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Topic Title: Regulation 411.5.1
Topic Summary: Why?
Created On: 03 May 2013 10:34 AM
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 03 May 2013 10:34 AM
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AJJewsbury

Posts: 11251
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A recent discussion got me looking at regulation 411.5.1 again:

Every exposed-conductive-part which is to be protected by a single protective device shall be connected, via the main earth terminal, to a common electrode. However, if two or more protective devices are in series, the exposed-conductive-parts may be connected to separate earth electrodes corresponding to each protective device.


Can anybody enlighten me as to what this regulation is trying to achieve?

I can't be to ensure that two different exposed-conductive-parts that might be simultaneously accessible are connected together, as that's already covered by the second paragraph of 411.3.1.1 and the introduction of a second protective device would avoid the requirement anyway.

RCDs (or indeed fuses/circuit breakers) don't care about the route the fault current takes to earth as they only monitor the live (or line) conductors.

I guess it makes it easier to test and state a single Zs for a circuit, but that seems a poor reason restricting designs in this way.

I can see that it could be quite useful to have a circuit part-TN and part-TT with one 30mA device covering both - e.g. a circuit running through a house but extending outside. The reg could be satisfied by installing a second RCD at the point the TN earth is dropped, but I can't see any physical advantage for doing so.

The only thing I've been able to think of where the described arrangement makes sense is if the protective device was a voltage-operated ELCB - where the c.p.c. must be routed back through the protective device to provide protection - but they've not been recognised by the regs for donkey's years. Surely it can't be a hang-over from such a long time ago?

What would be dangers be if we didn't comply with 411.5.1? Would it be justifiable just to note it as a deviation (implying no safety issue) and leave it at that?

- Andy.
 03 May 2013 03:52 PM
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DOUGIE1000

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As electrode im asuming its TT, is it not explaining that exposed conductive parts must be earthed. im amusing the protective devise is more referring to supply tails for example,

-------------------------
Dougie
Power Plus Electrical.co.uk

My mission is to live as long as possible......so far so good!
 03 May 2013 04:03 PM
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dg66

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It relates to seperate electrodes forming part of a TT system ,ie a sub main to a remote building having its own electrode and not relying on the main electrode at the supply origin.Thats how it looks to me anyway.
I dont see where TN systems can be relevant as 411.5.1 is under the TT system heading.

-------------------------
Regards

Dave(not Cockburn)
 03 May 2013 04:10 PM
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weirdbeard

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Originally posted by: AJJewsbury

A recent discussion got me looking at regulation 411.5.1 again:



Every exposed-conductive-part which is to be protected by a single protective device shall be connected, via the main earth terminal, to a common electrode. However, if two or more protective devices are in series, the exposed-conductive-parts may be connected to separate earth electrodes corresponding to each protective device.




Can anybody enlighten me as to what this regulation is trying to achieve?


Not me! But anyway......some blundering thoughts.......




I can see that it could be quite useful to have a circuit part-TN and part-TT with one 30mA device covering both - e.g. a circuit running through a house but extending outside. The reg could be satisfied by installing a second RCD at the point the TN earth is dropped, but I can't see any physical advantage for doing so.


Just wondering if you had part-TN / part-TT, would 411.5.1 apply, as the reg is only relevant to TT only systems, ie if you had an RCD in the DB on the TN side of things, you wouldn't necessarily need another RCD to satisfy the regs - an example, (maybe?), you want to TT a garden lighting circuit, where the supply is taken from a fused spur off a TN system 30mA RCD protected ring main?

The only thing I've been able to think of where the described arrangement makes sense is if the protective device was a voltage-operated ELCB - where the c.p.c. must be routed back through the protective device to provide protection - but they've not been recognised by the regs for donkey's years. Surely it can't be a hang-over from such a long time ago?


How about a TT house, with an outbuilding supplied via overhead singles? (although thinking about it 1 RCD at the origin would provide protection)



What would be dangers be if we didn't comply with 411.5.1? Would it be justifiable just to note it as a deviation (implying no safety issue) and leave it at that?
.


I'm minded of the regs that say power, eg non-lighting circuits must be wired in 1.5mm even if they are only powering a few watts of load, and that on a farm you can't use 30mA SRCDs unless the circuits supplying the SRCDs are also 30mA protected - some things just don't seem to make sense!
 03 May 2013 04:20 PM
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AJJewsbury

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Sorry, yes, it's part of the TT requirements (411.5).

is it not explaining that exposed conductive parts must be earthed

I thought that 411.3.1.1 already said that for ADS in general - this seems to be saying specifically that I must have one single electrode unless I have another protective device (usually an RCD according to the next reg).

amusing the protective devise is more referring to supply tails for example

I'm not sure about that - the whole section is about achieving ADS within certain time scales (e.g. 0.2s).

