Barn/workshop And Earthing

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LesNewell

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Oh no, not another 'how do I wire up my garden shed' question...

Well, hopefully this one should be a bit more thought provoking.

I recently purchased a small farm yard and I will need to rewire it. As this is a commercial installation I don't have to worry about part P. I'm an electronics designer by trade and I do a fair amount of repair work on large 3-phase industrial machines so i'm pretty familiar with working on mains voltage wiring. However I'm not so clear on the wiring regs.

So here's the setup. I have a 50m long concrete+wood barn (was a chicken barn but will now be a workshop). At one end is a 100A single phase supply that I think is TNC-S, fed from a transformer 20m away. I'm basing that assumption on the earth terminals built into the meter and the fact that there is no earth rod. At the other end of the barn is a 20m steel framed covered yard then a smaller wooden barn that I'm converting to an electronics workshop.

Currently only the main barn has any wiring and it's pretty dodgy so I will scrap it and start again from scratch. My current plan is as follows:

From the meter I'll install a 100A fused isolator. The meter needs to be relocated so I'll get it connected when they do that. After the isolator I'll have a CU to feed various supplies at that end of the barn (lights + a couple of sockets). The isolator will also feed a 30m long 25mm or 35mm cable to the middle of the barn where I'll have another CU to feed various machines, welders etc. From there I'll run 50m of 10mm to the office. I won't need more than 20A at the office (computers, test gear, lights + night storage heater). Does that sound like a workable plan?

Is it acceptable to have both a 32A round socket and 13A sockets on the same 4mm spur, as long as it is all on 32A MCB? I can't see any reason why this should be a problem.

I have a 30m length of new 35mm twin (not armoured) cable that I've had lying around for a few years. Could I use this with a separate earth cable? It would be nice to use this cable as I have it but it's not a major issue to buy some SWA if need be.

The bit i'm not so sure about is earthing. Do I change to TT? I'm also not sure about what size my earths need to be. I assume I need to bond the steel building. Would I need to bond each upright or just one as they are all connected through the steel framework? Any advice would be greatly appreciated.

 
With greatest of respect les, whilst you may have worked with Mains voltage there is a lot more to the design of an install. I would therefore recommend you employ the services of a suitably qualified electrician. At least this way you an sure to have a safe install. They may even permit you to do so e of the donkey work.

 
I take your point and I may end up going that way. However I also have a tendency to obsessively research everything I do. Even if I get someone in it would be nice to know why they make the design decisions they do. Watching over an electrician's shoulder and continually asking questions is likely to make me very unpopular.

 
Part P is an irrelevant issue ...  no idea why you even mention it ???  

(its only notifying the council for them to issue a building regs compliance cert once correct work has been done.)

It has little or no impact about how the electrical work should be undertaken...

Domestic or commercial the principal is the same....

The installation should be designed around the expected loads you need to supply..

Suitable cable sizes should be installed for the loads you need..

(and taking into account of any external influences)

Appropriate protective devices must be installed to protect the cables... 

The installed work must be dead tested before energising anything.

Full live testing of all circuits need to be done once energised...

The testing should confirm that disconnection times will be met in the event of a fault..

All test results must be documented onto the correct electrical certificate..

All proper signage and warning labels etc must be added..

etc..

etc..

All  you have told us is about the size of the building and some cables you have...

BS7671 wiring regs at the moment is a 496 page book...

it is a bit more involved to correctly design an installation than can be done remotely over the internet with a group of strangers who have never seen your site...

As Sharpend says...

best solution first of all is get someone to come and visit site to work out a proper installation design....

Guinness

 
my method for each circuit is,

 Decide on

-installation method

-length

-Cable type

-calculate the following

 :R1+R2

 :Ib

 :In

 :Iz

 :Zs

 :It

 :volt drop

 :fault current

 :thermal constraints

Change design on any parts and recalculate everything if needed.

Do that for every circuit. check it all works out ok.

Then you will have an on paper design.

