Earthing Second (Spurred) Consumer Unit

[regedit]

I have 2 consumer units. 1st consumer unit is upstairs. From the Double Pole Switch there is a spur off the live and neutral running to supply a second CU downstairs. 1st CU appears to be earthed (although I can't find the earthing rod). 2nd CU has no earth spike. To earth this 2nd CU is it just a matter of running cable from the 2nd CU to a new earthing rod ? Should I use 4mm or 6mm cable ? Thanks.

[Crossy]

You can add a rod to the second CU no problem. Ensure that all circuits are protected by an RCD since this is now a TT system.

Do NOT add a MEN (N-E) link to the second CU, if your original CU has a link between E and N then you would be better extending this MEN system to the second CU by running an earth between them.

Use 6mm cable to your rod.

[regedit]

Thanks for the advice.

Both CU's have only Circuit Breakers. Can these CB's simply be swapped with an RCD in each CU or is it a matter of replacing both CU's with Split Service CU's ?

Edited January 1, 2012 by regedit

[Crossy]

You need to replace the incoming breaker with an RCBO, that provides both over current and earth leakage protection.

A cheaper option if you have space is to simply add an RCD immediately after the incoming breaker in each board.

[regedit]

Crossy, you say to 'ensure that all circuits are protected by an RCDs' - do you really mean ALL circuits should be protected ? I'm wondering about nuisance tripping and whether it would be ok to set-up both CU's to be split-load CUs ? Thanks again.

[Crossy]

You should at least protect any circuits with Class-1 fixed appliances (water heaters) and all outlets with RCDs.

Nothing to stop you using split load CUs with RCBOs on things like the freezer.

[electau]

Appears is not good enough, visually check the main earth and connection first (and test). If this is OK you can run a 4 or 6 sqmm earthing conductor from the first switchboard to the next DB. This is connected to the earthbar. There must be no connection between the neutral and earth in the second DB if the MEN system is used, ie an N-E bond at the main switchboard.

If you are using the TT system you can run another main earth and electrode from the second DB if you wish but you should bond them together (equipotential bonding). Easier to run an earth between the two switchboards.

RCD protection. TT systems. All circuits.

RCD protection TN-C-S (MEN). All socket outlets, all lighting points. Permanently connected equipment on a risk assessment basis. All permanently connected equipment with exposed conductive material must be earthed.

[regedit]

Appears is not good enough

Quite right. When I got to the end of the earth cable I discovered that someone had 'forgotten' to attach an earth rod, leaving the end of the cable just buried in the soil. That was quickly fixed.

There is no MEN link on either DB.

Running an earth conductor from the 1st DB to the 2nd would mean going through ceilings, walls and a concrete floor, something I'd like to avoid if possible, at least until the next round of decorating. Is there any safety benefit of doing this vs a TT system ?

The L&N connecting the DB's have been there for several years - unfortunately they forgot to earth either DB. So I'm trying to fix that. Both DB's can be RCD/RCBO protected - I'm prepared to replace both DB's to accommodate them. I should also mention that there are 2 showers each with their own earth rod. Attached diagram shows the current set-up. Other than the missing RCD protection, would there be any other concerns with this that I should consider ?

Thanks for all the help and advice.

CU.pdf

[Kwasaki]

Appears is not good enough

Quite right. When I got to the end of the earth cable I discovered that someone had 'forgotten' to attach an earth rod, leaving the end of the cable just buried in the soil. That was quickly fixed.

There is no MEN link on either DB.

Running an earth conductor from the 1st DB to the 2nd would mean going through ceilings, walls and a concrete floor, something I'd like to avoid if possible, at least until the next round of decorating. Is there any safety benefit of doing this vs a TT system ?

The L&N connecting the DB's have been there for several years - unfortunately they forgot to earth either DB. So I'm trying to fix that. Both DB's can be RCD/RCBO protected - I'm prepared to replace both DB's to accommodate them. I should also mention that there are 2 showers each with their own earth rod. Attached diagram shows the current set-up. Other than the missing RCD protection, would there be any other concerns with this that I should consider ?

Thanks for all the help and advice.

[Kwasaki]

Appears is not good enough

Quite right. When I got to the end of the earth cable I discovered that someone had 'forgotten' to attach an earth rod, leaving the end of the cable just buried in the soil. That was quickly fixed.

There is no MEN link on either DB.

Running an earth conductor from the 1st DB to the 2nd would mean going through ceilings, walls and a concrete floor, something I'd like to avoid if possible, at least until the next round of decorating. Is there any safety benefit of doing this vs a TT system ?

The L&N connecting the DB's have been there for several years - unfortunately they forgot to earth either DB. So I'm trying to fix that. Both DB's can be RCD/RCBO protected - I'm prepared to replace both DB's to accommodate them. I should also mention that there are 2 showers each with their own earth rod. Attached diagram shows the current set-up. Other than the missing RCD protection, would there be any other concerns with this that I should consider ?

Thanks for all the help and advice.

What I meant to say was interupted

What I mean to say before was I thought is was 10mm cable from earth bar to rod and 4mm to equipment.?

[regedit]

There's probably a typo in .pdf but it's not the cable size I'm asking about now (yes, 10mm to the rod).

[Crossy]

If at all possible you should link your grounds together, either inside or at the rods.

This will reduce the possibility of shock between the two grounds during a fault and will also reduce the overall resistance to ground which can never be a bad thing.

[GavinK]

Wouldn't it be better to disconnect the independent earths of the showers and run them to the earth bar in the consumer unit ?

Edited March 19, 2012 by GavinK

[Crossy]

Indeed it would ^^^. Unfortunately some installations make it difficult or impossible to do this in a neat manner.