Metal light switches

[autolycus]

Hi,

I am just in the process of replacing all the metal (i.e. brass and chrome effect) light switches in our house with plastic ones because most of our five lighting rings do not have an earth.

I was also planning to replace the metal patress screws with nylon ones.

Two questions for the experts please:

1. All of the back boxes are flush mount ones - i.e. the back box is buried in the wall. In most cases the walls are brick as only a couple of our internal walls are plasterboard. I thought back boxes in brick walls were always netal, but a number of ours are plastic. Am I right in saying that where I have plastic back boxes I can get away with using metal screws on the faceplates instead of nylon ones?

2. I had assumed that the existing metal faceplates were electrically conductive. But when I put a continuity tester across them it appears as if they are not after all. Does this mean they have some kind of plastic film coating on them and, if so, does that mean I could get away without replacing them with plastic faceplates?

Before I get shouted at, I will be replacing them anyway, because I hate brass switches with fancy edging. question 2 is being asked purely for interest's sake.

Cheers,
Dave.

 

[luminaire]

Hi Dave
If you are replacing the metal switches with plastic ones, that's fine. I wouldn't worry too much about replacing the back boxes as well. Nylon screws are a good idea though.
Where you have replaced the knock-out boxes with dry lining boxes, metal screws will be ok as well.
Just make sure you haven't any metal lamp fittings.

 

[333rocky333]

2. I had assumed that the existing metal faceplates were electrically conductive. But when I put a continuity tester across them it appears as if they are not after all. Does this mean they have some kind of plastic film coating on them and, if so, does that mean I could get away without replacing them with plastic faceplates?

They should have, or still need to be earthed , and if there is fitted on the back an earth terminal this would confirm this, some are coated to prevent discolouring
Try continuity on the back or between the two screw holes, rather than the face edge

 

[autolycus]

Where you have replaced the knock-out boxes with dry lining boxes, metal screws will be ok as well.
Just make sure you haven't any metal lamp fittings.

I haven't replaced any of the KO boxes with dry-lining ones myself. The plastic boxes that we do have are some kind of old-fashioned looking plastic things - not the modern day things designed for plasterboard walls. With that in mind, does your advice about metal screws being okj still hold?

Re the metal lamp fittings - I know I'll get shouted at for this, but the house is full of them. Most of them are awful looking and will be replaced once I have sorted out the switches.

The two I wish to retain are sufficiently high up to mean they will not be touched by accident. Even if they are touched by accident during a fault condition, they are RCD protected so from what I understand this is not actually too bad (even though it is not in accordance with the regs).

At some point when time and funds allow, a full rewire will no doubt be a good idea, but at the moment I am simply trying to do away with the most dangerous bits such as the wall switches.

 

[autolycus]

2. I had assumed that the existing metal faceplates were electrically conductive. But when I put a continuity tester across them it appears as if they are not after all. Does this mean they have some kind of plastic film coating on them and, if so, does that mean I could get away without replacing them with plastic faceplates?

They should have, or still need to be earthed , and if there is fitted on the back an earth terminal this would confirm this, some are coated to prevent discolouring
Try continuity on the back or between the two screw holes, rather than the face edge

Yes, there is continuity between the screw holes and on the back. And yes there is an earth terminal.

But if there is no continuity across the face edge, then where is the danger of a shock?

 

[333rocky333]

But if there is no continuity across the face edge, then where is the danger of a shock?

Proberly minimal.
The earth is more for if the plate became live under fault, the fault current would return via earth and disconnect the protective device.

Although not a regular fault it is not unknown for the switch to short to the plate

Without the earth the plate could just sit there live and if the wall was damp could even liven that up

The coating was proberly not added for electric insulation.
And is only a fraction of a millimeter thick.
Not that you would , but if you were going to touch something that could be live, but proberly 99.9% or more likely not to be , would you wear Thick rubber gloves or cheap thin free ones from the petrol station

Although it does withstand your tester, it may not hold off a possible fault current of well over 100 amp at 230 volt through your body to earth.
This is why insulation is tested usually with 500volt testers to break down that resistance to give an accurate reading

To be fair many people have still got wiring like yours, but at least you are temporary sorting it for now, whereas many choose to ignore it

 

[RF Lighting]

a possible fault current of well over 100 amp at 230 volt through your body to earth.

Hmm I'm not sure that is factually accurate.

 

[333rocky333]

Hmm I'm not sure that is factually accurate.

Do explain please

 

[RF Lighting]

Due to the resistance of the human body combined with your shoes, and whatever you are stood on, a fault current of 100mA would be much more realistic.


Your body would need to be properly earthed, and have a resistance of 2.3 ohms to allow 100A to flow in the event of direct contact.

If your body had a resistance of 2.3 ohms then touching both ends of an AA battery would deliver a potentially fatal sock of 652mA.



*these are estimated figures, as I got bored of trying to IR test myself

 

[333rocky333]

Yes.
I somehow knew that was the reply.
Your spot on as always,

But ignoring the resistance of the body

The potential current available to the premises is 1000's of amps , and to blow the 5amp fuse if fitted which may not even be 5 amp , it could easily generate in excess of 100 amp in seconds possibly before it disconnects.
As you say your body will limit this current ,but I dont think anyone has tried it in real life.
Less than 1 amp can kill some people as you know
(Is it ok for me to replace the batteries in my tv remote then)
Is a fatal SOCK bad or does it just smell
Do you have a pic

 

[RF Lighting]

The potential current available to the premises is 1000's of amps , and to blow the 5amp fuse if fitted which may not even be 5 amp , it could easily generate in excess of 100 amp in seconds possibly before it disconnects.

Yes, its called the prospective short circuit current (Ipsc), and is required as one of the supply characteristics on an Electrical Installation Certificate.


I bet this supply would go with a bang

 

[daytona]

 

[333rocky333]

Yes, its called the prospective short circuit current (Ipsc)

RF, I never doubted you knew , it was more for the OP's benefit, people underestimate the potential danger sometimes and too rely on a thin plastic coating would not be in his best interest ,I would not have thought.

 

[ban-all-sheds]

But ignoring the resistance of the body

But you can't - the resistance of the human body, and its inevitable current limiting effect is why we get the 50V touch limit, and why we've decided that a CT 110V supply is safe for building sites etc.

Less than 1 amp can kill some people as you know

1A will kill anybody.