now it doesn't make a sound against the metal faceplate when hot. Is this normal?

I'm not sure how flat i got the backbox. I tried my best and everything seemed to screw together without issue. Should i be concerned at all?

The faceplate cover moves... i am wondering if it's because i left some wallpaper under one end and it's made it uneven. See video:

- Concerning?

Anything i've missed?

Some Pics:

Lovely crumbly brick made the job easier. Ended up using a screwdriver and a hammer.

I noticed the L1 wire is slightly protruding. Will sort that tomorrow.

Finished Product. Yes i need to tidy up the wall around it:

What you reckon guys.. Successful first job, or accident waiting to happen?

There are two ways to test the earth, one uses a low ohm meter and is done dead, the meter must use at least 200 mA for the test, but you need to know the earth loop impedance at the consumer unit to work it out.

The other uses a loop impedance meter, and is done live.

The impedance (called resistance with DC) has to be low enough to work the overload within a set time, a B type MCB will trip on the magnetic part of the trip in the time allowed if the current exceeds 5 times rated current plus 5% so with a 6 amp MCB that is 230/30x95% = 7.28Ω the problem is these meters are expensive, and any other method will not give you the reading to filling in the test certificate.

View attachment 275350 It is unlikely many DIY people bother with the Zs reading or the paperwork, they just trust to the lord, in real terms if doing DIY no option.

I had a look at those meters.. yes very expensive lol best start saving up. Thank you for the info - So it would seem if i want it properly tested i will need to call an electrician. Will ring around tomorrow and see how much they will charge. I want to learn all the above though so i can eventually buy the equipment and do it myself. One step at a time lol

There seems to be a lot going on here, so trying to keep things simple...



Are you sure only half of the consumer unit is RCD protected, can you post a picture of it and we may be able to confirm!

You currently have a surface mounted switch and back box.



Personally, I would go for a 25mm back box (if you are happy to drill a bit further into the wall). It will give you more space to play with to get the wires to fold back into the box neatly.
Where you knock out a hole for the wire to enter the back box, you will need to use a rubber grommet to protect the cable.
Switches are usually supplied with new screws - you dont want nasty chewed up screws spoiling the look of your new switch!
There won't be any need to cut screws to size.



You don't need to work on any live wires to check the earth continuity. Just use the continuity setting on the multimeter, connect one probe to the earth in your rewired switch, and the other to ideally the main earth terminal of the supply (probably next to the CU). If you can't achieve that, try the earth of a nearby socket - hopefully the multimeter will beep! A resistance measurement can also be taken.

Lastly, if it really worries you - stick with a plastic switch on the metal back box. It won't give you sleepless nights and you would have achieved something that will give you confidence.
...or consider having an installation inspection by a qualified electrician at some point soon!

That's our CU and then we have the earthing thing and the meter. Do you know, is it possible / easy for an electrician to switch the light switch to the RCD side?

Forgive my ignorance but where about is the main earth terminal of the supply and would it be okay to run a 1.5mm2 earth wire from there to the multimeter probe? Mine doesn't beep for some reason so do i just look for a value less than 1?

I did go for the 25mm back box in the end, wires weren't too difficult to get in although it worries me about the pressure on them and them coming loose.

Advice if you must work live is - Wear insulating footwear, make sure arms and legs are covered, plus work with only one hand - hopefully dry and not sweaty. Keep the other in your pocket. Idea is to prevent current flowing through your body, especially from one arm, to the other - because your heart is in the middle. A shock across your one exposed hand, is less of an issue, it just hurts.

Solid advice, thank you.

I find this all very strange, lots of folk use metal faced light switches, they are everywhere these days and perfectly safe unless seriously interfered, which the average child would be incapable of unless he were to set about using a hammer with all of his might. It's true that "only half" of modern consumer units are protected with an RCD - the power and cooker circuits which can carry a heavy load due to the thickness of the cables used but all circuits (including those for lighting) are never the less protected by MCBs these days which switch off instantly if there is a problem and are far safer than relying on just an earth wire. Only twice in the past 20 years have I seen old style units protected by fuse wire although I guess there may still be a few about. I was taught at tech college that the speed of current cut by an MCB was such that it would not be able to kill a two year old. Think about it, if metal light switches were at all dodgy, they would not sell very many would they?

