Discussion:
Earthing a metal backbox
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Grumps
2021-09-14 22:14:05 UTC
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In my house I have a few metal backboxes used for light switches that
don't have an earth terminal.
Is there a best and approved way of making an earth connection without
having to replace the box with a modern one?
E.g. drill and tap a hole and secure the CPC with a screw/bolt.
Ta.
nightjar
2021-09-14 22:39:28 UTC
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Post by Grumps
In my house I have a few metal backboxes used for light switches that
don't have an earth terminal.
Is there a best and approved way of making an earth connection without
having to replace the box with a modern one?
E.g. drill and tap a hole and secure the CPC with a screw/bolt.
Ta.
If the back box has fixed lugs for the screw holes and the screw holes
in the switch are earthed, that is an adequate system of earthing.
Adjustable lugs cannot be relied upon to make a safe connection though.
--
Colin Bignell
alan_m
2021-09-14 23:16:11 UTC
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Post by nightjar
Post by Grumps
In my house I have a few metal backboxes used for light switches that
don't have an earth terminal.
Is there a best and approved way of making an earth connection without
having to replace the box with a modern one?
E.g. drill and tap a hole and secure the CPC with a screw/bolt.
Ta.
If the back box has fixed lugs for the screw holes and the screw holes
in the switch are earthed, that is an adequate system of earthing.
Adjustable lugs cannot be relied upon to make a safe connection though.
back boxes tend to have one adjustable and one fixed lug.
--
mailto : news {at} admac {dot} myzen {dot} co {dot} uk
nightjar
2021-09-15 00:44:09 UTC
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Post by alan_m
Post by nightjar
Post by Grumps
In my house I have a few metal backboxes used for light switches that
don't have an earth terminal.
Is there a best and approved way of making an earth connection
without having to replace the box with a modern one?
E.g. drill and tap a hole and secure the CPC with a screw/bolt.
Ta.
If the back box has fixed lugs for the screw holes and the screw holes
in the switch are earthed, that is an adequate system of earthing.
Adjustable lugs cannot be relied upon to make a safe connection though.
back boxes tend to have one adjustable and one fixed lug.
Modern ones do. None of the ones in my house do. However, so long as you
have one fixed lug and the hole in the faceplate the screw to that goes
through is earthed, a separate earth is merely recommended.
--
Colin Bignell
Grumps
2021-09-15 06:04:04 UTC
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Post by nightjar
Post by alan_m
Post by nightjar
Post by Grumps
In my house I have a few metal backboxes used for light switches
that don't have an earth terminal.
Is there a best and approved way of making an earth connection
without having to replace the box with a modern one?
E.g. drill and tap a hole and secure the CPC with a screw/bolt.
Ta.
If the back box has fixed lugs for the screw holes and the screw
holes in the switch are earthed, that is an adequate system of
earthing. Adjustable lugs cannot be relied upon to make a safe
connection though.
back boxes tend to have one adjustable and one fixed lug.
Modern ones do. None of the ones in my house do. However, so long as you
have one fixed lug and the hole in the faceplate the screw to that goes
through is earthed, a separate earth is merely recommended.
Thanks.
It's probably my lack of knowledge about metal faceplate light switches.
Do they have an earth contact that has the CPC of the T+E connected
directly to it, or does it rely on earthing through the mounting screws
to the earthed backbox?
nightjar
2021-09-15 07:59:40 UTC
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Post by Grumps
Post by nightjar
Post by alan_m
Post by nightjar
Post by Grumps
In my house I have a few metal backboxes used for light switches
that don't have an earth terminal.
Is there a best and approved way of making an earth connection
without having to replace the box with a modern one?
E.g. drill and tap a hole and secure the CPC with a screw/bolt.
Ta.
If the back box has fixed lugs for the screw holes and the screw
holes in the switch are earthed, that is an adequate system of
earthing. Adjustable lugs cannot be relied upon to make a safe
connection though.
back boxes tend to have one adjustable and one fixed lug.
Modern ones do. None of the ones in my house do. However, so long as
you have one fixed lug and the hole in the faceplate the screw to that
goes through is earthed, a separate earth is merely recommended.
Thanks.
It's probably my lack of knowledge about metal faceplate light switches.
Do they have an earth contact that has the CPC of the T+E connected
directly to it, or does it rely on earthing through the mounting screws
to the earthed backbox?
The only metal faceplates I have used were industrial ones and those
were on surface mounted boxes with an earth terminal. However, the
plastic switches I use at home have the earth connected to metal plates
on the back of the switch that carry it to the screw holes. They also
have metal grommets in the hole, to ensure that the screw connects to
the earth.
--
Colin Bignell
Theo
2021-09-15 11:49:04 UTC
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Post by nightjar
The only metal faceplates I have used were industrial ones and those
were on surface mounted boxes with an earth terminal. However, the
plastic switches I use at home have the earth connected to metal plates
on the back of the switch that carry it to the screw holes. They also
have metal grommets in the hole, to ensure that the screw connects to
the earth.
I installed one of Toolstation's finest ('Axiom') white light switches
yesterday. No earth terminal on the switch at all, I had to screw the CPC
into the backbox (which was plastic and had an earth terminal that went
nowhere). So those screws weren't earthed.

