Earthing of metal boxes

[Jupiter01]

Hi All,

When replacing a plastic socket which sits in a metal box, do I need to connect (and sleeve) and earth cable from the socket to the earth terminal on the metal box (aka flylead I believe)? This would be in addition to the two earth cables that are already connected into the earth terminals of my socket. Their appears to be different interpretations on the need for this.

Thanks in advance

 

[Jackrae]

yes

 

[John D v2.0]

Putting the regs aside, the way I see it is in normal use the bolts will earth the box and themselves via the socket.
However the most likely time for there to be a short to the box would be when tightening down if the cable is trapped (I've seen a burnt cable and screw in my house) or when the cable are disturbed when the bolts are out.

 

[JohnW2]

Putting the regs aside, the way I see it is in normal use the bolts will earth the box and themselves via the socket. However the most likely time for there to be a short to the box would be when tightening down if the cable is trapped (I've seen a burnt cable and screw in my house) or when the cable are disturbed when the bolts are out.

Indeed - that's why I personally don't really like relying on the faceplate screws - but, as the OP said, opinions vary.

Kind Regards, John

 

[plugwash]

When replacing a plastic socket which sits in a metal box, do I need to connect (and sleeve) and earth cable from the socket to the earth terminal on the metal box (aka flylead I believe)? This would be in addition to the two earth cables that are already connected into the earth terminals of my socket. Their appears to be different interpretations on the need for this.

AIUI If the box has at least one fixed lug (most do) and the socket has earthing bars to the screwholes (most do) then the fixing screws are considered adequate earthing for the box.

AIUI If both lugs on the box are adjustable or the socket doesn't have earthing bars to the screwholes or an earth link between box and socket is required for purposes other than merely earthing the box (for example MICC or steel conduit installations without dedicated earth conductors) then the flylead is needed.

 

[JohnW2]

AIUI If the box has at least one fixed lug (most do) and the socket has earthing bars to the screwholes (most do) then the fixing screws are considered adequate earthing for the box.

As you say, that is "considered adequate", and probably is (adequate) when the socket is installed. However, as has been said, the greatest potential risk exists when the faceplate screws are removed and the socket pulled forward (albeit that should not normally be done with the circuit 'live'!) - which is one of the reasons why, for the sake of a couple of minutes to install a flylead, I personally do not favour relying on the screws/lugs.

Kind Regards, John

 

[Jupiter01]

Thanks guys. I will fit a flylead.

 

[Iggifer]

It's not required, so long as one of the lugs is fixed

 

[Jackrae]

Why would you not want to fit it - on the basis that it isn't compulsory ? ; even though it makes sense since you are providing a direct fault path rather than relying on a possible path.

 

[JohnW2]

It's not required, so long as one of the lugs is fixed

Yes, I think we are all agreed that is is not "required" in such a situation. However, in addition to what I've already said (about the situation when the faceplate is removed, with circuit still live), if the screws are not tightened enough to achieve good electrical contact between box lug and the socket's earth, and if (very bad luck!) there is coincidentally a short between L and the box, then the screws could become 'live' - so, as I've said, I'm personally more comfortable with a flylead.

Kind Regards, John

 

[Iggifer]

I think in that very unlikely scenario, you also run the chance of the fly lead popping out. Some of the knockout boxes I've seen, I'd rely more on the earth through the pins than the lug. I have had many just twist off as I've been tightening the screw.

And obviously, if you're taking a socket off live without knowing the state of what's behind it, you deserve to get a belt.

 

[JohnW2]

I think in that very unlikely scenario, you also run the chance of the fly lead popping out.

Possibly - although, I'd say, even more unlikley, provided the flylead was of a sensible length. What we must always remember is that so many of the things we do, either by virtue of perceived "good practice" or regulation, are designed to guard against incredibly rare eventualities - i.e. 99.99% of the time, one would 'get away' with not doing them!

Some of the knockout boxes I've seen, I'd rely more on the earth through the pins than the lug. I have had many just twist off as I've been tightening the screw.

What "pins" and what "lug" - does the latter perhaps relate to the earth terminal in box?

And obviously, if you're taking a socket off live without knowing the state of what's behind it, you deserve to get a belt.

Indeed - but, as above, many practices/regulations seems to exist because of the possibility of people doing silly things.

Kind Regards, John

 

[Iggifer]

Sorry:

Pin = M3.5 socket screw
Lug = earth terminal in knockout

 

[JohnW2]

Sorry: Pin = M3.5 socket screw Lug = earth terminal in knockout

Thanks - as I implied, that's what I rather suspected (although I'm not sure I would call it a "socket screw"!).

I agree that the earth terminals in backboxes have been known to 'twist off', or at least loosen (giving rise to uncertainties about the quality of the electrical connection). However, it's not a case of "either/or" - a flylead to that terminal will usually just be additional 'belt and braces' since (assuming at least one fixed lug on the box and appropriate configuration of the socket), there will still be an earth connection to the box via the faceplate screws (which connection you have said that you are happy to rely upon).

Kind Regards, John

 

[Echo the husky]

I've never thought about it but we call them socket screws too!