I mean, yeah they are safe.. unless the live comes loose and the earth isn't working / also came loose.

Anyway, i think that's everything for now lol. Thank you all for your advice, it's been very useful

 

[JohnW2]

It's true that "only half" of modern consumer units are protected with an RCD - the power and cooker circuits which can carry a heavy load due to the thickness of the cables used but all circuits (including those for lighting) are never the less protected by MCBs these days which switch off instantly if there is a problem and are far safer than relying on just an earth wire. ...

I'm afraid that there is quite a lot of (potentially misleading) nonsense in the above ...

It's true that "only half" of modern consumer units are protected with an RCD - ...

I suppose that it depends to some extent on what you mean by 'modern', but it is now many years since such an arrangement ceased to be the norm (for new CUs) and, many people would argue that it's also many years since they were compliant with current regulations (albeit there is no obligation to bring existing installations up to current standards).

It's true that "only half" of modern consumer units are protected with an RCD - the power and cooker circuits which can carry a heavy load due to the thickness of the cables used ...

Neither the size of the load nor the thickness of the cables has got anything to do with whether or not a circuit does (or should0 have RCD protection - and, in any event, those circuits do not carry a heavy load "because of the thickness of the cable" but, rather, because the items connected to the circuit may draw a heavy current (which is why 'thick' cables {at least, thicker than those used for lighting circuits} may be needed.

.... but all circuits (including those for lighting) are never the less protected by MCBs these days which switch off instantly if there is a problem and are far safer than relying on just an earth wire. ...

If there is no, or an inadequate 'earth connection', then an MCB will not do anything to protect a person if, for some reason, the face of a metal switch becomes live. Furthermore, there are plenty of other situations in which an RCD might protect people from the potentially serious consequences of an electric shock, whereas an MCB would do absolutely nothing to protect them.

Kind Regards, John
Edit: crucial missing word added. Apologies.

 

[flameport]

Do you know, is it possible / easy for an electrician to switch the light switch to the RCD side?

Nothing can be added to the RCD side in that consumer unit.
It may be possible to obtain an RCBO to replace the MCB for the lighting circuit, but given it's age and the small number of circuits any money spent would be far better put towards replacing the whole consumer unit.
The label suggests it's more than a decade overdue for an inspection.
If your smoke alarms are of a similar age, they also require replacement.

Multimeters will often show some voltage on disconnected wires due to the proximity of other wires which are connected.
Non contact indicators are unreliable in many situations and should not be relied on for anything.

 

[RandomGrinch]

That's our CU and then we have the earthing thing and the meter. Do you know, is it possible / easy for an electrician to switch the light switch to the RCD side?

Flameport has said above, far more than I could ever tell you!

I did go for the 25mm back box in the end, wires weren't too difficult to get in although it worries me about the pressure on them and them coming loose.

Others may disagree, but I have found adding a couple of bends to the wire beforehand (like a concertina), allows the switch to sit back neatly with out to much 'pressure' on the terminals.

Forgive my ignorance but where about is the main earth terminal of the supply and would it be okay to run a 1.5mm2 earth wire from there to the multimeter probe? Mine doesn't beep for some reason so do i just look for a value less than 1?

As ericmark has said, without a full electricians toolkit, as 'amateurs', we are limited in our approach to testing.
There have been plenty of discussions on here about the pitfalls of using a 'voltage pen' - but you have a multimeter, that's all you need for now
Connecting one probe of your multimeter to any of those earth points (...and the other probe to your switch's earth), or as I said earlier, the earth of a nearby socket - should give an indication of earth continuity. And again, you can always take a resistance measurement with your meter to confirm.
Though I am surprised that your meter doesn't beep in continuity mode, what model is it?