Theo
ARW
2021-09-15 17:24:09 UTC
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Post by nightjar
The only metal faceplates I have used were industrial ones and those
were on surface mounted boxes with an earth terminal. However, the
plastic switches I use at home have the earth connected to metal plates
on the back of the switch that carry it to the screw holes. They also
have metal grommets in the hole, to ensure that the screw connects to
the earth.
I have never seen that on plastic light switches. Sure on FCUs, 20A DP
switches etc.

Are you sure?

--

Adam
nightjar
2021-09-16 07:40:14 UTC
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Post by ARW
Post by nightjar
The only metal faceplates I have used were industrial ones and those
were on surface mounted boxes with an earth terminal. However, the
plastic switches I use at home have the earth connected to metal
plates on the back of the switch that carry it to the screw holes.
They also have metal grommets in the hole, to ensure that the screw
connects to the earth.
I have never seen that on plastic light switches. Sure on FCUs, 20A DP
switches etc.
Are you sure?
They were installed over 25 years ago and I've not looked at the back of
them since. I prefer not to disturb wiring if I don't have to. However,
loosening one screw does show a metal grommet behind it, so I'm probably
sure :-).
--
Colin Bignell
John Rumm
2021-09-15 12:24:58 UTC
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Post by Grumps
Post by nightjar
Post by alan_m
Post by nightjar
Post by Grumps
In my house I have a few metal backboxes used for light switches
that don't have an earth terminal.
Is there a best and approved way of making an earth connection
without having to replace the box with a modern one?
E.g. drill and tap a hole and secure the CPC with a screw/bolt.
Ta.
If the back box has fixed lugs for the screw holes and the screw
holes in the switch are earthed, that is an adequate system of
earthing. Adjustable lugs cannot be relied upon to make a safe
connection though.
back boxes tend to have one adjustable and one fixed lug.
Modern ones do. None of the ones in my house do. However, so long as
you have one fixed lug and the hole in the faceplate the screw to that
goes through is earthed, a separate earth is merely recommended.
Thanks.
It's probably my lack of knowledge about metal faceplate light switches.
Do they have an earth contact that has the CPC of the T+E connected
directly to it, or does it rely on earthing through the mounting screws
to the earthed backbox?
Although it's not unknown to have the backbox earthed via contact with
the accessory via the screws, it's not at all usual for the faceplate to
rely on contact with an earthed backbox to provide it's earthing.

Many plastic light switches won't provide any termination for an earth
wire at all.

Metal ones that are Class I will do, and those will include the annulus
of the screw holes as normal.

Note however there are a number of Class II (i.e. double insulated)
metal clad switches that do not require earthing. These may lack
continuity between screw hole and backbox.
--
Cheers,

John.

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Sysadmin
2021-09-15 04:05:11 UTC
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Post by Grumps
In my house I have a few metal backboxes used for light switches that
don't have an earth terminal.
Is there a best and approved way of making an earth connection without
having to replace the box with a modern one?
E.g. drill and tap a hole and secure the CPC with a screw/bolt.
Ta.
Ensure there is no paint around the connection and use a star washer for a
good contact.
n***@aolbin.com
2021-09-15 08:49:29 UTC
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Post by Grumps
In my house I have a few metal backboxes used for light switches that
don't have an earth terminal.
Is there a best and approved way of making an earth connection without
having to replace the box with a modern one?
E.g. drill and tap a hole and secure the CPC with a screw/bolt.
Ta.
It's not a problem that I've had to solve (because the back boxes here
are very shallow so I'm replacing them with deeper modern ones as I work
through the house) but I would add a crimp tag and star washer to one of
the fixing screws if I had to solve it without replacing the box. I have
always earthed back boxed but many people seem to rely on the faceplate
mounting screw.
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