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Barn/workshop And Earthing


LesNewell

Question

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.

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Any use?

 

https://library.ukpowernetworks.co.uk/library/en/g81/Miscellaneous/Other/OHL+Design+Manual+-+Section+1+-+Aerial+Bundled+Conductor+%28ABC%29+Low+Voltage+Mains+and+Services.pdf


To be honest, I forgot that some of the DNO's are now using Concentric CNE for overheads, same as they use underground, skimming that manual reminded me.

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The barn supply is underground from the tranformer pole. Supply to the other properties is two separate wires, not ABC. That manual was quite interesting. I noticed that for PME each pole should have an earth spike. I just checked and this is the case with my installation.

Sidewinder, my DNO is Western Power as you thought.

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Not every pole will always have an earth rod, your Tx will probably have two unless it is a hot site, but doubtful for a WPD pole mount AFAIK.

 

The UKPN manual only applies to UKPN areas, WPD will have their own, which may well be different, each DNO has their own rules.

 

So, you can't base decisions on WPD stuff on UKPN rules.

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Excuse my mind blank, what do you mean by "hot site" sidewinder?

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Shared HV & LV earthing

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Well never heard it referred to like that, cheers Sidey.

Every day a school day.

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Shared HV & LV earthing

 

i was thinking something along the lines of this

 

 

WE12.jpg

 

 

coming to a house near you shortly... thats if youre Jamals neighbour

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alright enough of the snidey remarks, I think jamal is now well aware we think he's possibly making a mistake in the opinions of ,many of this forum. As my granny used to say. 'if you can't say something nice, say nothing at all'.

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alright enough of the snidey remarks, I think jamal is now well aware we think he's possibly making a mistake in the opinions of ,many of this forum. As my granny used to say. 'if you can't say something nice, say nothing at all'.

 

 

Jamal?? Wasn't he on about a 40A socket??

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Forgive Binky, he's not been right since he ate them special cakes the hippies gave him ;)

 

 

:lol:

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just trying to keep the forum friendly as it is meant to be

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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.

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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.

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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

 

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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

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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

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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

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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.

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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. 

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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.

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Zs anyone?

Or am i being a bit too picky a bit too early?

Or is it my hangover?

Just

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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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In first post op said it is fed from its own pole mounted transformer, which is common on farms or large buildings out in the sticks.

Grounded at base of pole,

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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.

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