Worth doing the 2394, 2395, 2396  courses to help

 
or,

just have a walk round,

guess how far it is,

well,

its 2.5 for sockets, cos it always is, 1mm might be ok for the lights, but if you are having halogen floods then 1.5

6mm for the cooker, so that'll be fine for the welder too,

and if the leccy board havent left an earth terminal we can open it up and make one,

thats how my mate drunken Bob does it,,,,

and a lot of other people too, tbh

 
I take your point and I may end up going that way. However I also have a tendency to obsessively research everything I do. Even if I get someone in it would be nice to know why they make the design decisions they do. Watching over an electrician's shoulder and continually asking questions is likely to make me very unpopular.
And continually asking questions in here will wind up a few members on here too

 
Looking at my post again I didn't make a very good job of explaining. I wasn't sure how much info to put in the post. Too much and you get a wall of text that no-one will read, too little and you look like an idiot. I ended up more on the idiot side by posting the questions that were foremost in my head at the time without enough background.

Part P is an irrelevant issue ...  no idea why you even mention it ???  

(its only notifying the council for them to issue a building regs compliance cert once correct work has been done.)
True in theory, but if my neighbor's experience is anything to go by it can be a real pain. Building control wanted an electrician to sign it off, elec didn't want to sign it off as he didn't do the work...
The installation should be designed around the expected loads you need to supply..

Suitable cable sizes should be installed for the loads you need..

(and taking into account of any external influences)
Of course. This is my reasoning for the cable sizes I quoted. Due to the length of the cables, voltage drop is pretty much the deciding factor for cable size rather than current handling capacity.For the CU in the middle of the barn I'm not sure of the max load. I have some pretty beefy machines and I'm likely to get more. Therefore I allowed for the max available supply of 100A. 25mm would give an acceptable voltage drop to the middle CU on full load but if the office is in use the total voltage drop over the 25mm cable plus the office feed is marginal.

For the office CU I am allowing 20A, based on my current office.

Computers + test gear 1kw

Small free standing heater 3kW - this probably won't be used as the new office has even better insulaton than building regs require.

LED lighting 100W (every light on)

total 4.1kW = 17.8A @230V

I won't bother to post the voltage drop figures unless someone wants me to.

Appropriate protective devices must be installed to protect the cables...
For overload protection the 100A fused isolator should protect the 35mm cable. The 10mm cable would be on a 32A MCB. The cables are so over size heating isn't going to be an issue under normal load or fault conditions. I can of course crunch the numbers for completeness. As I mentioned I am good at researching and all of this information is available online. I don't know all of the regs but I can learn the parts I need.The rest of the wiring is pretty standard stuff, 4mm spurs for sockets, 1.5 lighting with appropriate MCBs I will need to check voltage drop on some of the sockets and increase cable size if needed. Separate RCDs for lighting and sockets. In the barn all cables would be surface mounted with no bunching so I don't have to worry about derating due to heating. There will be some bunched cables in the office but the calcs aren't difficult and there are plenty of on-line calculators that will do it for you.

The installed work must be dead tested before energising anything.
Of course. I have a lot of test gear so I have most of the kit I would need. Any kit I don't have I can probably borrow.
it is a bit more involved to correctly design an installation than can be done remotely over the internet with a group of strangers who have never seen your site...
I don't expect anyone to design it for me. I am aware that you are donating your time freely and you also have to be careful about liability. I would like to try designing this system even if it is just an intellectual challenge and I don't end up installing it myself.While I can find most of the information I need online some areas seem to be a bit grey, especially when it comes to earthing. The choice of earthing scheme has a fairly big impact on the design. From what I can find in the regs agricultural should be TT. However this is no longer agricultural and it appears that the original wiring wasn't TT. Is it practical to try to maintain an equipotential over 80 metres of assorted buildings? The floors are concrete with no DPC or buried grid so it is theoretically possible for some areas of the floor to have a leakage path to true ground rather than being at the equipotential voltage.

 
If your incoming supply is TNCS, you need to find out if it is actually PME, or simply DNO implied PME, this is only something you can measure, and calculate onsite

on reading again

I still don't see where you gave said what earthing you actually have at present. 

 
If it was a small farm yard and you are using the old chicken shed then wouldn't the supply be more TNS than TNCS. As said you will need to test and take readings to establish true earth arrangements.

 
I don't expect anyone to design it for me. I am aware that you are donating your time freely and you also have to be careful about liability. I would like to try designing this system even if it is just an intellectual challenge and I don't end up installing it myself.

While I can find most of the information I need online some areas seem to be a bit grey, especially when it comes to earthing.

Crack on matey, you clearly want to use "free expertise", so when its installed and tested and the tester condemns it, don't come here bleating and crying.

