Tt Installation Probs!

[apprentice87]

Hi all,

I am trying to sort out an installation at a farm for a friend. Yep, you guessed it, "farm" says it all!!!! I will add here, that i know that there are certain requirements for agricultural installations, but this is not one, it is a large field with horses, and horses are not an agricultural animal.....

Anyway, this place is supplied by overhead, and as the DNO have made no provision for earthing it is TT time, [or at least it should be...]

Now, over the years this place has grown and grown [and grown!] Problem is, i can find no earth rods anywhere. I asked the owner, and he alternates between; "there is no earthing" and; "i am sure there was a rod here once"!!!

All the various buildings [eight of them] are supplied by SWA cable of dubious origin, nothing is co-ordinated, and it is all a complete mess. The only provision for earthing in each building is from the cable armour, no other rods, nothing. Trouble is, there are no RCD's anywhere and Zs is 10 ohms.......  not good.... Not good at all.....

10 ohms seems very good for a TT installation, but as there are no rods [or earthing conductors!!!] that i can find, i can only assume that the one large steel framed building is acting as an earth rod for the whole installation, as, like i say, it is all linked by the cable armour. Even this is by chance, as the CU in the barn is a metal one, and merely happens to be bolted to a stanchion of the building, not deliberately bonded to the steelwork or anything like that!!

Now, i installed an installation, [eh] to a new building there the other day. I used an insulated, IP rated CU, rodded it, and the new building now has its own, discrete, TT, rod and RCD setup. The owner was amazed that a proper job could be done so cheaply, and now wants the rest improving...

Now to my question...

In my opinion, having all the buildings linked together by the cable armour is a huge mistake and is dangerous. You have a TT setup yes?? Even if you DID have RCD protection, let us assume for a second that the RCD has failed. If you now have an earth fault in any ONE building, ALL exposed conductive parts in ALL the buildings, will now become live, the whole lot... Even less good.....

So, here is what i propose to do....

300mA RCD in the meter tails to protect all distribution circuits. Then, at each individual building, insulate the incoming SWA from the CU, [plastic box, insulating gland, plastic CU, whatever.] and then individually rod and 30mA RCD each building on its own. This way, at least if you have an earth fault and the RCD fails to work, the shock hazard is confined to the one building, and not transmitted to the entire complex.

What you all think????

john....

 

[NozSpark]

IIRC a 300mA RCD won't comply on a TT installation,,, don't you have to have 100mA as a minimum for protection of livestock and against fire,,,, 'S' type obviously

I've also done what you propose with the armour to stop circulating earth currents

 

[steptoe]

I'll throw my tuppence in,

• 100mA TD upfront, why the 300 ? Ive hardly ever used a 300, either 100 or 500, if you do use a 300 it MUST be TD
• rod every building
• dont disconnect the armour, why would you? its NOT TNCS, its TT
• more rods in parrallel the better, usually,,,,,,,,,,,
• mmm, its getting late, Im sure I'll think of more in a bit,
• John, not having a go, I hope you'll understand, but, do you realise how TT works in comparison to TN systems.?
•  

 

[binky]

seem to remember reading 300mA RCD was required for circuits like lighting on farm buildings containing livestock / hay storage as a fire measure. However, 30mA everything is obviously better. Double pole RCBOs for the individual circuits probably a good idea. If i remember correctly, there' nothing wrong with using buildings steel framework as rod with a nice bit of earth cable and big clamp connected to an upright steel, suitabley labelled of course.

Like the idea of separating the buildings to make life simple - it's the way I tend to think, but think Steps has a point, the more earthing connected, the better the system, and not relying on single rod per building which can corrode over time, isn't a bad idea.

 

[apprentice87]

Hi Steps!!! Of course i realise you are not having a go!!!!!!

Why i suggested isolating the earthing from one shed to another, is, although i know it would be nice to have loads of rods in parallel, because there is plenty of tin sheeting and the like on these sheds, i thought that the "niceness" of this, would be outweighed by the fact that an earth fault in one shed would then give a shock hazard in all the sheds [presumably!] What i mean is, anyone leaning on any of the sheds will have a huge shock as a result of a fault in another shed. As there are eight sheds, that means that in the event of a fault anyone touching a shed will get a shock, but if they were separated, this chance would reduce down to one in eight!!! [i think!]

As to the choice of RCD, i am a bit confused here. True, in the OSG they do state a 100mA upfront RCD, but in the regs they merely call for a 300mA one. I would prefer a 300mA one though, as there is rather a lot of stuff all connected up, so with all the leakage currents all adding up, i do not want the owner to have to walk 70 yards in the dark in the rain and then have to unlock a shed and reset it all, while his missus fumes in the darkness!! I would definitely fit an "S" type one though! forgot to mention that!

Was only asked about it late last night, so i only started to think about it on the five minute drive home, so just kicking round some ideas at the moment.....

john...

