Consumer unit upgrade TT system

[monkeyhanger]

Appreciate a bit of advice regarding this that I'm doing in the morning-

Consumer unit upgrade in house with 6mm gas & water bonding existing (leaving bonding as it is). Existing rod in house with 6mm up to CU.

(Existing install) From a TT system in the house there is a 2.5/1.0 T&E running to a detached outhouse (perhaps, four meters from the back of the house) feeding an existing fuseboard in there with BS3036 fuses for 15A sockets radial and 5A one light.

The outhouse has a copper water pipe supplying a washing machine. This isn't bonded where it rises from the concrete floor in the outbuilding and there isn't a seperate rod in there. The only earthing of anything in there is the 1.0mm CPC of the T&E submain & CPCs of light/sockets.

Am I right in thinking that the CPC in the T&E won't suffice in the requirements for equipotential bonding in the outbuilding (it appears that the T&E is ran under the concrete back yard in plastic conduit.. many years ago). If I install a rod & earth conductor to the existing BS3036 unit in the outbuilding and bonding between this unit & the copper pipework in the outbuilding, should I insulate the 1.0mm CPC in the T&E submain from the outbuilding installation (at the outbuilding end)? Or leave it connected at both ends- in the house and the outbuilding. If either is acceptable would it be best practice to leave it connected both ends?

I am quite aware of requirements when exporting TNS and TNCS but I've never came across TT in the main building with an outbuilding where bonding is required.

This is hopefully obvious to one of you who may read this before tomorrow

 

[davesparks]

Something is certainly obvious, but i'll limit my comments to the job you are asking about.

I would not be happy with a T&E underground in a pvc pipe for a start. you are right that 1.0mm is too small for a binding conductor. Ideally you would get a 6mm to the outbuilding and add a rod there connected back to the MET to help the Ra of the installation. seperating the outbuilding and adding earth electrodes there will require the installation of RCD protection to the outbuilding.

 

[monkeyhanger]

Thanks dave. It's for an agent and unless absolutely necessary they wouldn't have it done. We are moving some sockets around as there is a new kitchen going in, and the existing consumer unit is damaged so needs replacing.

The existing T&E submain will be 30mA RCD protected in the house. It is in plastic conduit but below the concrete, the IR test on it is fine (>299 @ 500V) & you can see where it comes out of both ends indoors. Despite outbuilding being very close to the house, it wouldn't be easy to get back to the CU in the house from the outbuilding (opposite end, laminate everywhere etc).

A 6mm can't be ran to the outbuilding unfortunately.

If I install a rod to the outbuilding, what should I do with the 1.0mm CPC in the submain at the outbuilding end? I can't see any reason to disconnect it as both sides are TT, but I'm not sure.

 

[davesparks]

What do you mean A rod? you'll need more than one to achieve and accepetable Ra!

 

[davesparks]

Is there enough space to get the earth electrodes at the outbuilding far enough away from those you install for the house?

 

[monkeyhanger]

Something is certainly obvious, but i'll limit my comments to the job you are asking about.
I don't mind if you elaborate!

 

[davesparks]

Something is certainly obvious, but i'll limit my comments to the job you are asking about.
I don't mind if you elaborate!

Your OP reads as though you haven't got a clue how to install a TT system properly. It sounds like you are planning to have just a single rod for the main earth and that you don't understand either the regulations concerning earthing of multiple buildings nor the theory behind them.

 

[monkeyhanger]

I am not installing any new in the house, there is an existing rod in the house is at the opposite end of the building, below the CU, I would hazard a guess that its 30-40m in a straight line from there to where the outbuilding is.

I would guess that (terraced housing) the rod in the house next door is perhaps 2m away from the rod in the house we are working in, are there any definitions / good practice measurements of being far away enough?

I don't see any reason to need more than 1 rod at the outbuilding to acheive <100 ohms

If the outbuilding wasn't in use I would have left it disconnected, but they will want it connecting to the new CU in the house due to the washer being out there, but I want to make sure the arrangements in it are acceptable first!

 

[davesparks]

What is the Ra of this one existing rod? How long is it and what diameter is it? Is it really inside the building?

What has 100 ohms got to do with anything? The point of having multiple rods is to achieve stability through depth, a low Ra and the ability to safely handle the maximum current which may flow under fault conditions.

In a scenario where an earth fault occurs in the house, what do you think would happen to the cpc in that submain if you connected it to the electrode at each end?

 

[monkeyhanger]

just a single rod for the main earth = measurement at this existing rod was about 35 ohms, why would I need more?

and that you don't understand either the regulations concerning earthing of multiple buildings nor the theory behind them
= this is an existing install with outbuilding 4m away with two sockets and a light. I'm aware that the existing arrangement isn't satisfactory but thats why I'm asking on here.

:49: I'm well aware that some on these forums will bang in multiple rods until their reading is as good as TNS! However if the reading is acceptable, it's acceptable. Especially for a letting agent

 

[davesparks]

How did you measure the 'about 35ohms' ? A 3 wire fop test? Or was it a loop test with the rod disconnected from the installation?

What do you define as being an acceptable reading? Bearing in mind there is no definition in the regs, it's up to a professional to make their own assessment.
What do you mean by 'especially for a letting agent'? Are their lives worth less than other peoples and so it's ok to install substandard earthing ?

 

[monkeyhanger]

I don't know for sure, but I would say 5/8" and probably 4 ft if it isn't extended.
Yes it's inside (under the hallway floor as you come in the front door), as will be every other electrode in the 2000 identical houses in that area!

I've always used 100 ohms as a guide for acceptable / likely to be stable as it's what I was taught during my apprenticeship, 2391 course, the way it was done at NIC companies ive worked for & it seems it's acceptable with NAPIT who I'm with.

I don't think anything would happen to that submain as RCDs would operate.

 

[Engineer54]

My best advice is to check with the local DNO to see if a PME connection is available, but he'll still be at a loss as to what to do about the outbuildings extraneous pipework... He'll probably just bung a twig in and call it a day.... Hasn't even bothered giving us the existing Ra value for the the main house.....

EDIT....Ah i see he's just given the house Ra value...

 

[monkeyhanger]

Loop test with rod disconnected when I made a brief visit the other day.

"What do you mean by 'especially for a letting agent'? Are their lives worth less than other peoples and so it's ok to install substandard earthing ?"

What I mean is, if something doesn't need doing, it won't be paid for. If I can install a rod and bonding to the outbuilding, and that is acceptable, then that will get done. If that isn't acceptable, and a new submain is needed e.g. 6mm3C SWA, lifting of laminate floors from front to back etc, and they ask someone else for a quote who goes and puts a rod & bonding in the outbuilding AND THAT IS ACCEPTABLE, I may lose out on future work from the agent.

 

[davesparks]

What is the point in asking questions if all you want to do is the bare minimum you possibly can and scarper?