what size

[steptoe]

we probably should be

 

[Apache]

we probably should be

Probably? Are you ok?

:coffee

 

[ADS]

we probably should be

Why on earth should we?

Forget the TN-C-S / PME argument - we know where we both stand on that (we've flogged it to death.)

But if you had a TN-S (distributor supplied) earthing system, with a nice o.8 ohm Ze, why would you TT an outbuilding, at say 50+ ohms Ra, instead of running the supplied earth to it?

That makes no sense whatsoever.

 

[steptoe]

Why on earth should we?Forget the TN-C-S / PME argument - we know where we both stand on that (we've flogged it to death.)

But if you had a TN-S (distributor supplied) earthing system, with a nice o.8 ohm Ze, why would you TT an outbuilding, at say 50+ ohms Ra, instead of running the supplied earth to it?

That makes no sense whatsoever.

at the risk of sounding ignorant, and please dont take me as being anything other than being inquisitive on this question, as I really would like to hear an educatated answer,

why would you risk taking an already unstable earthing system outside the EZ without any guarantee of the safety of the system.

by the way,l DNO NEVER guarantee they will supply you with an earth

that is why I have my own,

and IMPO any spark that rely's on DNO for an earth is very naive IMHO.

 

[ADS]

The way I see it is this:

Like I said, forget TN-C-S / PME.....that presents a different set of problems.

If we are looking at a property with a TN-S earthing system and this property had an attached garage.

The owner asks me to run power and lighting into the garage for him.

Now I wouldn't dream of sinking a rod and putting the (attached) garage on it's own earthing system - I'd utilise the DNO-provided earthing system.

Now, change the scenario, the garage is 3 meters away from the house.

How does this fact alter my decision making, with regards to how I earth the garage.....it doesn't - it is perfectly ok to once again use the DNO-provided earth.

If you are saying it is a personal preference of yours to ignore DNO earths and use a rod everytime, that's fine, but it is just a preference and not a necessity.

 

[Megaohm7]

I've got a similar spec to quote for. TLC-direct have a volt-drop calculator http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html which says 10mm for anything between 4.5 and 7kw but to be honest I was looking at 2-core. Would anyone else use 3-core?

Some people prefer to install 3 core as they don't want to rely on the armour for a cpc if using 2 core it all comes down to cost though at the end of the day .

 

[batty]

Personally as a rule I use 10mm 2 core swa for most external buildings gives you enough power for whatever you need and as it a seperate building and not part of the existing installation I TT outbuilding. Now I have done probably nearly a hundred this way with no problems. I do this whatever the earthing system is in the house and happy to carry on doing it this way. This could end up being a ten page thread but whats the point. You can comply with BS7671 many ways thats not to say the way you do it is wrong or right.

 

[ADS]

Personally as a rule I use 10mm 2 core swa for most external buildings gives you enough power for whatever you need and as it a seperate building and not part of the existing installation I TT outbuilding. Now I have done probably nearly a hundred this way with no problems. I do this whatever the earthing system is in the house and happy to carry on doing it this way. This could end up being a ten page thread but whats the point. You can comply with BS7671 many ways thats not to say the way you do it is wrong or right.

That's well said, Batty.

I'm not being a smart ar*e, but I do have a question. (for either you or Steptoe) :

If the outbuilding in question was going to require a power circuit not on an RCD - say incubation for nesting parrots.....where nuisance tripping could kill the birds.

How would you go about this when a TT install requires RCD earth fault protection, and you refuse to use the suppliers earthing system?

 

[Lurch]

If the outbuilding in question was going to require a power circuit not on an RCD - say incubation for nesting parrots.....where nuisance tripping could kill the birds.How would you go about this when a TT install requires RCD earth fault protection, and you refuse to use the suppliers earthing system?

You design the system to minimise disruption and also maybe build in other safeguards. There are many supplies to many levels of critical systems and these will be found with RCD's on where required, but it's doubtful you'd just find a 30mA RCD covering the whole installation as this would be inconvenient.

 

[ADS]

You design the system to minimise disruption and also maybe build in other safeguards. There are many supplies to many levels of critical systems and these will be found with RCD's on where required, but it's doubtful you'd just find a 30mA RCD covering the whole installation as this would be inconvenient.

But if the shed is TT'D, in order to sattisfy disconnection times under earth fault conditions you would have to install RCDs covering all circuits......not good for the parrots.

Use the provided TN-S earth and it's not a problem.......no RCD required on a dedicated circuit, MCB providing earth fault protection.

 

[Lurch]

But if the shed is TT'D, in order to sattisfy disconnection times under earth fault conditions you would have to install RCDs covering all circuits......not good for the parrots.

If you design it properly and put in other safeguards it isn;t an issue. I'm with you on the exporting earth bit, I'd do it and negate some of the problems, but if you had to use an RCD then the optimum solution isn't to just bang a radial on a 30mA and call it done so you can;t really say that the parrots have a problem with TN-C-S or TT.

 

[batty]

But if the shed is TT'D, in order to sattisfy disconnection times under earth fault conditions you would have to install RCDs covering all circuits......not good for the parrots. Use the provided TN-S earth and it's not a problem.......no RCD required on a dedicated circuit, MCB providing earth fault protection.

Rcd's come in all sizes you could use a 500mA one not likely to trip. I did an outbuilding with a kiln in it and put that on a 300mA trip as Kilns are a bit earthy not been a problem yet.

 

[steptoe]

as said, if you can negate and minimise the risks, and the installation is under the supervision of a skilled or instructed person then you only have to protect property so a higher rated RCD is permissable,

would take a mighty fault to trip a 500mA RCD.

remember also that even on TNCS/TNS if the installation is NOT under supervision of a skilled or instructed person then all 13A socket outlets still need to be on 30mA RCD.

 

[ADS]

Yeh, I suppose there's always ways around things.

As Batty said, there's many ways of complying with the regs, you just have to make that judgement call at the time.

 

[ADS]

We must all be coming down with something - I don't think we've ever agreed so much (or been so polite) - the Mods will be pleased.

 

[Doc Hudson]

We must all be coming down with something - I don't think we've ever agreed so much (or been so polite) - the Mods will be pleased.

I do enjoy reading constructive healthy debate, especially over these points that do not have black and white answers. Also I personally think it helps a lot of the less experienced members who just read and rarely post, but come away from our forum better informed and more knowledgeable than when they joined. Once again thank you to all our regular contributors who put up some very good discussion.

Doc H.

 

[Doc Hudson]

That's well said, Batty. I'm not being a smart ar*e, but I do have a question. (for either you or Steptoe) :

If the outbuilding in question was going to require a power circuit not on an RCD - say incubation for nesting parrots.....where nuisance tripping could kill the birds.

How would you go about this when a TT install requires RCD earth fault protection, and you refuse to use the suppliers earthing system?

I think this was an interesting point for debate, Thank you ADS for raising it.

Doc H.

 

[SPECIAL LOCATION]

Although a bit late :_|

I will just add..

surface wiring, to equipment directly connected via an FCU doesn't need 30ma RCD.. even if TT'd

:coffee