 

[apprentice87]

Hi Canoe!

300mA it is then!!!

Thanks for that!!! Does my general plan of action with separating the buildings earthing systems sound a good one to you?? or should i leave them all linked??

I would like to separate them for the reasons i explained above. Apparently a few years ago a horse owner there was playing about with the wiring in one shed, touched a metal bit with a "live" phase conductor, and nearly killed the owner who was opening a steel barn door in another building!!!! As i say, there is no RCD protection at all, despite it being TT, [or meant to be!]

There is one of them old VOELCB things, but i do not think it works anyway, and besides, the owner has jammed the "switchy bit" with a piece of wood!!!!

john...

 

[Sidewinder]

john,

Keep the "mesh" earthing, I am resisting calling it PME, but, think about it.

Yes 300mA up front, S type, £lots!

However, no real other option.

I would also go down the double pole RCBO route, look at the SBS units from SBSDave?

Put some thought into the design then you can do it once and do it right.

Got a job up your way as soon as I can too, if the customer still wants me, they have been waiting a while, been so busy and not so well!

 

[batty]

Yes definitely go for double pole RCBO's if you use single pole it may well take out the time delay upfront  if there is a neutral to earth fault. I would go for RCBO's being a farm have done a few and generally electrics are diabolical. 

 

[steptoe]

john,

Keep the "mesh" earthing, I am resisting calling it PME, but, think about it.

Yes 300mA up front, S type, £lots!

However, no real other option.

I would also go down the double pole RCBO route, look at the SBS units from SBSDave?

Put some thought into the design then you can do it once and do it right.

Got a job up your way as soon as I can too, if the customer still wants me, they have been waiting a while, been so busy and not so well!

:slap

I should have known someone would come up with that,

but, yes, I totally agree with that train of thought, 

 

[apprentice87]

Hello all!

Ok, so i can see that those that know FAR more than me agree that all buildings should remain linked together, yes, a PME type arrangement if you like, and who am i to argue.... Thing is though, my train of thought goes something like this..

Say you are in the house there; Now there is a fault to earth, lets say a wire rubs through on to a metal part of one of the sheds. The loop impedance in all the buildings is about 10 ohms, so you will have a fault current of 23A This is not going to trip the OCPD for the circuit, nor is it going to trip the none existant at the moment, [or possibly faulty in future] RCD. So, you are in the house, what do you care?? You are on an insulated floor and exposed conductive bits and any extraneous ones have been bonded, you do not notice anything amiss. Meanwhile, a person or horse out in the yard touches the top metal part of any one of the eight sheds, Zap, full mains voltage appears across them until such times as the proposed [if it ever materialises] 300mA "S" type front end rcd decides it wants to open. You would not be a happy bunny!!!!

Is my thinking miles out?? Please tell me HONESTLY if you think i am worrying too much or talking rubbish, I will not be offended!!! [Remember the agreed definition of "competant" included knowing when to ask and not being afraid to!!]

Meanwhile, i was in the house yesterday. I was trying to explain the the owner about earthing when they asked if i could test in the house. Well ok, got my meter in the car, I asked them what earthing arrangements they had for the house, [i know full well it is TT, but i thought i would try to winkle some details out of them, as they built the thing themselves you see] and they said "an earth rod but it is "all covered over, but we think the wire has fallen off it anyway!!"

Ok........ so, i could not have a look for myself, as it is all boxed in, and so i thought i would have a test with the meter. Now yes, i do know that you need to make sure of proper bonding first, or otherwise the testing itself is liable to shock anyone that is in the house at the time. Still, i KNEW that there was no-one else there, so i tested Zs at a socket. The same around 10 ohms as the rest of the place.. Yep earth coming in down the SWA armour. I knew the place had only been built a few years ago, so i asked if they wanted the RCD's testing, they said ok then and they asked if i could test them/it at a socket too. Hmmmm, ok then.... better than nothing i suppose.

I asked if i could see the CU to see what protected where, so they showed me.. I was not able to take the cover off as it was rather inacccessable, and i did not have a screwdriver, but anyway, it is an insulated one. Ok, so on with testing the RCD [more in a second!] That went ok, but while i was doing it the owner was moaning on about "then trip things are awful sensitive moan moan" I asked him when the place had been wired and he assured me only a few years ago and by "proper electricians too". Trouble was, i was expecting to see a split load board, only this one, only had a single RCD, and worse, a gap in the middle, [no blanks] about 2 modules wide...... Ah........ I think that owing to these "blasted sensitive trips" some muppet has removed one and just bridged the busbar and neutral bars..... Oh deary me!!! I will have another look tomorrow......

john...

 

[Steve3948]

Well my thoughts are first all buildings should have RCD protection as a priority, also if they are periodically checked as required there should be no problems.

If you fulfill all your requirements you have done all you can to offer a safe installation.