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earthing SPARX

Postby 333rocky333 » Wed Jan 02, 2008 11:12 pm

Moved here to not hijack other post.
Bugging me now about the earth link.
Just cant find where info came from.


Regarding using trunking and conduit as earth
543-02-02 and I agree with you.

However look at reg543 -01-01 it says you must either

543-01-03 work out the area of the steel etc.
Maybe BS made conduit exceeds that any way.
(ALSO formula is REASON FOR 1.5 earth in 2.5 cable)

543-01-04 use table 54G which maybe why they go for the earth wire,
or for people that cant do tight joins on conduit

AS for the pig tails

Would the metal box be a conductive part
Would conductive part need a protective conductor.
543-02-02 states what can be used as protective conductor.
Hence the copper cable link between.

Regarding size
Back to 543-01-01
As link not part of a cable
Socket box is not an enclosure
Then csa not less than 2.5 mm

Best I could come up with
I am sure you will prove me wrong
Regards R
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Simply Build It

Postby ericmark » Thu Jan 03, 2008 8:27 pm

Thank god I'm not only one. I also looked at regs when SparX wrote the reply and he is right it says you can use steel if over 10mm equivalent copper conductor. Took me ages to find a web page with resistance in then when I posted link it was removed. But I calculated it to be 27mm in steel. Looking for thickness of conduit walls also took ages but finally found to be 1.5mm then amount threader cuts into that and seems 60 degrees at 1.5mm pitch which leaves about 0.5mm so taking area of 19.5mm from 20mm gave me 15.5mm which since under the 27mm required makes me believe that we can't use 20mm conduit as an earth? Please prove me wrong.
Next I looked at that formula and stuck it into excel to test. Here the PSC was stumbling block but also K and which I should use but a PSC of 2000 Amps comes up with something like 27.5mm CSA required in steel which further complicates matters.
I would think on a 150mm SWA cable one may not need a copper earth but on the smaller cables it seems one would?
I was told as an apprentice you needed a copper earth and I never questioned it. Once I was indecisive now I'm not so sure!!!!!!!!!


Postby 333rocky333 » Thu Jan 03, 2008 11:46 pm

I think the bs numbers that swa and conduit are made too, may already taken this into account

From table 54G under 16mm phase conductor, the steel area only has to be k of the phase conductor ( table43A ) divided by k of the steel ( tables 54B to 54E ), times the size of phase conductor.

K for steel appears no less than one third of k for copper so i would say there is easily 3 times as much steel as there is phase copper


I guess the easiest way is to measure res of a length of conduit ,swa armour and a length of copper cable and compare each.

Looking at it there is a lot more steel and the bigger the swa the bigger the steel wire.

Same with trunking and conduit

Proberly the largest cable you would find in 20mm pipe would be 6mm maybe 10mm
543-01-02 regarding size if more than one circuit
I would say there is more than 30 mm sq of steel in 20mm conduit.

Suppose its going the way of just put an earth wire in pipe, just to make sure even if not required.
Standard of conduit and trunking I have seen lately is pretty rough
So maybe a good thing
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Postby ericmark » Fri Jan 04, 2008 2:56 am

I agree with your just in case statement. I have only once come across a real problem with SWA and Earthing. It ran under a concrete road and the earth loop impedance was getting worse each year. Since the shed it feed was ear marked for replacement I was not permitted to replace the cable. I did fit a 100ma RCD but it was something I worried about. I also fitted earth rods. never did get it dug up to find real fault. But steel will rust and we can lose the current carrying capability as a result. There have been a few sites where copper slip, Cold galve, etc have been used every time steel is cut but many more where it was never made available and also very few sites where one can view the previous reading to see a trend. I think I would fail to mention steel can be used as an earth conductor and continue using copper. I have on tray work taken a 50mm earth cable and crimped on the 6mm tails rapped in that special water resistance 3M tape earthing each section of tray. But I have also been forced to link one bit of tray to the next with little 6mm links in spite of the trays being bolted with many 6mm roofing bolts together (i.e. not clipped) which we all know is a complete waist of time in fact it increases the chance of electrolysis and the resulting corrosion yet we also know we just have to do as we are told or we are trouble makers. So unless it's all in stainless I will continue telling people it needs to be a copper earth under 10mm and pretend I missed the bits saying otherwise. As you rightly say there seems a general lack of information as to cross sectional area of both conduit and SWA and conduit from the EU can have push fit bends with clamps and painted rather than galve and even free air joints harmonizing seems to be all one way. Yet saying that the Germans do use 5 core SWA or 4 core with a bare copper conductor under the steel braid with their braided type cables. As you say lets see what the 17th edition comes up with.

