what if you had two cpc
feed to DB has earth through a seperate bush to L&N????????
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Hi, Poni
Not going to shoot you down at all!! But the question in hand has nothing to do with the efficiency or otherwise of the earth connection, but the fact that for very sound engineering reasons [plus it says in the regs!!] ALL conductors in a circuit [including the CPC] MUST enter a ferromagnetic enclosure THROUGH THE SAME HOLE.
See 521.5.2
john...
Not going to shoot you down at all!! But the question in hand has nothing to do with the efficiency or otherwise of the earth connection, but the fact that for very sound engineering reasons [plus it says in the regs!!] ALL conductors in a circuit [including the CPC] MUST enter a ferromagnetic enclosure THROUGH THE SAME HOLE.
See 521.5.2
john...
but the trunking and enclosure are part of the circuit,
how do you get the sheath of a SWA into the enclosure when it is used as the CPC?
how do you get the sheath of a SWA into the enclosure when it is used as the CPC?
Brass is non-ferrous. Can you get ferrous bushes ?Brass bush
Not sure i understand... When a SWA goes into an enclosure all the conductors ARE going through the same hole, the armour [used as CPC is concentric with them] the gland acts as a continuation of the armour/CPC conductor as it goes through the hole, so all is well.
Even if you dispensed with the gland and stuffed the SWA through a hole and tied a knot in it, it would still comply, so long as, that is, you used a suitable grommet strip to line the hole and observed minimum bending radii when tying the knot!! Well, not really, but it sounded funny i suppose!!!!!!!!!!!!!
john...
Even if you dispensed with the gland and stuffed the SWA through a hole and tied a knot in it, it would still comply, so long as, that is, you used a suitable grommet strip to line the hole and observed minimum bending radii when tying the knot!! Well, not really, but it sounded funny i suppose!!!!!!!!!!!!!
john...
I have got some steel reducing bushes!
However if the brass bush is still in the wall of the steel enclosure then the "wire" is still entering the enclosure at basically "90 degrees"
---------- Post Auto-Merged at 15:54 ---------- Previous post was made at 15:52 ----------
Steps,
The wire armour should be terminated at the gland at the entry to the enclosure then the other conductors enter through the same hole.
This now brings up the question of an SWA with parallel cpc i.e. a g/y cable tied to the outside, this should according to the reg, enter through the SWA gland, which it cannot thus it must be terminated on the exterior of the enclosure if you take the reg word for word.
Nicky Tesla
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Your all missing the point with 521.5.2
If you supply the db with two single core swa cables through the same hole, by the fact they are swa, effectively means they have gone through different holes as they will be separated by the swa and eddy currents will be formed in the swa itself.
any cpc does not have to go through the same hole if the enclosure is bonded, as the metal the hole is cut into is at the same potential as the cpc and will, (being bonded), become part of the cpconductor, and any fault current to any circuit will also include a current flowing in the enclosure itself, which is why it does not matter which hole the cpc goes through as it becomes one, like a big terminal block.
If you supply the db with two single core swa cables through the same hole, by the fact they are swa, effectively means they have gone through different holes as they will be separated by the swa and eddy currents will be formed in the swa itself.
any cpc does not have to go through the same hole if the enclosure is bonded, as the metal the hole is cut into is at the same potential as the cpc and will, (being bonded), become part of the cpconductor, and any fault current to any circuit will also include a current flowing in the enclosure itself, which is why it does not matter which hole the cpc goes through as it becomes one, like a big terminal block.
Hi all, Errrm, No....
Scenario 1,
"If you supply the db with two single core swa cables through the same hole, by the fact they are swa, effectively means they have gone through different holes as they will be separated by the swa and eddy currents will be formed in the swa itself"
Single core SWA for an ac circuit is banned outright anyway... [by the same reg]
Scenario 2,
"any cpc does not have to go through the same hole if the enclosure is bonded, as the metal the hole is cut into is at the same potential as the cpc and will, (being bonded), become part of the cpconductor, and any fault current to any circuit will also include a current flowing in the enclosure itself, which is why it does not matter which hole the cpc goes through as it becomes one, like a big terminal block"
Yes the cpc will be sharing the fault current with other parallel paths, so i can see what you are saying, but there is no knowing what proportion, and so the cpc in a different hole MAY, and i say MAY [as i am no expert on this sort of thing] still be subject to damaging electromechanical forces as a result of not being grouped with the other circuit conductors in the event of a fault, which is one of the reasons for this reg, not just the elimination of eddy currents.
Apart from this, the regs are quite clear, they MUST all go through the same hole!!
john..
