feed to DB has earth through a seperate bush to L&N????????

[james KEI]

Hi ppl,

A DB sitting on metal trunking, it's 25mm bush/lock ringed on the trunking, the submain feeding it has L+N through it's own bush and the Earth goes through a seperate bush with the circuits coming off that board.

I remember one of the comapny's i used to work for pulling this up on PIRs and we used to make sure they all went through the same hole in remedial work. (so that the main feed L('s)+N+E all through the same hole)

Is this against regs, or against good practice. i'm having trouble finding a specific reg no. wanted to double check so that i can back it up before i flag it up.

thanks for any help!!

 

[SLIPSHOD & SLAPDASH]

There is nothing wrong with this at all. No regs broken & not even classed as bad workmanship. In fact years ago on large jobs all the earths were brought in together seperate from other cables. Only L & N should be kept together to avoid Eddy currents.

 

[sparkytim]

You may be thinking of L and N going thru different holes. This is a NO NO unless a slot is cut between them OR its non ferrous enclosure.

 

[kerching]

I'd like to know what Code they put against this on a PIR... :coat

 

[kme]

+1 to all the above

 

[apprentice87]

Hi all,

I am going to well and trully stick my newbie neck out again...Try to be kind to me!!

As far as i am aware, this sort of thing is definitely NOT allowed!!!!!

Reg 521.5.2 states; "Single-core cables armoured with steel wire or tape shall not be used for an ac circuit" [so that is them out of the way]

But what about other conductors??? well, the paragraph goes in to state;

"The conductors of an ac circuit installed in a ferromagnetic enclosure shall be arranged so that all the line conductors and the neutral conductor, if any, and the appropriate protective conductor are contained in the same enclosure"

Well, yes, the arrangement the original poster described complies with this; they all [the conductors] end up in the same box...

BUT, the next sentence reads;

"Where such conductors enter a ferrous enclosure [remember they have already specifically mentioned "protective" conductors] they shall be arranged such that the conductors are only collectively surrounded by ferrous material"

In other words, they must all go through the same hole!!

Remember now, they have used the word "circuit", so obviously, [before anyone thinks of it!] they do not mean that EVERY cable that enters an enclosure must all go through the same hole, but that every cable associated with a particular CIRCUIT [including the cpc] must all go through the same hole!!!!

"hole" by the way, so far as i know, [and as has already been pointed out] can include a series of holes joined by a slot..

john...

 

[philbas]

While I agee with the points made and the regs do say that, however they also fail to define a Ferromagnetic enclosure.

A quick google tells me it would be made of Iron, Cobalt, Nikel or some alloy of.

So if we are going into a db made from one of the above it would be in contravention of the regs.

Other than that I see no problem

 

[SLIPSHOD & SLAPDASH]

Good points. However you could argue in the OP the cables are already in a ferrous enclosure and collectively surrounded, not entering into one.

---------- Post Auto-Merged at 17:19 ---------- Previous post was made at 17:18 ----------

While I agee with the points made and the regs do say that, however they also fail to define a Ferromagnetic enclosure.A quick google tells me it would be made of Iron, Cobalt, Nikel or some alloy of.

So if we are going into a db made from one of the above it would be in contravention of the regs.

Other than that I see no problem

Steel is classed as ferromagnetic & ferrous.

 

[Sidewinder]

As per Slips, Steel is an Iron alloy.

 

[philbas]

I did say it was a quick google

However in this case we have a spilt of the cables for a few inches as they enter through seperate holes, the one that is seperated should in normal circumstances have no current, If it is carrying current then we have more important things to think about than the electromagnetic effects over the small length we are considering

 

[Sidewinder]

It is simply the entry into the enclosure through the steel plate that causes the problems, the length of run is not relevant in this situation.

My initial thoughts were lives only, earths did not matter, however, having red App87's post & checked the brb, then I am now on his side, though I will have to check further.

 

[kme]

Furthermore, the enclosure is already at earth potential - what would the difference be if the earth cable was in the same hole? Zero, unless cpc is carrying current.

If a metallic enclosure, which does NOT form part of the earthed metallic containment system was the subject, then I could (to a point) accept & agree with such a stance.

Having said all that, 521.5.2 does indeed state exactly that. Well posted (I have "scoobed" you, just for finding, understanding, and posting that )

 

[Sidewinder]

KME,

I't not to do with potentials it's to do with magnetic fields, induced currents etc. rather than anything electrical

 

[kme]

I know - I don`t think I implied otherwise, did I?

 

[philbas]

not disputing if steel is a ferromagnetic material i had to look it up

But is splitting the earth out for a few inches really a fail on a pir.

As i said it shoudnt have current, if it has you have more problems than electromagnetic effects

There is the reall world and the theoretical world

 

[Sidewinder]

KME,

Sorry the way I read the post it seemed you were concerned more with the earth potential of the enclosure.

philbas,

If there were a few kA flowing through the earth conductor in the event of a fault then more damage or a more sustained fault or other issues could arise.

According to the letter of the reg posted by App87 then YES it absolutely IS a fail on a PIR.

There is something uneasy with this that means I am not going to come down on either side of the fence yet!

 

[kme]

It would fail as a code 4 ; given apprentices` reg. post up there - although I don`t think I`d generally look for, or have an issue with this

 

[apprentice87]

Hi all,

Not all steel is "ferromagnetic" I am no metallurgist, but i presume "ferromagnetic" means a ferrous material, in other words an iron based one, and the "magnetic" part refers to its ability to be magnetised or attracted to a magnet.

A steel which contains ferrite, would be obviously ferromagnetic, but one that did not, say an austenitic grade of stainless steel, would not, so i suppose you could have an austenitic stainless steel enclosure and stuff the wires in anyway you liked.

I suppose you could sum up by saying, that if a magnet sticks to it, it is ferromagnetic, so all cables must go through the same hole. If not, stick the wires where you like!!

One thing i cannot understand is though, is that although aluminium for example, is obviously not ferromagnetic, [it is neither ferrous nor magnetic] so far as i know you could still induce eddy currents in it, as this is the principle on which car speedometers work or so i believe..

Over to Sidewinder on this one i think!!!!!

john...

 

[Sidewinder]

John,

I know about Austenitic Stainless steel, and no generally it is not magnetic, so in the case of an Austenitic enclosure then you perhaps could ignore this rule.

However, the material is still ferrous, and can be heat treated to be magnetic.

As far as the Al bit goes, you are again correct, it is also how electricity meter work.

I have a former colleague who got burned on the finger when some very high currnent kit he was working with induced currents into his wedding ring.

Not quite simple to explain so I need some time to research.

 

[apprentice87]

Another thing that occurs to me [and i am sticking my neck out even further here] is this;

If you run the cables in a circuit in a group, the magnetic fields induced as a result of the current flow, will cancel themselves out.

Now, the CPC normally just sits there and does not do a lot.

In the event of fault currents flowing though, which can obviously be thousands of amps, the CPC will now well and trully form "part of the circuit"

Could it be that if it is run separately, there is, at least in theory, a chance that the CPC could actually be mechanically damaged as a result of the electromechanical forces induced, [due to it not being part of the same group of conductors] which i believe in theory at least, can be very substantial.....

john...