Socket Backboxes Back-to-Back

Section62

Apodemus said:

fenwick458 said:

Ah!  Thanks!  Yes that makes sense.  So in the OP's case, it would be safest to make sure the earth wires for the two back-boxes are linked?

No.  Not unless the OP knows that both back boxes should be electrically connected.

On the limited information we have, it is impossible to tell either way.  The first piece of information we need is whether the two backboxes are for sockets on the same circuit.

Ectophile

Apodemus said:

Ectophile said:

Section62 said:

grumbler said:

Section62 said:

JohnB47 said:

If everything is secure and comprehensively earth strapped together, I can't see a problem.

It could be a problem if the two boxes are on different circuits and not meant to be "earth strapped together".

Can this be found in domestic wiring?

With two or more RCDs  neutrals do have to be separated, but not earths.

Anything can be found in domestic wiring. (not necessarily compliant with current regs)

Having more than one earthing system within one installation is not that uncommon. In which case care has to be taken to avoid accidental interconnection.

You should never have two different earthing systems within touching distance of each other.  That's usually only a problem in commercial premises divided into individually metered units.  Most houses only have one supply.  It only really gets to be a problem when running supplies outdoors (sheds, hot tubs, etc.) when you need to think about what earthing system to use.

This is all good stuff, which I was previously unaware of.  Can you explain why two different earthing systems touching each other is a problem?  I would always have (wrongly it seems!) assumed that the more earthing the better!

All the earth wires in a house should ultimately be connected to the same Main Earth Terminal (MET).  That can be the earth bar in the consumer unit, but it's often a terminal block near the supply fuse.

If there are metal water or gas pipes (known as "extraneous conductive parts" in electrician speak), then they should also be bonded back to the MET.  That's the green and yellow stripy wires you see clamped to metal pipes.

That way, you know that everything in the house that's "earthed" is actually at the same voltage.  In a severe fault, that voltage may drift considerably away from the true Earth voltage, maybe as much as 120V above Earth for a short period.  But if every earthy thing in the house is connected together, then there's no way to be electocuted.

It all gets more complicated when you look at outdoor appliances, such as hot tubs.  People can be standing on the real Earth, while touching an appliance connected to the house earth.  The safety rules for electric car chargets are becoming increasingly complicated, as they include more systems to stop people being electrocuted while unplugging the car.

Eldi_Dos

Apodemus said:

Ectophile said:

Section62 said:

grumbler said:

Section62 said:

JohnB47 said:

If everything is secure and comprehensively earth strapped together, I can't see a problem.

It could be a problem if the two boxes are on different circuits and not meant to be "earth strapped together".

Can this be found in domestic wiring?

With two or more RCDs  neutrals do have to be separated, but not earths.

Anything can be found in domestic wiring. (not necessarily compliant with current regs)

Having more than one earthing system within one installation is not that uncommon. In which case care has to be taken to avoid accidental interconnection.

You should never have two different earthing systems within touching distance of each other.  That's usually only a problem in commercial premises divided into individually metered units.  Most houses only have one supply.  It only really gets to be a problem when running supplies outdoors (sheds, hot tubs, etc.) when you need to think about what earthing system to use.

This is all good stuff, which I was previously unaware of.  Can you explain why two different earthing systems touching each other is a problem?  I would always have (wrongly it seems!) assumed that the more earthing the better!

If my recollection is right the main suspect for the York Minster fire was the earth for the lights in roof space having less resistance than the lightning protection system.

Eldi_Dos

fenwick458 said:

Apodemus said:

Ectophile said:

Section62 said:

grumbler said:

Section62 said:

JohnB47 said:

If everything is secure and comprehensively earth strapped together, I can't see a problem.

It could be a problem if the two boxes are on different circuits and not meant to be "earth strapped together".

Can this be found in domestic wiring?

With two or more RCDs  neutrals do have to be separated, but not earths.

Anything can be found in domestic wiring. (not necessarily compliant with current regs)

Having more than one earthing system within one installation is not that uncommon. In which case care has to be taken to avoid accidental interconnection.

You should never have two different earthing systems within touching distance of each other.  That's usually only a problem in commercial premises divided into individually metered units.  Most houses only have one supply.  It only really gets to be a problem when running supplies outdoors (sheds, hot tubs, etc.) when you need to think about what earthing system to use.

This is all good stuff, which I was previously unaware of.  Can you explain why two different earthing systems touching each other is a problem?  I would always have (wrongly it seems!) assumed that the more earthing the better!

he didn't say 2 different earthing systems touching each other was the problem, the problem could be having 2 different earthing systems.
its potentially dangerous as you could get a potential difference between the 2, and if an unsuspecting person was to touch both at the same time, they would get a shock

When travelling on the railway you will see that the cabinets containing signalling equipment and ones with telecomms are distanced because of this.

Ben1989

So they’re in a structural wall with one supplying one bedroom and the other the adjoining bedroom. The supply is on the same circuit “upstairs sockets”.

I’ll try and take a picture soon

fenwick458

Section62 said:

Apodemus said:

fenwick458 said:

Ah!  Thanks!  Yes that makes sense.  So in the OP's case, it would be safest to make sure the earth wires for the two back-boxes are linked?

No.  Not unless the OP knows that both back boxes should be electrically connected.

On the limited information we have, it is impossible to tell either way.  The first piece of information we need is whether the two backboxes are for sockets on the same circuit.

I'm struggling to think of a reason (in a domestic environment) why 2 circuits can't share the same earth?
most metallic things in a wall are connected to earth anyway, thats just how it is. if you were to remove the CPC's from a socket and a lightswitch within 1m of each other and do an insulation resistance check , you'd get a reading 0.3-1 Mohm depending. so that means they are connected without any cables, should we be taking steps to ensure that all CPC's are kept separate, and if so why?