Scenario 1,
"If you supply the db with two single core swa cables through the same hole, by the fact they are swa, effectively means they have gone through different holes as they will be separated by the swa and eddy currents will be formed in the swa itself"
Single core SWA for an ac circuit is banned outright anyway... [by the same reg]
Scenario 2,
"any cpc does not have to go through the same hole if the enclosure is bonded, as the metal the hole is cut into is at the same potential as the cpc and will, (being bonded), become part of the cpconductor, and any fault current to any circuit will also include a current flowing in the enclosure itself, which is why it does not matter which hole the cpc goes through as it becomes one, like a big terminal block"
Yes the cpc will be sharing the fault current with other parallel paths, so i can see what you are saying, but there is no knowing what proportion, and so the cpc in a different hole MAY, and i say MAY [as i am no expert on this sort of thing] still be subject to damaging electromechanical forces as a result of not being grouped with the other circuit conductors in the event of a fault, which is one of the reasons for this reg, not just the elimination of eddy currents.
Apart from this, the regs are quite clear, they MUST all go through the same hole!!
john..
philbas
Well-known member
I have to agree the regs are clear, but just out of interest can anyone tell me what we are protecting from?
rubbish
if you use SWA the cpc doesnt even enter the enclosure, it simply becomes a part of it.
the reg you keep referring to is when the cable/s enter a metal enclosure,
they entered the metal enclosure when they entered the trunking,
Hi Sidewinder,
You say;
"This now brings up the question of an SWA with parallel cpc i.e. a g/y cable tied to the outside, this should according to the reg, enter through the SWA gland, which it cannot thus it must be terminated on the exterior of the enclosure if you take the reg word for word"
Well, no.... The reg actually says, "they shall be arranged such that the conductors are only COLLECTIVELY surrounded by ferrous material"
If you drill a hole for the gland, and then an adjacent hole for the CPC, and then join the two holes by means of a narrow slot, [to stop those nasty eddy currents!!] then i believe this achieves much the same thing. From an electrical point of view, it is still one hole, [thanks to the slot] but merely elongated!!!
[so far as i know!!!!]
john...
You say;
"This now brings up the question of an SWA with parallel cpc i.e. a g/y cable tied to the outside, this should according to the reg, enter through the SWA gland, which it cannot thus it must be terminated on the exterior of the enclosure if you take the reg word for word"
Well, no.... The reg actually says, "they shall be arranged such that the conductors are only COLLECTIVELY surrounded by ferrous material"
If you drill a hole for the gland, and then an adjacent hole for the CPC, and then join the two holes by means of a narrow slot, [to stop those nasty eddy currents!!] then i believe this achieves much the same thing. From an electrical point of view, it is still one hole, [thanks to the slot] but merely elongated!!!
[so far as i know!!!!]
john...
sparkytim
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Well Guys take a look at the new BGB and it has an ammendment to 521.5.1 on page 121
"These requirements do not preclude the use of an additional protective conductor in parallel with the steel wire armouring of a cable where such is required to comply with the requirements of the appropriate regulations in Chapters 41 and 54. It is permittted for such an additional protective conductor to enter the ferrous enclosure individually.
"These requirements do not preclude the use of an additional protective conductor in parallel with the steel wire armouring of a cable where such is required to comply with the requirements of the appropriate regulations in Chapters 41 and 54. It is permittted for such an additional protective conductor to enter the ferrous enclosure individually.
Hi Steps,
From the description given originally, it seemed to me that what they meant was that this was a DB sitting on top of trunking, and that the submain and cpc entered through a hole elsewhere on the DB, and then presumably, the cables disappeared of into the trunking and not the other way about.
Might be wrong though!!!! A photo is what is needed!!!
john..
---------- Post Auto-Merged at 16:54 ---------- Previous post was made at 16:53 ----------
Hi wozz,
521.5.2 "Single core cables armoured with steel wire or tape shall not be used for an AC circuit"
john..
---------- Post Auto-Merged at 16:58 ---------- Previous post was made at 16:54 ----------
Hi Sparkytim,
Well, i guess that puts an end to all that then!!!!!
We will all have to buy a new book now......[at a crazy price]
Thanks for posting that!! Are there any conditions attached to it though??
Well done that man!!!!!
john..
From the description given originally, it seemed to me that what they meant was that this was a DB sitting on top of trunking, and that the submain and cpc entered through a hole elsewhere on the DB, and then presumably, the cables disappeared of into the trunking and not the other way about.
Might be wrong though!!!! A photo is what is needed!!!
john..
---------- Post Auto-Merged at 16:54 ---------- Previous post was made at 16:53 ----------
Hi wozz,
521.5.2 "Single core cables armoured with steel wire or tape shall not be used for an AC circuit"
john..
---------- Post Auto-Merged at 16:58 ---------- Previous post was made at 16:54 ----------
Hi Sparkytim,
Well, i guess that puts an end to all that then!!!!!
We will all have to buy a new book now......[at a crazy price]
Thanks for posting that!! Are there any conditions attached to it though??
Well done that man!!!!!
john..
Nicky Tesla
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Hi errrm YE...