It relates to seperate electrodes forming part of a TT system ,ie a sub main to a remote building having its own electrode and not relying on the main electrode at the supply origin.

They might have had that in mind, but it's an odd way to write it down - there's no mention of buildings (unlike 411.3.1.2) and seems like a very blunt instrument when you start thinking of outdoor electrics.

- Andy.
 03 May 2013 04:43 PM
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AJJewsbury

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Just wondering if you had part-TN / part-TT, would 411.5.1 apply, as the reg is only relevant to TT only systems

I think it would - as I read it 411.4 (TN) would apply to the TN bit and 411.5 (TT) to the TT bit - I'm sure there used to be a reg that said something to that effect, but I can't find it now...

How about a TT house, with an outbuilding supplied via overhead singles? (although thinking about it 1 RCD at the origin would provide protection)

That's one example of my thought - if the outbuilding has its own rod does it really need its own RCD as well if the upstream one already provides protection?

- Andy.
 05 May 2013 10:31 AM
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Christof

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Is it not just pointing out that in a TT the exposed conductive part's may be connected to more than one Earthing Conductor within the installation.
 05 May 2013 11:19 AM
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John Peckham

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I am also thinking of a distribution circuit connected to the suppliers TN-C-S electrode (the neutral) supplying a remote building where the installation in that building is connected to an earth electrode only separating the two means of earthing.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 05 May 2013 01:29 PM
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Zs

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Originally posted by: AJJewsbury

Every exposed-conductive-part which is to be protected by a single protective device shall be connected, via the main earth terminal, to a common electrode. However, if two or more protective devices are in series, the exposed-conductive-parts may be connected to separate earth electrodes corresponding to each protective device.


Interesting Andy, I read that clause with my installers head on and not as an instruction but as permission. Thus providing the installer with the option of a separate earth electrode in circumstances where a cable route back to the MET is impossible, long or difficult for example. Provide the protective devices as stated and you may knock in a rod for your exposed-conductive parts.

gotta go to work.

Zs
 05 May 2013 04:53 PM
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ebee

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I think that Zs has it.

a Permission to do so. Not an instruction.

an MET (or EMT) for each protective device , ie not separates but in groups.

If groups are paralleled then one electrode (or group of electrodes) per RCD but in series each group may have its own but does not have to, the most upstream one will suffice.

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 07 May 2013 11:08 AM
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AJJewsbury

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I think that Zs has it.

a Permission to do so. Not an instruction.

Yes - I agree that's what the 2nd sentence says, it's the first sentence - 'thou shalt have have all exposed-conductive-parts protected by the same device connected to the same electrode' bit that puzzles me.

Why can't I have several electrodes? Why is having an additional electrode conditional on having an additional RCD?

Can any of our history keepers trace the wording back to pre-RCD days? I'd be interested if it existed in the regs that allowed voltage-operated ELCBs.

- Andy.
 07 May 2013 01:14 PM
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ectophile

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Here's my thought on the subject.

Imagine a house where the electricity supply (let's say TN-S) and the gas supply enter at the front of the house, and the water supply enters at the back. Both the gas and water are supplied in metal pipes, and so are extraneous conductive parts.

The consumer unit will be connected to the supplier's earth, and it's easy enough to bond the gas supply to it as well.

But what about the water pipe? Easy - just whack an earth rod into the back garden, and bond the water pipe to that!

My reading is that 411.5.1 disallows such a "solution", whilst still allowing an outbuilding to be TT'd if necessary by adding an additional RCD.

-------------------------
S P Barker BSc PhD MIET
 07 May 2013 01:25 PM
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Christof

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Originally posted by: AJJewsbury

I think that Zs has it.



a Permission to do so. Not an instruction.


Yes - I agree that's what the 2nd sentence says, it's the first sentence - 'thou shalt have have all exposed-conductive-parts protected by the same device connected to the same electrode' bit that puzzles me.

Its just stating that ECP's protected via a single CPD shall be connected to the MET, the Met connected to an Electrode.If you have CPD in series, then you may segregate the two sets of ECP and connect to two separate Electrodes which are not common.



Why can't I have several electrodes? Why is having an additional electrode conditional on having an additional RCD?

