What's the difference?

[thomp1983]

Hi all,

Today I've been fitting some new metal sockets and light switches today, in our lounge the back boxes are a combination of plastic dry line and metal ones.

For the sockets the earth goes into the faceplate and if fitted to a metal back box a fly lead is fitted from the earth to the earth point in the box, dry line boxes the earth's just go in the faceplate.

For the light switches the earth goes into the lug on the metal back box and then a fly lead goes from there to the earth point on the light switch, in the dry line boxes the earth just goes to the faceplate.

Why is a fly lead needed between a metal backbox and the faceplate but not in a dry lining box? If a fault occurred and the faceplate became live it would surely be irrelevant if the back box in the wall is live or not?

I understand it for the light switch as the earth goes to the lug on the back box so the fly lead is needed to provide the earth path from the faceplate but for the sockets the earth is already connected to the faceplate?

Thanks
Chris

 

[davelx]

There is no difference.

A dry lining box is made of plastic - there is nothing to earth, so no link required for sockets or light switches.

For the light switches the earth goes into the lug on the metal back box and then a fly lead goes from there to the earth point on the light switch, in the dry line boxes the earth just goes to the faceplate.

The earth on light switches should be connected the same way as the earth on sockets when there is a metal back-box, to the switch/socket faceplate, and then a fly lead to the metal back box.

 

[SUNRAY]

Hi all,

Today I've been fitting some new metal sockets and light switches today, in our lounge the back boxes are a combination of plastic dry line and metal ones.

For the sockets the earth goes into the faceplate and if fitted to a metal back box a fly lead is fitted from the earth to the earth point in the box, dry line boxes the earth's just go in the faceplate.

For the light switches the earth goes into the lug on the metal back box and then a fly lead goes from there to the earth point on the light switch, in the dry line boxes the earth just goes to the faceplate.

Why is a fly lead needed between a metal backbox and the faceplate but not in a dry lining box? If a fault occurred and the faceplate became live it would surely be irrelevant if the back box in the wall is live or not?

I understand it for the light switch as the earth goes to the lug on the back box so the fly lead is needed to provide the earth path from the faceplate but for the sockets the earth is already connected to the faceplate?

Thanks
Chris

As long as the metal front plates and metal back boxes are adequately earthed it doesn't really matter too much.

I prefer to run the earth to the metal front first to remove one point of failure but that's just me being me.

 

[thomp1983]

What I was trying to ask is why do we need the fly lead? Surely if a fault makes the face plate live its irrelevant if the back box is live or not?

I can't imagine a likely fault that would make the back box live first which would then make the faceplate live

Chris

 

[SUNRAY]

What I was trying to ask is why do we need the fly lead? Surely if a fault makes the face plate live its irrelevant if the back box is live or not?

I can't imagine a likely fault that would make the back box live first which would then make the faceplate live

Chris

Look at it the other way and assume the front is correctly earthed but some fault (Let's not get into any discussions about what the faultis or could be) makes the unearthed back box live.

Now the thing is the 2 screws holding the plate in place are connected to the back box and now there are live parts. Let's not also get involved with discussions about the screws being in contact with the metal front plate.

Yes I know it sounds like clutching at straws but it will have happened to someone somewhere and adding the strap will stop any potential problem.

 

[ericmark]

With the isolate else where rule there should be no need to earth back box as the 3.5 mm brass screw does that, it is only when an error is made and the switch is removed when still live that there is a problem, but that does happen a lot, I have thought the socket or switch is dead, and wrong label or other has resulted in it still being live. In one case it was connected to next doors supply, so even with the main incoming isolator off it was still live.

We all tend to consider our own house or where we work, and forget other options, where the back box can be touched with socket or switch in place, there are still some homes with surface wiring, really it is safer, unlikely to hit a wire drilling if all are surface, but the regulations try to cover all situations, including those which change.

It is easy to look at what is in your hand, and forget the rest, my odd metal faced switch is still class II so no earth to switch, same with light fittings, and the metal light fitting with plastic pipes inside each metal tube giving double insulated looks for outside the same as the model without the tube, and until 1966 we did not run earths to lights.

So safe method is to earth when ever you can, so you don't miss it when required.

 

[JohnW2]

Why is a fly lead needed between a metal backbox and the faceplate but not in a dry lining box? If a fault occurred and the faceplate became live it would surely be irrelevant if the back box in the wall is live or not?

A dry lining box is plastic, and therefore cannot 'become live' - or, put another way, there's nothing (conductive) in a (plastic) dry lining box to which one could connect a fly lead.

