Can you use flexible metal conduit as a circuit protective conductor (CPC) - BS7671
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- Опубликовано: 9 май 2021
- Flexible metal conduit is used for many different electrical installation applications such as final connections to motors and machinery.
In this electrician Q&A Joe Robinson explores BS7671 and if you can flexible metal conduit as a stand-alone circuit protective conductor (CPC / earth connection)
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❌ This content on this channel is for electrical professionals.❌ ==================================
📹 Presented by
Joe Robinson - Technical Editor eFIXX
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#efixx #electricalinstallation #bs7671 
Наука
I am glad my random thoughts to the answer ended up being "the answer" for once.
Normally I am surprised by some randomly little addendum somewhere in the regs.
Thanks for great video Joe .. hearing the science behind WHY this is not suitable, is really useful 💡 🙏
Excellent job of explaining that Joe. Hope you and your family are well. #StaySafe
Channel is growing amazingly roll on 50k gents
Thanks for the support 👍
Always welcome been watching this since 500 subscribers
In the US, we’re allowed to use FMC as a grounding conductor but only on circuits up to 20A.
I think it's fairly logical from them. UK doesn't wanto make any difference.
As soon as i saw the question i could see the answer in 7671
Gratified that my gut reaction *"No"* was in fact the correct answer.
If properly earthed via the metal back boxes, is this flexible conduit suitable for use when a cable is run in a wall outside the prescribed zones?
Thanks for the video.
If the flexible conduit was earthed one end,
And silecon wires was used.
Would that protect the wire from high frequency,
As on a plasma cutting table.
Would that be like using screened cable.
Never come in to my mind to use the metal of the Flexi conduit , run a CPC cable inside full stop,just like earthing the back of metal ko box ,what's the problem to do it .
Thank you
👍
As i understand it you cant use the sectioned conduit as shown in the clip however as long as its a single extruded corrugated flexible conduit then this would be acceptable as I think it is in the US.
After watching your explanation, it does make sense. IET would be too nity gritty if they were stated "unless it's only a meter long" or something.
FMCs are good in harsh environment condition. I have seen plastic conduit literally degraded into tiny peaces.
good question , but frankly its far easier and quicker to just pull a CPC and be sure rather then guess or hope
I remember the metal conduit being used as the cpc but that was 30 years ago and it was rare then .
My flat built in 1964 has all circuits in metal cinduit acting as the cpc, all checked out recently by UKPN and given a thumbs up.
@@nw5835 as long as it hasn't corroded it should be fine . Good initial installation means a long service life .
You say about high resistances with the spiral but rcd protection omits us to 1667 with 30mA protection so is this still no longer compliance if within 1667?
can't guarantee that the value won't vary over time and use
The 30mA is for additional protection, you still need to meet the requirements for ADS. If it's a TT supply so you need an RCD to meet ADS then it wouldn't be the same RCD.
It certainly isn't okay to be depending on the RCD for ADS in a TN-S or TN-C-S system.
How about armoring of the cable? Copper of course can be used but what about steel?
Look at 1:00 in the vid for what is acceptable as a CPC: reg 543.2.1 (v) "A metal covering, for example, the sheath, screen or armouring of a cable"
So yes.
The question was asked by the one and only Nick Bundy.
CPC is a bond across the pond, same rules tho
Great to know - cheers 🍻
I might be wrong but is metal flexible conduit not made of one unbroken length of metal wrapped around itself at an angle so it interlocks?
I’ve done it before and if you were to pull on it you would end up with a long length of metal. In my eyes it has the potential to be a CPC as it has an uninterrupted path to earth providing the correct glands have been used.
I’m not saying I would use it as a CPC as I never use it but I think no matter what way the conduit was bent it would still offer the same if not better if it was making more contact while being bent than a 1.5mm or 2.5mm separate CPC.
That is exactly as they describe in the video. Sure the interlocking is in reality going to make some contact and reduce the end-to-end resistance, but as it's not fixed you can only really rely on it as it would be in the unravelled state, which is a lot less cross-sectional area and a lot more length than a solid conduit.
Remember also that steel has a higher resistivity than copper, therefore you have a relatively high and unmeasurable resistance (as it can change easily from small movements).
The result is that while in practice it would PROBABLY provide an adequate earthing path, it's unreliable for ensuring suitably low Zs readings so you cannot guarantee protective devices will operate in the required time.
TLDR: Like with many things in the electrical field, it would probably do the job, but stick with the regs or it's your ass on the line if it fails!
In the US, we’re allowed to use FMC as a grounding conductor but only on circuits 20A or less.
Impact Driver
So why not gas pipe ?
The pipe could be disconnected/altered at any point without knowing it was used as a cpc.
@@jamiebourne8047 LIVE GAS PIPE ?????
Metal gas pipes can be replaced or repaired with plastic pipe.
@@robertburrows6612 plastic pipe can only be used underground. If the incoming main was renewed in plastic the earth return path would be lost .
In America ya because we live dangerously
🤣👍