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C

CableTie

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Hello all,

Can anyone tell me why steel cable tray must not be used for the cpc? I know its full of slots and holes but it must have a minimum csa.

Thanks.
 
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S

Spudnik

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  • #2
Main reason i think is that it cannot be relied upon as a cpc, due to various factors.

Im sure one of the commercial/industrial lads will be along in a mo with a better explanation.
 
G

Guest123

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  • #3
Hey mate.

I cant see why you would want to use tray as cpc?? as surely the cableing you are laying on the tray would already have it's cpc in it??

If not then you would run a seperate cpc next to it - if it was a large swa for example.

I dont think you could guarantee every joint without earthing straps also continuing the earth off the tray into your switchgear/accessories would pose a problem and every joint would need to be accessible for inspection

I would bond the tray but not use it for cpc.
 
Last edited by a moderator:
Hello all,

Can anyone tell me why steel cable tray must not be used for the cpc? I know its full of slots and holes but it must have a minimum csa.

Thanks.
It CAN(sorry guys)and is a recomendation of the ECA.I read this in their book called guide to the wiring regulations-17theddition wiring regulations(BS7671) PAGE 174 FIG.E6.3 The book is downloadable from torrent.
 
G

Guest123

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  • #5
Hey.

I never said it couldn't, was only asking why you would want to?? as surely most cableing laid on a tray would already have it's cpc with it. Also that it would probably more hassle than it's worth both cost and labour wise.

Obviously you wouldn't be running pvc singles on it so I cant see why you would need to use the tray as such.

Also I couldn't find any mention of tray being used as cpc in the regs only conduit, trunking, ducting...
 
S

Spudnik

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  • #6
Agree with Lenny on this one.

Does seem daft to use it as a CPC alongside a cable with it already in.

Still needs bonding though.
 
Agree with Lenny on this one.

Does seem daft to use it as a CPC alongside a cable with it already in.

Still needs bonding though.
oops seems I've missread the book.The item I was refering to utilizing the cable tray as protective conductor,was talking about using it as protective bonding conductor (not cpc)from which you could cross bond to various services and structures rather than installing loads of green and yellow runs. A thousand appologies.:eek:
 
G

Guest123

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  • #9
oops seems I've missread the book.The item I was refering to utilizing the cable tray as protective conductor,was talking about using it as protective bonding conductor (not cpc)from which you could cross bond to various services and structures rather than installing loads of green and yellow runs. A thousand appologies.:eek:
That makes more sense mrloy. I thought Iwas going mad especially when when I couldn't find it in "ol red" after.
 

benji

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Arms
cable tray/ladder/basket are support mediums and thats it.If you used it as a cpc and somebody further down the route cut and fitted a tee into it for the duration of fitting the tee you would have no cpc ,and how do you know if the tray been tested for continuity after fitting the tee
your benji
 
S

Spudnik

  • Thread Starter Thread Starter
  • #11
Isn't metal conduit an additional CPC if a seperate CPC is run to a circuit?
One could use it as an additional cpc as it would require bonding but im not sure of the need really.
 
G

Guest123

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  • #12
Isn't metal conduit an additional CPC if a seperate CPC is run to a circuit?
I agree with what you've said, but the point I was trying to make was that a cable tray would not be run to every switch, socket, isolator etc and to rely on it as cpc would mean earth strapping every joint and also tapping off with earth cable to go to the switches, sockets, and isolators.

Running a cpc with an armoured is just easier.

With regard to conduit then this would be run to every switch, socket , isolator as this would be your containment for the pvc singles you were installing.

Do you see where I'm coming from?

Cheers.:D
 
D

desertbootz

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  • #13
Tray is generally steel, steel generally rusts, rust doesn't conduct.

Tray should be bonded against Indirect Contact (or whatever it's called in the 17th... secondary?) back to the Main Earth.

I generally don't bond back to the tray but bond cubicles or other extraneous to the nearest supply cpc if poss.

If anyone should ever come across the Commissioning Engineer that complained about pigtails being used as to link tray sections because he felt they would create too great an impedance in case of a fault please do what I should have...
 
Tray is generally steel, steel generally rusts, rust doesn't conduct.

Tray should be bonded against Indirect Contact (or whatever it's called in the 17th... secondary?) back to the Main Earth.

I generally don't bond back to the tray but bond cubicles or other extraneous to the nearest supply cpc if poss.

If anyone should ever come across the Commissioning Engineer that complained about pigtails being used as to link tray sections because he felt they would create too great an impedance in case of a fault please do what I should have...
I recon your tray should definatly be earth/bonded(but often isn't)as it can be quite common that a cable can be nicked or the sheath be burned through by pulling a another cable accross an existing one,and so a tray can become live,unless there is a satisfactry route to earth.I also recon that copper bonding links should be used.I don't know about that rusty metal theory--most metal trays here are galvanized and where cut they should also be tuched up with cold galv paint.As for the pigtails,I suppose the sizeing of them should take account of maximum fault current.And I recon your extraneous bonding should be sized as per main bonds(10mm+)
Only my take on things.
 
G

Guest123

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  • #15
Agree with mrloy, the tray is galv so rusting is not that great an issue and also all cuts/joints are always sprayed with galv paint after.

Personally I always bond my tray to the panel in whatever the largest size of cpc conductor run on it is i.e 25, 35mm etc.

:D
 
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