Section62

fenwick458 said:

Section62 said:

Apodemus said:

fenwick458 said:

Ah!  Thanks!  Yes that makes sense.  So in the OP's case, it would be safest to make sure the earth wires for the two back-boxes are linked?

No.  Not unless the OP knows that both back boxes should be electrically connected.

On the limited information we have, it is impossible to tell either way.  The first piece of information we need is whether the two backboxes are for sockets on the same circuit.

I'm struggling to think of a reason (in a domestic environment) why 2 circuits can't share the same earth?
most metallic things in a wall are connected to earth anyway, thats just how it is. if you were to remove the CPC's from a socket and a lightswitch within 1m of each other and do an insulation resistance check , you'd get a reading 0.3-1 Mohm depending. so that means they are connected without any cables, should we be taking steps to ensure that all CPC's are kept separate, and if so why?

For example because -

Circuit 1 = your socket inside the house on a TNCS/PME system.  Circuit 2 = Outbuildings on a TT system, including your lightswitch on the exterior side of the wall.

In that situation "comprehensively earth strapp[ing]" the socket to the light switch would link the TNCS/PME system to the TT system (and therefore to the TT earth electrode) which would represent a potentially dangerous situation.  In this case 'more earthing' means less safe.

AIUI it is prohibited to connect the earthing arrangements of a TNCS/PME system to a local earthing electrode (e.g. one provided for the TT part of the installation).

fenwick458

Section62 said:

fenwick458 said:

Section62 said:

Apodemus said:

fenwick458 said:

Ah!  Thanks!  Yes that makes sense.  So in the OP's case, it would be safest to make sure the earth wires for the two back-boxes are linked?

No.  Not unless the OP knows that both back boxes should be electrically connected.

On the limited information we have, it is impossible to tell either way.  The first piece of information we need is whether the two backboxes are for sockets on the same circuit.

I'm struggling to think of a reason (in a domestic environment) why 2 circuits can't share the same earth?
most metallic things in a wall are connected to earth anyway, thats just how it is. if you were to remove the CPC's from a socket and a lightswitch within 1m of each other and do an insulation resistance check , you'd get a reading 0.3-1 Mohm depending. so that means they are connected without any cables, should we be taking steps to ensure that all CPC's are kept separate, and if so why?

For example because -

Circuit 1 = your socket inside the house on a TNCS/PME system.  Circuit 2 = Outbuildings on a TT system, including your lightswitch on the exterior side of the wall.

In that situation "comprehensively earth strapp[ing]" the socket to the light switch would link the TNCS/PME system to the TT system (and therefore to the TT earth electrode) which would represent a potentially dangerous situation.  In this case 'more earthing' means less safe.

AIUI it is prohibited to connect the earthing arrangements of a TNCS/PME system to a local earthing electrode (e.g. one provided for the TT part of the installation).

seems like the most unlikely scenario imaginable but ok I get the point

Section62

fenwick458 said:

Section62 said:

fenwick458 said:

For example because -

Circuit 1 = your socket inside the house on a TNCS/PME system.  Circuit 2 = Outbuildings on a TT system, including your lightswitch on the exterior side of the wall.

In that situation "comprehensively earth strapp[ing]" the socket to the light switch would link the TNCS/PME system to the TT system (and therefore to the TT earth electrode) which would represent a potentially dangerous situation.  In this case 'more earthing' means less safe.

AIUI it is prohibited to connect the earthing arrangements of a TNCS/PME system to a local earthing electrode (e.g. one provided for the TT part of the installation).

seems like the most unlikely scenario imaginable but ok I get the point

I could describe some far less likely scenarios if you like   Some of which I've seen over the years.

But the key point was that DIYer's shouldn't assume that everything 'earthy' should be connected together, and/or that making random 'earth' connections between things improves safety.  There is a real possibility that their household earthing arrangements use more than one earthing system which shouldn't be interconnected, and since things like EV charging points, garden lighting, hot tubs/pools, garden rooms etc are becoming more popular, the chances of someone having more than one earthing system within their property will be on the increase.

Ectophile

Section62 said:

fenwick458 said:

Section62 said:

fenwick458 said:

For example because -

Circuit 1 = your socket inside the house on a TNCS/PME system.  Circuit 2 = Outbuildings on a TT system, including your lightswitch on the exterior side of the wall.

In that situation "comprehensively earth strapp[ing]" the socket to the light switch would link the TNCS/PME system to the TT system (and therefore to the TT earth electrode) which would represent a potentially dangerous situation.  In this case 'more earthing' means less safe.

AIUI it is prohibited to connect the earthing arrangements of a TNCS/PME system to a local earthing electrode (e.g. one provided for the TT part of the installation).

seems like the most unlikely scenario imaginable but ok I get the point

I could describe some far less likely scenarios if you like   Some of which I've seen over the years.

But the key point was that DIYer's shouldn't assume that everything 'earthy' should be connected together, and/or that making random 'earth' connections between things improves safety.  There is a real possibility that their household earthing arrangements use more than one earthing system which shouldn't be interconnected, and since things like EV charging points, garden lighting, hot tubs/pools, garden rooms etc are becoming more popular, the chances of someone having more than one earthing system within their property will be on the increase.

Within a house I would assume that earthy things can be connected together.  Outbuildings, hot tubs, EV chargers and other things outside the house can have their own earthing arrangements.  But there is never any good reason to have multiple eathing systems within a house.  It's likely to be dangerous, and against Wiring Regs.