What I was trying to ask is why do we need the fly lead? Surely if a fault makes the face plate live its irrelevant if the back box is live or not? I can't imagine a likely fault that would make the back box live first which would then make the faceplate live

It's probably 'more likely a fault' than for the primarily problem being the faceplate becoming live - if a live conductor 'comes lose', it probably more likely that it will come in contact with the back box than the faceplate. However (whilst the faceplate is present) if the faceplate is earthed (but with no fly lead to box) then, if the box becomes live, it's electrical contact with the back box (by direct contact and/or the screws), should cause the circuit's breaker to trip (or fuse to blow).

As far as I can see, the only real issue/danger exists if/when some very daft person gets into a situation in which the faceplate screws have been removed and the faceplate pulled forward AND the circuit is energised (still 'live') AND the back box is live due to a fault - in which case they could simultaneously touch a life backbox and earthed faceplace. However, as you imply, that seems to be an iuncredibly 'unlikley' scenario.

Kind Regards, John

 

[JohnW2]

So safe method is to earth when ever you can, so you don't miss it when required.

That's not really the ideal (safest) approach.

What one should do is ascertain whether something 'metal and touchable' needs to be earthed and only earth it if it is needed. To 'unnecessarily earth' pieces of touchable metal within a home (or wherever) actually slightly increases the risk of electric shocks.

'Earthing' touchable metal is, in some situations, essentially a 'necessary evil' but should really be avoided when it is not 'necessary'.

Kind Regards, John

 

[winston1]

A flylead is not required if at least one of the backbox lugs is fixed (which is almost always the case).

 

[333rocky333]

A flylead is not required if at least one of the backbox lugs is fixed (which is almost always the case).

Do you have anything to support that claim.

Flyleads only appear to be mentioned in Reg 543.2.7

 

[JohnW2]

A flylead is not required if at least one of the backbox lugs is fixed (which is almost always the case).

I'm not sure in what sense you intend "required" (i.e. "required by what?") since, as Rocky has said, BS7671 appears to say nothing about them ever being required, other than in the situation (per 543.2.7) in which the circuit does not have a CPC in the form of a dedicated 'wire' (i.e. when the CPC is provided by metal conduit, trunking or ducting or the metal sheath and/or armour of a cable). That is a pretty unusual system in domestic installations these days - and in that one case (and contrary to your suggestion) it does not say that the fly lead may be omitted if the accessory is attached by a screw to a fixed back box lug.

So, BS7671 does not, in general, require there ever to be a fly lead, except in that one situation, in which it does (always) require a fly lead, even if there is a fixed lug in the back box.

 

[EFLImpudence]

What I was trying to ask is why do we need the fly lead? Surely if a fault makes the face plate live its irrelevant if the back box is live or not?

Strictly speaking you are correct.

The back box is not an exposed-conductive-part but people like to earth it and be extra safe but the circuit should not be live when the faceplate is removed, so...

 

[Harry Bloomfield]

What I was trying to ask is why do we need the fly lead? Surely if a fault makes the face plate live its irrelevant if the back box is live or not?

I can't imagine a likely fault that would make the back box live first which would then make the faceplate live

Chris

If you pulled the faceplate forward, the faulty insulation on the cables, nicked insulation or similar - might make contact with the box. Everyone knows you should never investigate such with the power on, but sometimes the work needs the power to be on.

 

[JohnW2]

If you pulled the faceplate forward, the faulty insulation on the cables, nicked insulation or similar - might make contact with the box. Everyone knows you should never investigate such with the power on, but sometimes the work needs the power to be on.

Indeed - but, as I wrote:

.... As far as I can see, the only real issue/danger exists if/when some very daft person gets into a situation in which the faceplate screws have been removed and the faceplate pulled forward AND the circuit is energised (still 'live') AND the back box is live due to a fault - in which case they could simultaneously touch a life backbox and earthed faceplace. However, as you imply, that seems to be an incredibly 'unlikley' scenario.

Although, as you say, "sometimes the work needs the power to be on", I would personally say that the most prudent approach (which I would probably practise myself) would be to de-energise the circuit before "removing the screws and pulling the faceplate forward" and then to visually inspect the conductors/connections ('nicked insulation', conductors pulled out of terminals etc.) before re-energising the circuit and doing whatever has to be done "with the power on".

Kind Regards, John

 

[Harry Bloomfield]

Indeed - but, as I wrote:
Although, as you say, "sometimes the work needs the power to be on", I would personally say that the most prudent approach (which I would probably practise myself) would be to de-energise the circuit before "removing the screws and pulling the faceplate forward" and then to visually inspect the conductors/connections ('nicked insulation', conductors pulled out of terminals etc.) before re-energising the circuit and doing whatever has to be done "with the power on".

Kind Regards, John

Not every one is prudent. When they make something fool proof, they invent a better fool