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- Thread starter spadge47
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Leesparkykent
Well I wouldn't want a 32mm grommet on the bottom of a CCU in a cupboard that a kid could easily get in to.
I keep repeating the word REQUIRED because that is what is REQUIRED by BS7671.
I stand by my post, the regs never have "allowed" a separate cpc to enter an enclosure unless collectively surrounded by ferrous material.
Now if the regs "REQUIRE" this, then that is a different matter.
Look, the interpretation is all down to the language, and the language is subtle but, specific.
One must understand the differences.
You can only bring the additional protective conductor in through a different opening if said conductor is REQUIRED under BS7671 to be installed in parallel with an SWA, so it's NOT a red herring, read the reg FFS.
If you just decide that you are going to run a separate "earthing" conductor in parallel with an swa just because you fancy it, then it MUST enter the enclosure collectively surrounded by ferrous material.
IF you have sat down and done all the design calculations and these show that your SWA is inadequate as a cpc and you must run a separate "earthing" conductor then it is OK to enter through a separate entry into the ferrous enclosure, thus not collectively surrounded by ferrous material.
HOWEVER, if your calculations show that your SWA is inadequate as a cpc, then you must bring your design calculations into question as you must ensure that your circuit complies with BS7671, and if it requires a separate "earthing" conductor then it is doubtful it does.
Look into it, do the calcs and check it out, then look back at the reg and it's associated regulations.
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davesparks
I'm really not following you here? Are you suggesting that we need to take account of bad parents when installing CUs?
(It's hard to resist making pedantic comments about goats here)
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Leesparkykent
Haha... I don't see the harm of it being considered in a domestic situation for the sake of £13
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You need to look at the definitions of earthing conductor, bonding conductor and circuit protective conductor.
On any electrical installation there must only be one means of earthing, thus the earthing conductor connects the installation to the means of earthing.
Now that could be any of the supply systems.
However, said earthing conductor then becomes part of the "sub-main" supplying the distribution board at the origin of the installation.
Now, regardless of where the supply originates from, or the means of earthing goes to, the Line, Neutral and earthing conductor connected between the means of earthing for the installation and the MET of the "first" "DB" in the installation all form part of the same supply circuit, thus they all must collectively enter any ferrous enclosure.
The same applies to any outgoing circuits.
Now bonding conductors are something totally different, they exist simply to create an equipotential zone in the installation.
They are not there by design to carry any fault currents within a supply circuit.
The distinction MUST be understood.
Thus they do NOT form part of any supply circuit, thus they are not part of the requirements of 521.5.1.
Therefore the earthing conductor of an installation is not an equipotential bonding conductor, thus, there are differing requirements for the three kinds of conductor.
Come on this is basic stuff FFS.
On any electrical installation there must only be one means of earthing, thus the earthing conductor connects the installation to the means of earthing.
Now that could be any of the supply systems.
However, said earthing conductor then becomes part of the "sub-main" supplying the distribution board at the origin of the installation.
Now, regardless of where the supply originates from, or the means of earthing goes to, the Line, Neutral and earthing conductor connected between the means of earthing for the installation and the MET of the "first" "DB" in the installation all form part of the same supply circuit, thus they all must collectively enter any ferrous enclosure.
The same applies to any outgoing circuits.
Now bonding conductors are something totally different, they exist simply to create an equipotential zone in the installation.
They are not there by design to carry any fault currents within a supply circuit.
The distinction MUST be understood.
Thus they do NOT form part of any supply circuit, thus they are not part of the requirements of 521.5.1.
Therefore the earthing conductor of an installation is not an equipotential bonding conductor, thus, there are differing requirements for the three kinds of conductor.
Come on this is basic stuff FFS.
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davesparks
Why now though? For more years than I've been alive electricians have used a grommet or a bush for the job. If there was a problem with children (or goats) then I'm sure the IET would have given us a daft regulation to deal with it, they have after all create this nonsense about non combustible boards
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davesparks
The earthing conductor performs the functions of both cpc and bonding conductor. It is even sized with consideration to fulfilling both requirements.
It's late and I'm off to bed, you're not making sense to me.
Spin stated
You replied
That's quiet clear to me.
It's not uncommon for a separate cpc to be required with swa when you get to larger csa armoured.
Spin stated
You replied
That's quiet clear to me.
It's not uncommon for a separate cpc to be required with swa when you get to larger csa armoured.
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Leesparkykent
True but Its personal preference and Imo the wiska gland provides for better Ingress protection. Also the gland supports the tails which an open grommet wouldn't.
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So what does it bond dave?
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davesparks
Which regulation says there must only be one means of earthing?
They have Dave, openings to IP2X
I use 32mm grommets, once the tails are fitted they comply with that.
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davesparks
Depends on the earthing system I guess, but if it wasn't a bond then it wouldn't be required to comply with the requirements of a bond would it?
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davesparks
Yes it is uncommon, a seperate cpc is almost never required with SWA.
The only time a seperate protective conductor is normally required is to fulfill the requirements of a main bond.
It doesn't it must comply with the requirement of an earthing conductor, and depending on the means of earthing this has differing requirements, derived from ESQCR & BS7430.
Seriously?
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davesparks
Where PME conditions apply the earthing conductor must meet the requirements of a main bond.
For the other earthing systems the earthing conductor automatically meets the requirements obviously
It's not uncommon with larger sizes Dave, over 50 or 70mm.