Postby sparx » Sat Jan 05, 2008 10:29 pm

Hi Rocky, eric & others,
this is great!!!!
Thanks for moving here, I almost didn't answer original post but get fed up with all the billy-B-S being put about by various people.
As leckies we are quite at liberty to employ any method we like to achieve disconnection times etc, providing able to show test readings to confirm, this has always been a factor of regs. eg notwithstanding detailed calcs of CSA if the loop reading shows a low enough path to trip supply in required time then it complies.
As Winston Churchill once said " rules are for the guidance of wise men & the obediance of fools!"
An example I gave of 'pig-tail' earths was introduced by an NICEIC moron who misread regs. it has now become folk-law....
If wiring with Pvc/Pvc twin / cpc the circuit earth conductor should go to the front plate, when the plate is fitted with at least one fixed lug then the back box is deemed to be earthed, if the front plate is being undone the power should be OFF, HSE guide lines, Eletricity at work Act Ect. so box not a risk.
If we as Competent Persons require live circuit for testing etc. we must justify such action, take precations to reduce danger, thats what we do, it doesn't follow that other less competent people will have same duty-of - care as us so as Doc says be careful what we say to others,
I regularly go on 'trade only' forums, NAPIT ECA & the amount of debating that goes on between very experienced tradesmen is amazing,
regs so open to interpretation that as long as we are individually happy to defend our understanding of issues then it's fine for use but confusing for outsiders, Having just paid over £1400 to renew my annual insurances for leckie work I will be very picky as to which posts I answer.....
I don't want to try to prove anyone wrong, equally I don't want to give bad advice, as for buried SWA I was told by a cable maker it is wrong to run seperate earth conductor as it can unbalance cable under fault conditions but for some smaller sizes of cable particularly those using thinner XLPE insulation it is better to use an additional core & if used as a sub-main to disconnect at downstream end and use TT system, 45 years in trade & still having 'mass-debates' can't be all bad,
best wishes to all,
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Postby ericmark » Mon Jan 07, 2008 12:20 am

I agree live working was a big thing where it seems testing live items is not considered as live working only removing or replacing cables is considered as live. Doesn't seem right to me. One poor guy in the Midlands it seems switched off a large panel in the panel it's self to fit an internal lamp and having first fixed the lamp he tried to shake the flex down behind the plate and it touched the main incomer where the insulation had been caught with Stanley knife when originally fitted. The ionization caused the spark to be throne across the room braking limbs. The HSE fined him for live working and his firm tried to sue him for loss of production. His mate left and I met him in Chard where he told me story. There by the grace of god go I. I did before this story often isolate at the box I was working on rather than else where. And still I was got, I used electrical lock instead of personal lock as I had already used personal one. Another electrician removed it and turned on the machine while I was working on it. Mauled right hand never worked since wasn't even electric that got me it was a concrete recycling machine. I don't believe in recycling now!!!!!!!!!

Postby sparx » Tue Jan 08, 2008 8:50 pm

sorry to hear why you don't work due to industrial injury, seems a bum deal to me!
I too got caught; abt. 1972 was working in paper mills doing instrument control work, mainly pneumatic and electronic logic stuff, mainly ELVoltage seemed pretty safe....
was tracing fault on air signal line in panel full of copper microbore pipes, turned off panel isolator to make sure,
hand up back of panel, found exposed back of 240v remote panel indicator lamp with knuckles, couldn't remove hand, still got blue copper bits in one joint today!!!!
It worries me when people think 'it's only lighting' so safe to work on!!! if they only knew.
best wishes,
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