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Topic Title: RCDs in agricultural premises
Topic Summary: 705.411.1(iii)
Created On: 02 August 2012 08:43 PM
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 02 August 2012 08:43 PM
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weirdbeard

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Sorry if this is a daft question, but just wondering if supply tails are exempt from the requirement for RCD protection in agricultural premises, as it seems to me no matter how short the tails might be between the suppliers meter and an RCD device these tails won't be RCD protected, but would still constitute a circuit subject to BS7671?
 02 August 2012 08:47 PM
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slittle

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And I can show you many a site when the rodent population has visited the external meter box and chewed through both layers of insulation

I suppose it comes down to the fact that a P-P or P-N short (if the tails were that close to each other) would clear the cutout fuses and the chances of P-E should be remote given the RCD is normally in a plastic enclosure and you would expect a lump of fire rated chipboard behind the meter (rather than any earthy metal work)


Stu
 02 August 2012 08:59 PM
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weirdbeard

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Thanks for the reply Stu, wasn't expecting such a quick reply, been called in for tea! Though I have seen a pig farm where the rats had completely stripped off all the 3phase tails insulation, I was more wondering in a un-ratted install whether the lack of RCD to tails would be an departure on an EIC, or a codable observation on an inspection?
 02 August 2012 09:14 PM
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slittle

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If it had livestock or produce and didn't have a front end RCD then I'd code it.

Some of my sites are now only farms by name,having no animals and all industrial units I then tend to look at it a bit different as it's no longer an agricultural installation in my eyes, they are only industrial estates in the countryside.

Stu
 02 August 2012 10:42 PM
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UKPN

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you can always put your tails in trunking, but how it works from a DNO/
meter operator point of view is this--

when the polarity test of the consumers installation is done it is at the M/SW position (incoming) that tail is then marked up at the meter position, (3phase, rotation as well).

so the wiring regs come into effect FROM the M/SW position.

PNB--beware of imitations.

Regards.
 02 August 2012 11:52 PM
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AJJewsbury

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but would still constitute a circuit subject to BS7671?

I agree it seems poorly worded. While the intention seems ideal enough, in practice it seems impossible to comply unless the supplier substitutes an RCCD for their isolator, as we're inevitably provided with a terminals rather than cables at the end of the supplier's equipment.
- Andy.
 03 August 2012 07:51 AM
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slittle

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Originally posted by: UKPN

you can always put your tails in trunking, but how it works from a DNO/

meter operator point of view is this--



when the polarity test of the consumers installation is done it is at the M/SW position (incoming) that tail is then marked up at the meter position, (3phase, rotation as well).



so the wiring regs come into effect FROM the M/SW position.



PNB--beware of imitations.



Regards.



Unless it's really well sealed, and I mean really well. Tails in trunking is likely to be more of a feast for my little rodent cousins than in free air.

They tend to chew them to get past them or through a hole.

Stu
 03 August 2012 01:34 PM
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OMS

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PNB--beware of imitations.


Indeed - and beware of misinformation, misinterpretation and misunderstanding from people who really should know better !! -

OMS

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Failure is always an option
 03 August 2012 05:35 PM
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UKPN

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--exactly! PNB, as widely informed is not TN-S but similar
to TNC-S

Regards.
 03 August 2012 05:40 PM
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OMS

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Really - then I guess we are going to disagree on that - and it's going to cause a bit of alarm in the explosive and petrochem sectors who are well used to a transformer supplying a switchboard containing a single N-E bond and calling it PNB - which has the characteristics of TN-S - cos TN-C-S is sehr verboten

Regards

OMS

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Failure is always an option
 03 August 2012 07:06 PM
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weirdbeard

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Originally posted by: AJJewsbury

but would still constitute a circuit subject to BS7671?


I agree it seems poorly worded. While the intention seems ideal enough, in practice it seems impossible to comply unless the supplier substitutes an RCCD for their isolator, as we're inevitably provided with a terminals rather than cables at the end of the supplier's equipment.



Andy thanks for the reply, that was what I was thinking too, no wonder Perspicacious has yet to issue a satisfactory report - as it seems sometimes it's not always possible to comply with the regs as they are written!
 03 August 2012 07:07 PM
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weirdbeard

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To All,

This post is a follow on from my recent grain dryer topic, the farm have decided to go for a new transformer and the dno are going to re-route their cable to a new position adjacent to the main load, so the meters and switch gear will be housed in a new dedicated breeze-block switch room - so, the thought behind my question was to try and establish wether it would be ok to consider the dedicated switch-room as being a non-agricultural zone, with a view to planning the best implentation of RCD protection for all outgoing circuits, and also the issue of IP ratings for the electrical equipment within the switch room, as per 705.512.2

So to summarise the above waffle the question is; if there's a dedicated purpose built switch room, is it ok to treat it as as a non agricultural zone, as long as outside that zone section 705 is complied with?