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davesparks
It is uncommon for it to be required, this is not the same thing as not being uncommon to see one installed. If is very common to see a seperate cpc being installed unnecessarily
Now that is something I will agree to 1000%
If you read the actual words it says that it must meet the cross sectional area requirements for a main bonding conductor, not that it acts as one.
The weasel words are there to mean specific things, this is why it seems that there are so many interpretations of the regulations.
The IEC write them in a particular way to mean very specific things, the trouble that we have is that there are some changes when the document becomes an EN, then some more when it becomes a BS.
Also, the meaning of the words is exactly correct for what those who wrote them wanted them to say, however, they are the only ones who know what they wanted to say, the rest of us just have to guess!
Sometimes you need to look at the design intent of the regulation to see why they word it that way, and look back to try and second guess the FMEA's that have been done to come up with that regulation.
G
GEOFF S
I agree with dave, where a system is PME the eathing conductor it needs to meet the requirements for a bonding conductor also, size to the greatest, obvious reason's, broken pen, diverted neutral current will flow down it possibly, as it may the extraneous conductive parts.
Cheers
Cheers
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spadge47
Spoken to the NICEIC today. The context of the appropriate protective conductor in regulation 521.5.1 doesnt include a main earthing conductor as its not part of an a.c circuit in bs7671 as it is classed as a separate entity.
dont shoot me its just what ive been informed!
dont shoot me its just what ive been informed!
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davesparks
Forget the NICEIC for a minute and just think about the possibly scientific reasons behind this.
I am only basing this on my own conclusions about this. If a fault occurs which causes a significant current to flow though line and earth, then if they pass through seperate holes this will create its own significant eddy currents in the steel.
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spadge47
But the metal board will be earthed....
thats what i was getting at earlier.
there is no need to have the earth going in the same hole because it is at the same potential as the case.
unlike L and N which will be at 230+ potential to the case/earth
Potential of the enclosure has nothing to do with reducing eddy currents.
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spadge47
Its all starting to make sense... *eyes opened*
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spadge47
No but if the metal board is earthed it is exactly the same as having a cable running through the same hole as the metal surrouning the whole is the earth. Makes no difference.. Sorry for the terrible explanation but at least it makes sense in my head
No it's not, the metal enclosure would be at earth potential.
The CPC entering the enclosure would be carrying a fault current, and would be insulated from the enclosure as it passed through.
Thus there would be a potential between the case of the CU & the point at which the CPC passed through the case due to the resistivity of the cable.
Surely this is not that hard to grasp?
Now even though the cable resistivity would be low, hopefully the fault current would be high, which would make the eddy current generation high also, now this would generate a heating effect in the ferrous enclosure.
etc. etc.
Why try to find a way of not complying when complying is easier.
As far as the NIC technical helpline goes...
The CPC entering the enclosure would be carrying a fault current, and would be insulated from the enclosure as it passed through.
Thus there would be a potential between the case of the CU & the point at which the CPC passed through the case due to the resistivity of the cable.
Surely this is not that hard to grasp?
Now even though the cable resistivity would be low, hopefully the fault current would be high, which would make the eddy current generation high also, now this would generate a heating effect in the ferrous enclosure.
etc. etc.
Why try to find a way of not complying when complying is easier.
As far as the NIC technical helpline goes...
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spadge47
But we are on about the main tails and earthing conductor... Not a cpc from a circuit.
i could understand if a circuit from the board, but cant with the earthing conductor as i wouldnt class that as the appropriate protective conductor, as the main bonds are also assoiated etc..
i could understand if a circuit from the board, but cant with the earthing conductor as i wouldnt class that as the appropriate protective conductor, as the main bonds are also assoiated etc..
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Irrelevant. It is still the associated protective conductor, with the same effects.
Dear Lord,
Forgive me for my sins...
Forgive me for my sins...
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spadge47
Im going by what the niceic have stated.
Im sure there are plenty of people who believe they know better but as i work for a company registered in their scheme, i ultimately take their interpretation.
Thankyou to all who have posted their arguements, and its good to know that electricians differ completely.
ps. Hilarious comment -_-
Im sure there are plenty of people who believe they know better but as i work for a company registered in their scheme, i ultimately take their interpretation.
Thankyou to all who have posted their arguements, and its good to know that electricians differ completely.
ps. Hilarious comment -_-
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I don't think it's so much that electricians differ, it's that people have different levels of understanding. This has been an interesting thread to read and it certainly gets you thinking!
As ever, you shouldn't soley trust the advice of a forum and so I'll be getting the theory books out soon to make sure that I'm confident of my understanding.
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Paul.M
OK this thread has been reported due to a heightened state of debate that may turn personal. (Its not that bad btw lol)
I'll put 20 pence in the duke box so we can all listen to a nice bit of calming music, here goes:-
https://www.youtube.com/watch?v=OfJRX-8SXOs
I'll put 20 pence in the duke box so we can all listen to a nice bit of calming music, here goes:-
https://www.youtube.com/watch?v=OfJRX-8SXOs
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davesparks
Potential has got bugger all to do with it. The neutral is at substantially the same potential as the case and that doesn't stop eddy currents occurring.
It's actually quite a good thread paul.
Theres a bit of silliness and provoking language but it's not too bad and the main thing is everyone is actually contributing, unlike in the past where there would have been posts solely designed to cause offense.
Let's see if the music has helped :smile5:
ps... Although as for whether the Main CPC should go through the same hole as the tails I'm far more confused than when I first read the reg!! I think I'll stick to what the reg says and have all three together, thus it avoids any problems with those eddies and complies with BS7671.
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