large shop with an rcd at the origin

[hoover]

......Set at 500ma. Now this supplies several dist boards serving quite a few circuits. My question is do I install individual rcds set at 30ma at every board or to reduce incovenience when a circuit trips do I put all circuits on rcbo's( think of the expense of over 100 circuits). A lot of these circuits will supply fridges\freezers so might be a lot of nusieance tripping.Also cables running down stud walls serving light switches. 30ma? Sounds like its going to be an expensive excirise.

 

[Amp David]

Are you modifying all the circuits are have they asked for the 30ma protectipn for every circuit?

 

[hoover]

Modifing the supply from tns to tt system.

 

[telectrix]

why????

 

[Engineer54]

why????

Now that's a dammed good question Tel!!! I was wondering if he meant TT to TNC-S?? ..lol!!!

 

[Knobhead]

This is beginning to make no sense what so ever.

hoover can we have all the facts please?

 

[IQ Electrical]

Yes, come on Hoover, this sucks!

 

[hoover]

Well basically the supply authority say they can no longer supply an earth so it's up to their customer to supply one and so making the system a tt. So just to clarify my main point --do I need to put all circuits which incorporate cables in stud walls, supplying light switches etc, on individual rcbos at 30 ma? Obviously these weren't required when system was a tnc. Now I know sockets and the like will be rcd'd and there will be an rcd at the origin if the installation but this will be set to 500 ma's.

 

[SPARTYKUS]

Ahh......

well if they cant they cant. Is their excuse the theiving pikeys!!!!!

IDEALLY:

Armoured to each dist board and then RCBO's for each circuit.

Can that be done? How many DB's. how far. how much access to cable runs do you have. Is there a DB at the origin.

ps. just read "100 circuits". Hmm.

All down to what you have been asked to do / how much they are willing to pa. Do they want it saf-ER and cheaper, or future proofed?

 

[Deleted member 26818]

The requirements for 30mA RCD protection are the same for TT as for TN installations.
Just because the installation is now TT, doesn't mean that 30mA RCD protection must be provided.
Unless you can establish a very low and reliable Ra(Ze), you will have to use RCDs to provide the disconnection times required for a TT installation, 1s for distribution circuits and 0.2s for all other circuits.
You will also have to ensure discrimination between the RCDs.
If earth leakage is a problem, then you may have to provide individual RCD protection and a separate earth electrode to each DB, with the exposed-conductive-parts related to the DBs connected to the relevant earth electrode, double insulation would have to be provided for the part of the installation, between the origin and where the RCDs are situated.

 

[IQ Electrical]

'they can no longer supply an earth' implies that they have supplied an earth but can no longer guarantee it's integrity, have you reminded them of their duty to maintain a previously supplied earth connection (in writing)?

 

[Knobhead]

http://www.eon-uk.com/distribution/CiCdocs/01%20Technical%20Documents/CN%20Combined/Earthing%20Manual/E2%20Earthing%20Manual%20Earth%20Guidance%20Notes.pdf

 

[SPARTYKUS]

http://www.eon-uk.com/distribution/...l/E2 Earthing Manual Earth Guidance Notes.pdf

Wow, thats an interesting read - how'd you get that!

So according to that (is it up to date? Sept 2007) they must fix the TNS earth .....

 

[Knobhead]

I posted it in the sticky's at the top of the forum page ages ago.

Yep, the warm and smelly stuff has just landed in their lap!

 

[IQ Electrical]

Wow, thats an interesting read - how'd you get that!

So according to that (is it up to date? Sept 2007) they must fix the TNS earth .....

That's what I was getting at in post 11, this has as far as I can remember, always been the case with an existing suppliers earth connection.

 

[Knobhead]

hoover, let us know how you go on

 

[Guest123]

Going by a past experience I wouldn't hold your breath.

Same thing happened to me and no matter what I did or who I shouted at down the phone, I still had to make the install a TT, the whole street infact - 47 houses.

 

[malcolmsanford]

Hoover as this is a commercial enterprise you maybe able to evoke regulation 522.6.7 concerning cables buried in walls and 30mA RCD protection.

If you can prove that 500mA RCD will activate in 1sec for a distribution circuit and 0.2 seconds for a final circuit, then you could leave as is.

It is not ideal having such a large shop on a single up front RCD, but in these economic times often you have to make do with the hand your dealt with. I would be looking to make sure that 500mA RCD is operational and you may indeed need to change it for a new one to ensure safe operation.

 

[Engineer54]

Spin,


Quote from post#10 ...You will also have to ensure discrimination between the RCDs.
If earth leakage is a problem, then you may have to provide individual RCD protection and a separate earth electrode to each DB, with the exposed-conductive-parts related to the DBs connected to the relevant earth electrode, double insulation would have to be provided for the part of the installation, between the origin and where the RCDs are situated... Quote...

Why would you need a separate earth electrode to each DB??

 

[Deleted member 26818]

So that accumulated earth leakage doesn't cause an RCD to trip.

 

[Engineer54]

So that accumulated earth leakage doesn't cause an RCD to trip.

I would say, that if he were to provide multiple deep driven earth electrode positions, it would be better that he connected these positions together. It would/could provide a very decent earth path. It may also be very difficult to isolate building/service conductive parts, from one DB to others...

As this is a large store, it maybe that the surrounding ground to the property is tarmac or concreted over (car parking facilities) Which isn't particually good news when having to provide an earth electrode system, let alone in maintaining it....

Then there is the RCD device protection within this store, that needs sorting out!!! lol!!

 

[Deleted member 26818]

Yes using a number of electrodes would/could provide a very decent earth path.
However the OP has already indicated that earth leakage is likely to be a problem.
It mayt be that the only solution is to divide the installation into parts, giving each part it's own RCD and electrode.
The earth leakage in one part of the installation would then be kept separate from the earth leakage in other parts.

 

[Engineer54]

Yes using a number of electrodes would/could provide a very decent earth path.
However the OP has already indicated that earth leakage is likely to be a problem.
It mayt be that the only solution is to divide the installation into parts, giving each part it's own RCD and electrode.
The earth leakage in one part of the installation would then be kept separate from the earth leakage in other parts.

Very easy to say, but can be extremely difficult and often costly to achieve!!

First things first, ...the OP needs to start measuring any leakage problem areas. At least then, he will know the extent of any perceivable problems, and be able to to provide alternative suitable solutions.