Thanks
 03 August 2012 07:18 PM
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weirdbeard

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Originally posted by: UKPN

you can always put your tails in trunking,


BS7671 section 705 doesn't seem to give an exemption for tails in trunking?




so the wiring regs come into effect FROM the M/SW position.



Only if the main switch happens to be an isolation switch supplied by the electricity supplier, otherwise BS7671 applies from the meter terminals.

Thanks for the reply anyway!
 03 August 2012 09:12 PM
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slittle

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Not wishing to design it for you, but I'd put a 300ma TD RCD as the incoming device in the kiosk and go from there.

Bear in mind you won't get a PME connection anyway so you've got to TT it and the first protective device on a TT has to be an RCD.


Stu
 03 August 2012 10:15 PM
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UKPN

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--I think you are worrying too much about this "tails between
the meter and the M/SW, or as you say isolator" issue.

I know OMS will reprimand me over the PNB thing but many
farms, fed from O/H TXs with this type of earthing do not
have this problem.
that is because the farm S/W gear can sit on top of steel
trunking alongside the meter and with the cut-outs underneath
the trunking.
the "tails" are not exposed and should an earth fault arise, because
the SW gear is solidly earthed to the boards distribution system.

from a steel M/SW an earth-fault relay device feeds the grain drying
equipment.

a 100ma/300ma feeds the sockets/lighting etc.

i have digressed, i will look up the DNOs rules for tails between the
meter and 1st point of connection.

we have many situations where this happens in TT situations but the
DNO have steel equipment for services and metering.

if the tails are in free air, i still dont see a problem, if the tails are in
PVC trunking and double insulated before the RCCD/EARTH FAULT
RELAY i am sure the board are satisfied for ESQCR reasons.

it may well be you list it as a departure--

Regards.
 01 October 2012 03:46 PM
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AJJewsbury

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I agree it seems poorly worded. While the intention seems ideal enough, in practice it seems impossible to comply unless the supplier substitutes an RCCD for their isolator, as we're inevitably provided with a terminals rather than cables at the end of the supplier's equipment.

I've been thinking again about this one. 705.411.1 (by the numbering) is modification to the requirements of section 411 - i.e. ADS.

If the supply tails etc were protected by double/reinforced insulation (not uncommon), they'd be covered by section 412 rather than 411, so 705.411.1 wouldn't apply to them.

It'd be debatable whether it was entirely double insulated though - what happens with faults internal to the DB for instance? Could a fault from the supply tails energise a c.p.c. or earth bar for instance? Some suppliers produce TT kits for metal CUs to provide additional insulation prior to the incoming RCCD main switch - so perhaps a similar approach might be considered.

A 300mA device in a insulated enclosure with no earth connections within, and then double insulated tails (either insulated+sheathed or insulated + plastic trunking etc) to the meter seems like it might comply with the letter of the regs.

You're just as stuck if the supplier decides to use metal trunking around the meter though

- Andy.
 02 October 2012 10:55 PM
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weirdbeard

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Hi Andy, really appreciate your additional thoughts to this issue..... as you say the requirement in question does seem to apply to the ADS factor, my question now is to split tails of a circuit over 100A in two can you get henly type blocks >100A or will to I need a busbar chamber?
 03 October 2012 12:27 PM
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AJJewsbury

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my question now is to split tails of a circuit over 100A in two can you get henly type blocks >100A or will to I need a busbar chamber?

Not sure to be honest - I've never worked on anything over 100A. You might be able to do something with DIN rail mounted stud terminals, or others might have a better idea.

Is this before or after your front-end RCD? (it's just that I imagine that most busbar chambers will be metal cased, taking you back to square 1 on your original problem again if it's before).

- Andy.
 03 October 2012 09:39 PM
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slittle

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Last time I done it, RS do some really nice din rail mounted studs capable of about 300Amps.

4 of them with separating barriers in a nice enclosure did the trick.

Not sure why you want to split the tails. Why not go into a 4 way board and do it in there.

Depending on your outgoing loads you may get away with something like a Merlin type B board as they are rated at 250 amps incoming with I think a max of 63A outgoing. If not a small panel board with MCCB's ??

Stu
 04 October 2012 08:41 PM
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weirdbeard

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Thanks for replies, not entirely sure what I will be faced with until I get there, info is a bit hard to come by so far, but will hopefully get some details tomorrow, as in earthing arrangements, switching etc.

I was just mulling ideas for not having one rcd upfront, but having one dedicated non time delayed rcd for this new machine and a time delayed one for the other stuff, which will allow down-stream 30mA rcd, hence the splitting idea.
IET » Wiring and the regulations » RCDs in agricultural premises

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