Yes this is what im saying!....THIS IS THE REASON!... you are not allowed to use single core swa, being single core the L & N are both surrounded by bonded armour and therfore have effectively gone through different holes.
sparkytim
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+1
BGB Pg 122
521.5.2 "Single-core cables armoured with steel wire or steel tape shall not be used for an a.c. circuit
Note The steel wire or tape armour of a single-core cable is regarded as a ferr0magnetic enclosure. For single-core armoured cables, the use of aluminium armour may be considered.
BGB Pg 122
521.5.2 "Single-core cables armoured with steel wire or steel tape shall not be used for an a.c. circuit
Note The steel wire or tape armour of a single-core cable is regarded as a ferr0magnetic enclosure. For single-core armoured cables, the use of aluminium armour may be considered.
b:O !!
ks :^O of course the cpc enters the c/u, as its the third core ie the LIVE NEUTRAL AND CPC inside the main part of the SWA:innocent:tongue in cheek---------- Post Auto-Merged at 17:34 ---------- Previous post was made at 17:31 ----------
but yet the dno are allowed to use concentric cable which effectively creates a single core armoued type cable, although it has the neutral on the outer cores/sheath if pme'd or tncs then the outer sheth part is at earth potential / neutral potential anyway, so therefore no difference in single core armoured.
i await the flying bullets :^O
OK healthy debate here!
Can we ensure it stays that way please!
Since my last post.
Herr Tesla,
This is to do with electro magnetism, nothing to do with earth potentials,
Also it is nothing to do with shared fault currents as there is scant ways for us to determine these.
There is a research paper out there regarding this which does make interesting reading, but this is not really the debate here.
App87,
Yes in fault conditions there can be significant electromagnetic forces, however, by design these should be limited to less than 5 seconds in duration, in many cases less than 0.4 or even 0.2 seconds.
There is a cahnce of mechanical damage, however, minimal it cannot be discounted.
Philbas,
Electro magnetically induced eddy currents which can cause heating of the ferrous material
Steps,
I disagree slightly they should pass through every ferrous "bulkhead" via the same hole IF the reg is relating to EM effects as it seems to be.
App87,
You would decrease the IP rating of the enclosure in this scenario so this may not be possible.
However, it is a recognised solution in certain environments.
Sparktim,
I have not looked at the bgb yet wrt this, I was basing this on the brb, thanks for that, it seems that the issue has been realised in a better light.
Wozz,
The reg that App87 has referred to disallows single core SWA for a.c., however in which case why is it listed in the tables of the book!
App87,
None of us are exactly aware of the situation with the OP.
I believe that to the letter of the reg the number you quote seems correct.
As far as the bgb goes, yes the new book is now in force and you must design to it as of the 1/1/12.
It has the same sort of handover as the bbb/brb.
Wozz,
Al armour does exist.
Herr Tesla,
This is a reinforcement of the reg, so Sir, you are correct.
sparkytim,
Correct Sir.
Badger,
Stop winding us up please
The DNO do not work to 7671, is this DNO cable ferrous armour?
There is a difference to single core armoured if we consider single phase then the N return is via the armour which would be terminated at the ferrous enclosure containing the L conductor.
BTW if anyone else has posted since I started writing then this may seem a bit "wrong"!
Can we ensure it stays that way please!
Since my last post.
Herr Tesla,
This is to do with electro magnetism, nothing to do with earth potentials,
Also it is nothing to do with shared fault currents as there is scant ways for us to determine these.
There is a research paper out there regarding this which does make interesting reading, but this is not really the debate here.
App87,
Yes in fault conditions there can be significant electromagnetic forces, however, by design these should be limited to less than 5 seconds in duration, in many cases less than 0.4 or even 0.2 seconds.
There is a cahnce of mechanical damage, however, minimal it cannot be discounted.
Philbas,
Electro magnetically induced eddy currents which can cause heating of the ferrous material
Steps,
I disagree slightly they should pass through every ferrous "bulkhead" via the same hole IF the reg is relating to EM effects as it seems to be.
App87,
You would decrease the IP rating of the enclosure in this scenario so this may not be possible.
However, it is a recognised solution in certain environments.
Sparktim,
I have not looked at the bgb yet wrt this, I was basing this on the brb, thanks for that, it seems that the issue has been realised in a better light.
Wozz,
The reg that App87 has referred to disallows single core SWA for a.c., however in which case why is it listed in the tables of the book!
App87,
None of us are exactly aware of the situation with the OP.
I believe that to the letter of the reg the number you quote seems correct.
As far as the bgb goes, yes the new book is now in force and you must design to it as of the 1/1/12.
It has the same sort of handover as the bbb/brb.
Wozz,
Al armour does exist.
Herr Tesla,
This is a reinforcement of the reg, so Sir, you are correct.
sparkytim,
Correct Sir.
Badger,
Stop winding us up please
The DNO do not work to 7671, is this DNO cable ferrous armour?
There is a difference to single core armoured if we consider single phase then the N return is via the armour which would be terminated at the ferrous enclosure containing the L conductor.
BTW if anyone else has posted since I started writing then this may seem a bit "wrong"!
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