 
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Hey... Now come on.. That is a bit unfair.. The bloke is doing his best and has clearly put a lot of thought into this himself. He is certainly not one of the "how do i do it because i am passing myself off as an electrician and charging people when i do not have a clue" types, but is a genuine bloke seeking to improve his knowledge and wants to design and sort his own installation..

Good for him i say, besides, it would be good to have an electronics boffin here on the forum. i am sure we might find ourselves asking for design help one day.....

Telling people to **** off when they ask advice is a sure way to kill the forum......

john...

 
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Crack on matey, you clearly want to use "free expertise"
Yes, to a certain extent you are correct. If someone asks me for advice in my areas of expertise I am always willing to help, unless they take the mick and expect me to do all the work for them. It is of course totally up to you if you want to help or not.
so when its installed and tested and the tester condemns it
Technically I don't need the installation to be tested. It is not residential so it is covered under H&S. I am the only one using the premises and H&S is complaints driven. I'm not exactly going to put in a complaint against myself. I know an electrician who would be willing to cast an eye over my work if I pay his time, just in case I've done anything stupid. There is nothing dodgy about doing that. He wouldn't be signing it off, he would just be taking measurements that I can use to fill in my own paperwork.

don't come here bleating and crying.
I take responsibility for my own mistakes. My attitude towards life is that I can do anything unless proven otherwise. Of course I make mistakes but even then I have learned something new.

 
Les you are not quite correct there in that the install would have to be tested to ensure that it is integrally safe for service. out of interest do you have to inform planning for change of use for the building? Or are you just adopting the attitude of its my building I'll do what I like with it?

If you know of a spark who'll cast an eye,perhaps his other eye might give guidance on design?

 
7671 says work must be inspected & tested, so if you dont, then you have not complied with 7671. also, if you dont I&T, then you do not know it is safe, therefore breach of EAWR. and imagine if someone else was in there and got a shock from something

so yes, technically you do need it to be tested...

 
Les,

There is a little more to BS7671 than can be gleaned from the internet.

Also, as far as your home is your castle thing, then yes it is, and to a certain extent you can go what you like.

Good on you for coming for advice to fill in the gaps.

Possibly your only "official" issues would be planning change of use, and whether there would be a requirement from them for any "paperwork" and also your insurers, I'm guessing that you will insure the buildings, so they may have a requirement in the event of a claim.

I've not gone through the whole thread in detail, however, one thing that does jump out is your intended use of spurs, that is very poor design on a new install, correctly specified radial circuits are a better solution, with a ring if you wish, though, to be honest I would not bother with a ring.

Also a 32A EN 60309 socket sould really, be on its own dedicated circuit.

I'll have another read later.

 
If it was a small farm yard and you are using the old chicken shed then wouldn't the supply be more TNS than TNCS. As said you will need to test and take readings to establish true earth arrangements.
Thank you. The barn is supplied from a nearby tranformer on a pole. The tranformer supplies two other properties.I just dug though my pile of lesser used test equipment and found a Seaward DL750 earth loop tester. It's calibration is out of date but it should be good enough to get an idea of what is going on. The wiring in the barn was a scary mess with multiple CUs of various vintages, mostly half full of chicken s**t and dead insects. Luckily it has a big old cast iron isolator at the meter so a while back I dropped the power and scrapped everything apart from one CU that feeds one socket, all within 0.5m of the meter. The only earth bonding was to an LPG main. The main and tank have now been removed so I just have the meter, isolator, CU and socket as a completely self contained setup. I plugged the earth loop tester into the socket and got a reading of 0.1 ohm.

 
Possibly your only "official" issues would be planning change of use, and whether there would be a requirement from them for any "paperwork" and also your insurers, I'm guessing that you will insure the buildings, so they may have a requirement in the event of a claim.
I have the change of use in progress and I'm really not looking forward to having a visit from the rates assessor. As far as I can tell if it's not part P they aren't interested in the electrics. As far as insurance goes, I have never been asked about the electrics. A friend with a car repair workshop did have to come up with paperwork for his insurance but they were happy with a basic safety inspection from a local electrician.

I've not gone through the whole thread in detail, however, one thing that does jump out is your intended use of spurs, that is very poor design on a new install, correctly specified radial circuits are a better solution
Oops, my bad. I meant radials. As you say in this application ring mains wouldn't work very well.
Also a 32A EN 60309 socket sould really, be on its own dedicated circuit.
Thanks. I had a suspicion that was the case.
 
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