Does a CPC require mechanical Protection

[dodge26]

We have been asked to carry out some linking up eart bars on 5 blocks of flats, they want the CPC running in metal conduit, where I think that a CPC can be clipped direct, anyone know the answer and possible reg number.

 

[johnboy6083]

anything less than 4mm needs mechanical protection as per 544.2.2 (new green regs)

 

[oxocube]

If the client wants it in conduit then thats what you do, he who pays the piper etc.

 

[Engineer54]

If the job's specification calls for earth cables to be run in metallic conduit, then that is exactly what you provide the client. It doesn't matter a jot, what you think, or how you want to install these earth cables, the job's Specification is what you work too!! It's the same on all projects, where the Specification can and often does over rule Regulations, provided is of a higher order!!

 

[Simlec]

I would say they what it in conduit so as to stop it getting nicked!!

 

[johnboy6083]

to be honest, i dont like clipping cables anyway. It has its place, but that place is not anywhere near where people can pull it off the wall.

 

[Knobhead]

Right I’m going to pi$$ on everyone’s parade.

If the CPC is run through steel conduit you have circulating currents and reactance to take in to account. A single conductor through a magnetic conductive trunk is a NoNo! Plastic, aluminium or stainless steel protection is OK, but not plain conduit!

 

[telectrix]

but, tony, surely the only time a current will flow is during a fault condition, for a short duration till the ocpd operates.

 

[Knobhead]

I’m not quoting regulations but “pure physics”.

Remember what you were taught about induced circulating currents?

 

[Simlec]

Right I’m going to pi$$ on everyone’s parade.

If the CPC is run through steel conduit you have circulating currents and reactance to take in to account. A single conductor through a magnetic conductive trunk is a NoNo! Plastic, aluminium or stainless steel protection is OK, but not plain conduit!

Are you referring to Mr. Eddie currents ?

 

[Adam W]

What if you earth the conduit as per regs....
I would agree with Tel - current is likely to only flow for around 0.4 sec once in a blue moon.

 

[Engineer54]

Right I’m going to pi$$ on everyone’s parade.

If the CPC is run through steel conduit you have circulating currents and reactance to take in to account. A single conductor through a magnetic conductive trunk is a NoNo! Plastic, aluminium or stainless steel protection is OK, but not plain conduit!

Your not ****ing me off ...lol!! I took it, that this installation was in conduit anyway, and therefore the conduit for these earth link cables, will be earthed as a matter of course anyway. Your not going to get circulating currents if both the earth cable and conduit are at the same potential, and will remain almost at the potential even when there is a fault current flowing....

 

[Knobhead]

Re-read the original OP E54, it’s a single CPC run through conduit, no other circuit conductors included.

OK I know I’m being pedantic but I remember getting my ar$e kicked for running 4 x 95mm CPC’s through a 4” steam pipe, I thought it would be better than the 4” clay pipe specified. Earthing the conduit doesn’t stop the reactance in the CPC. And yes I know it’s for a few mS once every Preston Guild but the laws of physics don’t go away.

PS if any of you want to know what the Preston Guild is, look on Google. I’ve been to one. The next one is next year and the one after that I’ll probably be pushing up daisies.

 

[Adam W]

If the conduit were at the same potential as the wire running through it, surely that would at least negate the effect of the current running through the wire(s) in it?

... Or you could just use the conduit as the conductor with nothing in it.

If it has to be steel conduit then it has to be steel conduit, although I would suggest plastic, then suggest high impact plastic as a compromise.

 

[Adam W]

... And I would suspect the current running through a 10mm cable might be slightly less than that in a 95mm cable.

 

[Knobhead]

Reply to #14

No it doesn’t because the lines of magnetic flux in the conduit will be at 90° to the CPC current.

 

[Adam W]

Reply to #14

No it doesn’t because the lines of magnetic flux in the conduit will be at 90°to the CPC current.

That makes sense.

 

[Engineer54]

I think you'll find that this earth conductor is not a CPC (circuit protective conductor), but a bonding conductor linking the existing flats METs, making them in effect EMTs hopefully with a MET at the origin of the installation.


Now as both the conductor and the conduit are supposedly connected to these EMTs, any fault current flowing, will be carried by both the conductor and the conduit. At a disconnection time of a theoretical 0.4 sec, i doubt if a circulating current if any, would even have a chance to form.

 

[dodge26]

Sorry I asked, (only joking) brought up some intresting points. To be honest I just needed a get out as the spec is to link the MET'S and what is the cheapest way. Therefore as long as it's bigger than 4mm and clipped high enough that it can't be swung and nicked we will be fine.