I did not run an earth wire from the faceplate earth to the backbox.

Yes, still safe! As a belt and braces measure, some will include the wire to the back box - it's nice to have, but not strictly necessary. You may find a few discussions on this matter if you do a search of this forum!

my thinking was, if i try and squeeze two wires into the faceplate earth terminal it could compromise the holding power of the terminal

In the case of a socket on a ring, with a spur, there can be three wires in a terminal (plus one more if you want an earth fly lead to the back box!).
This isnt something to overly worry about

I'm not sure how flat i got the backbox. I tried my best and everything seemed to screw together without issue. Should i be concerned at all?

Nope, as long as the edges of the box are below the surface and the box is relatively level, it should be good!

All in all, a good start, well done!
Now on to bigger and better projects

 

[JohnW2]

Nothing can be added to the RCD side in that consumer unit.

Are you saying that just because it is an 80A RCD (given that there appear to be a couple of 'spare' ways available on the 'RCD side'), or whasst/why?

Kind Regards, John

 

[boringoldcodger]

Deleted as no longer relevant.

 

[JohnW2]

I believe John meant "will not do anything".

Whoops! Indeed I did. Now corrected - many thanks for noticing. My apologies.

In effecting that correction, I've also just noticed this ....

I was taught at tech college that the speed of current cut by an MCB was such that it would not be able to kill a two year old.

... which, of course, takes us back to my comment which I have now corrected - namely that an MCB does not necessarily ever trip in response to a faceplate becomes 'live', particularly if the earthing of the faceplate is absent or 'inadequate' (but, in some cases even if there is an adequate earth connection.

Kind Regards, John

 

[JohnW2]

Flameport has said above, far more than I could ever tell you!

He has, but he hasn't yet responded to my asking him why he wrote "Nothing can be added to the RCD side in that consumer unit.". As I implied, given that there appear to be two spare 'spaces', the only reason I can think of is that the RCD is rated at 80A - but, if that is his reason, I would personally regard that as rather 'pedantic'.

What did you think was the reasoning behind flameport's statement?

Kind Regards, John

 

[RandomGrinch]

What did you think was the reasoning behind flameport's statement?

I'm afraid CU's aren't in my skillset
I had noted the spare ways and that Legrand switches are still available, so I thought the circuit could be moved.
...and then I saw Flameports post and decided to defer to the expert!

 

[JohnW2]

I'm afraid CU's aren't in my skillset I had noted the spare ways and that Legrand switches are still available, so I thought the circuit could be moved. ...and then I saw Flameports post and decided to defer to the expert!

I'm no official expert on CUs (or anything else electrical, come to that).

However, given the existence of the spare ways, the only thing I could think of which might have 'explained' his comment was the fact that the RCD was rated at 80A. However, as I also said, if that were his reason then I would regard it as pretty 'pedantic', since there must be millions of CUs out there where an 80A (or even 63A) RCD is protecting circuits whose MCBs' rating add up to ore than 80A (or 63A), and I really could not get excited about 'the problem' of adding an extra 6A MCB to an RCD which was currently protecting two (32A) sockets circuits and one (40A) shower circuit.

Hopefully he will clarify his thinking before too long!

Kind Regards, John

 

[flameport]

Are you saying that just because it is an 80A RCD (given that there appear to be a couple of 'spare' ways available on the 'RCD side'), or whasst/why?

80A could be one of the reasons, as the MCBs total more than 80A. In this example the supplier fuse appears to be 60A so that may not be a problem if the fuseholder label is correct. However if it is 60A, then that 40A shower really should not have been installed.

Others are that it's a type AC RCD, which can't be used for anything other than resistive loads
moving the lighting there will result in most of the installation on a single RCD, see division of installation
and the 9mA leakage current per RCD is likely to be exceeded, and may already be.

For any of those, shoving the lighting in with the rest is making a mediocre installation worse.

If RCD protection is wanted for the lighting, it's new consumer unit time.
Or just leave as is, replacing a light switch does not require that anything else for the circuit is changed.