Electrical Setup in Farm Outbuildings

[Defcon]

I'm just looking for opinions here on my uncle's farm. He had several new large outbuildings erected within the past three years. At the source, the sparky fitted an 80A 30mA RCD as the main isolator which protects all the other outbuildings at one part of their farm.

From here, they take a supply from an mcb to another building which has an enclosure with a 100A 2-pole isolator. This shed then has a supply taken from it to another outbuilding. i.e A-B, B-C,C-D.

The problem here, is the 30mA RCD. If damp etc gets into a light or switch, the other lot goes down and their well pump goes out to and thats a big problem during frosty weather cause if the power is off, the pump will freeze.

The sparky that did the job is now working in China but I asked another sparky who was doing work at a factory I was visiting and he said their isn't much that can be done as it has to be on a 30mA RCD since its TT.

I have a good enough knowledge of understanding electrical work but there are areas I am not clued up on. Cheers

 

[monkeyelectric]

I'm just looking for opinions here on my uncle's farm. He had several new large outbuildings erected within the past three years. At the source, the sparky fitted an 80A 30mA RCD as the main isolator which protects all the other outbuildings at one part of their farm.

From here, they take a supply from an mcb to another building which has an enclosure with a 100A 2-pole isolator. This shed then has a supply taken from it to another outbuilding. i.e A-B, B-C,C-D.

The problem here, is the 30mA RCD. If damp etc gets into a light or switch, the other lot goes down and their well pump goes out to and thats a big problem during frosty weather cause if the power is off, the pump will freeze.

The sparky that did the job is now working in China but I asked another sparky who was doing work at a factory I was visiting and he said their isn't much that can be done as it has to be on a 30mA RCD since its TT.

I have a good enough knowledge of understanding electrical work but there are areas I am not clued up on. Cheers

That damn E54! You could put a time delayed 100mA upfront ,then submain before the 30mA rcd, then do the same at each of the other sheds with each having its own earth stake, good 5/8" rods coupled together would be best and link them all together, that shoud give you a decent Ra

 

[Defcon]

In other words, your saying to fit a 30mA RCD in each enclosure with the 100A Isolator. Like a split board with the 100A Isolator and mcb for supply to other shed, on one half and the 30mA RCD on the other half with that sheds circuits protected by it?

By linking the rods, I take it the armour/cpc of the swa between the sheds is the 'link'?

 

[monkeyelectric]

In other words, your saying to fit a 30mA RCD in each enclosure with the 100A Isolator. Like a split board with the 100A Isolator and mcb for supply to other shed, on one half and the 30mA RCD on the other half with that sheds circuits protected by it?

By linking the rods, I take it the armour/cpc of the swa between the sheds is the 'link'?

Yep all sounds good to me

 

[KAS1]

I think Defcon is a budding electrician perhaps just starting out on his own lol

 

[Defcon]

Nope, far from it.

Only problem now is if his enclosures have room for a split.

 

[Norwich]

Yes i agree with that, but why can't people do it properly in the first place...... stupid question I know the answer to that.

 

[monkeyelectric]

I think Defcon is a budding electrician perhaps just starting out on his own lol

I don't know about budding electrician but he is an ignorant git! I give advice and not a word of thanks! Don't know why i bother

 

[Defcon]

I wasn't finished yet and was still open to opinions from other people. On another electrial forum, a few sparks there have stated not to use a 100mA RCD but a 300mA S Type instead. I mentioned the earth rod at each shed to help with the Ra and they joked.

No point thanking you now after 'your' insult. btw, I am doing this between flights so my responses will be delayed.

 

[derek]

Your RA value will determine the RCD, but a time delayed RCD up front should be installed. Any idea what the Ra value is?

 

[monkeyelectric]

I wasn't finished yet and was still open to opinions from other people. On another electrial forum, a few sparks there have stated not to use a 100mA RCD but a 300mA S Type instead. I mentioned the earth rod at each shed to help with the Ra and they joked.

No point thanking you now after 'your' insult. btw, I am doing this between flights so my responses will be delayed.

Well why don't you just read the regs book and determine for yourself what it needs instead going from forum to forum getting different peoples opinions! but i know what i would do, i assume your not an electrician though

 

[Engineer54]

Let's get one thing clear straight away, ...it wasn't ME!! lol!!

Frankly, from what i can make of this installation, it's a mess, and has been designed and installed by someone that really hasn't a bloody clue!! One 30mA RCD to protect several buildings circuits on a farms TT system, ...Jesus!! And what happens if that single device ever fails?? lol!!

It doesn't matter much whether you use a 100mA or 300mA up front S type RCD, probably in this case better to utilise a 300mA if the the source supply is feeding several buildings. The problem then comes from discrimination between the buildings!! It would have been better to of had 2 DB's at source, one to supply the sub-mains to these other outbuildings and the other to supply the final circuits installation within the source building.


Anyway it seems that each supply is piggy backed off the DB's in other out buildings, which is going to make things very difficult unless you split these supply sub-main cables away from the individual outbuildings final circuit DB installation (which should all be provided, with 30mA protection), leaving the sub-main cables protected only by the 300mA source RCD, (final distribution circuits will have both 30mA and the 300mA back-up protection)


Earthing wise, there should be at least one deep driven earth electrode position at each of these outbuildings, connected to the EMT of each building, which should in turn be linked together preferably by a 3rd core in the SWA sub main cables, or via the armours. If these outbuildings are steel constructions, even better (depending on construction method) but first check if the steelwork is providing a decent Ra value and if so, link it with any made deep driven electrode position.


Basically without going in to too much depth, (not feeling too great today lol!!) the outbuildings 30mA final circuit protection should not be in series with the other upstream buildings.

 

[Defcon]

@derek I don't know the Ra but will have to find out.

@Engineer54 Many thanks for the detailed reply. The sheds (all are 100ft or more long) are all steel girder constructed with the post/trusses in each shed joined like one big steel frame.

 

[Engineer54]

@derek I don't know the Ra but will have to find out.

@Engineer54 Many thanks for the detailed reply. The sheds (all are 100ft or more long) are all steel girder constructed with the post/trusses in each shed joined like one big steel frame.

It depends more on whether the posts have been driven into the ground, or concreted into the ground, as to whether they will provide an additional earthing facility (even better if a steel re-bar mesh has been laid in the floor slab).

Whether they do or not, at the very least they will almost certainly be earthy, (extraneous earth) so these steel framed buildings will need to be bonded anyway, to the buildings MET/EMT positions!!

It will be interesting to know what sort of Ra we are talking about, on this existing installation??