Maybe it's how you define Electrode, this could be multiple Rods, Mat's, Tape or all three connected together. If I have two separate Electrode's for two CPD's in series but then decide to link them, I now have one Electrode, that's my take on it any how

Chris



Can any of our history keepers trace the wording back to pre-RCD days? I'd be interested if it existed in the regs that allowed voltage-operated ELCBs.

Can't help on that one?


- Andy.
 07 May 2013 01:31 PM
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AJJewsbury

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But what about the water pipe? Easy - just whack an earth rod into the back garden, and bond the water pipe to that!

My reading is that 411.5.1 disallows such a "solution",

Nice try, but 411.5.1 only talks about exposed-conductive-parts, not extraneous-conductive-parts, so doesn't really disallow that (but 411.3.1.1 would if they were simultaneously accessible).

whilst still allowing an outbuilding to be TT'd if necessary by adding an additional RCD.

but when is an additional RCD necessary? - 411.5.1 seems to say I need an extra RCD regardless if I want one section earthed via a separate rod - even if there's already an upstream device with suitable characteristics.

- Andy.
 07 May 2013 01:34 PM
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Christof

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Originally posted by: AJJewsbury

But what about the water pipe? Easy - just whack an earth rod into the back garden, and bond the water pipe to that!



My reading is that 411.5.1 disallows such a "solution",


Nice try, but 411.5.1 only talks about exposed-conductive-parts, not extraneous-conductive-parts, so doesn't really disallow that (but 411.3.1.1 would if they were simultaneously accessible).



whilst still allowing an outbuilding to be TT'd if necessary by adding an additional RCD.


but when is an additional RCD necessary? - 411.5.1 seems to say I need an extra RCD regardless if I want one section earthed via a separate rod - even if there's already an upstream device with suitable characteristics.

Andy 411.5.1 mention's no RCD?



- Andy.


Chris
 07 May 2013 01:39 PM
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AJJewsbury

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Maybe it's how you define Electrode, this could be multiple Rods, Mat's, Tape or all three connected together. If I have two separate Electrode's for two CPD's in series but then decide to link them, I now have one Electrode, that's my take on it any how

Agreed.

Its just stating that ECP's protected via a single CPD shall be connected to the MET, the Met connected to an Electrode.If you have CPD in series, then you may segregate the two sets of ECP and connect to two separate Electrodes which are not common.

Indeed - my reading is the same. So why can't I have a series of METs & corresponding means of earthing if I have one common protective device? Do RCDs 'care' which electrode the fault current goes through?

- Andy.
 07 May 2013 01:42 PM
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AJJewsbury

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Andy 411.5.1 mention's no RCD?

OK, it mentions 'protective device' in the context of ADS, which for 99.9% of practical purposes will be an RCD.
Apologies if my abbreviation caused any confusion.

- Andy.
 07 May 2013 02:07 PM
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Christof

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Originally posted by: AJJewsbury

Maybe it's how you define Electrode, this could be multiple Rods, Mat's, Tape or all three connected together. If I have two separate Electrode's for two CPD's in series but then decide to link them, I now have one Electrode, that's my take on it any how


Agreed.



Its just stating that ECP's protected via a single CPD shall be connected to the MET, the Met connected to an Electrode.If you have CPD in series, then you may segregate the two sets of ECP and connect to two separate Electrodes which are not common.


Indeed - my reading is the same. So why can't I have a series of METs & corresponding means of earthing if I have one common protective device? Do RCDs 'care' which electrode the fault current goes through?

Well by separating the two groups of ECP's, then in effect we have two electrical installations, the Origin of the latter in the series having no CPD or isolation etc?



- Andy.


Chris
 07 May 2013 02:10 PM
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Parsley

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In a TT installation that has several local DB's including RCD's for the final circuits, the chances of the additional remote earth rods being removed or damaged might be less likely than where there is only one RCD at the origin of the installation.

Regards
 07 May 2013 02:30 PM
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AJJewsbury

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Well by separating the two groups of ECP's, then in effect we have two electrical installations, the Origin of the latter in the series having no CPD or isolation etc?

Perhaps so (and wanting to avoid going off at a tangent about the definition of an electrical installation!) - although 411.5.1 doesn't seem to address that - e.g. a single pole switching RCBO (or even a fuse if Zs was low enough) would be acceptable as a protective device for ADS for TT (so satisfying 411.5.1), but isolation would have to be DP for TT, so that problem (if it is one) would still be outstanding.

As for not having a CPD - there's a general principle (e.g. 434.2.2) for faults etc. that devices aren't needed where an upstream device gives protection - so why the difference for ADS?

- Andy.
IET » Wiring and the regulations » Regulation 411.5.1

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