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Topic Title: conduit used as earthing conductor
Topic Summary: calculating restance of and other things.
Created On: 13 October 2008 10:02 PM
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 13 October 2008 10:02 PM
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dbullard

Posts: 1166
Joined: 02 March 2006

Evening all,

Did a test and report for the the optomerists i look after, they wish to rent out the mostly unused portion of one of the branches(credit crunch and all that).

None of the circuits have any earthing conductor and are using the embedded black enamel conduit as the CPC, now do i have to calculate this for the purpose of the report, i cant locate the ends of the conduit due to building modifications so cant get a relaible reading or verify continuity apart for the fact of the ZS reading on all the circuits if so where in the new regs are the calcs for this.
Also the cables installed are not stranded but stripped down t&e with the cpc ommited i cant fail the install on the cable as the readings are very good but one circuit i dont really like a solid cable being used for this purpose and drawn through condiut to feed circiuts.

Regards

Daren

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www.quest-electrical-sw.co.uk
 13 October 2008 10:18 PM
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thedan71

Posts: 834
Joined: 16 August 2007

To confirm CPC continuity can't you link Line and CPC at the CU and measure at each point and record the highest reading? I don't quite understand why you don't like the solid cable feeding circuits? Not too sure what you mean by that?
 13 October 2008 10:29 PM
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jamieblatant

Posts: 513
Joined: 11 January 2006

thats not stripped down twin and earth its just solid conduit cable popular in the 70,s i belive nothing wrong with it at all for use in conduit unless you have to install it and its a right b*****d

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 13 October 2008 10:38 PM
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alancapon

Posts: 5763
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The risk of putting solid conductors in conduit,is the initial installation, where the insulation can be damaged, particularly on bends. Once the conductor is in, it is unlikely that any further damage will occur. Insulation tests between the conductor and the conduit are likely to show any potential problems.

Regards,

Alan.
 13 October 2008 10:39 PM
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dbullard

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Hi thedan71, not a fan of slid cable being drawn through conduit of that age 20y +.

almost positive it is or was t&e due to still visible cpc in some parts of the circuit but far to short to crimp or terminate in anyway shape or form, maywell have been used as a draw i suppose.

regards daren

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www.quest-electrical-sw.co.uk
 13 October 2008 10:50 PM
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John Peckham

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As it has already been said solid singles came in around 1970 when cable went metric. So nothing wrong with that as such.

Have a read of GN3 in respect of continuity when performing PIRs. If you have nice low Zs readings you are proving earth continuity. In fact if you are using a loop impedance tester on a high current range this is a more demanding test than a continuity test using an ohmeter at 12V at 200mA. Just make sure there are flying leads from the back boxes to the accesories.

John Peckham

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 13 October 2008 11:03 PM
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dbullard

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Joined: 02 March 2006

Cheers chaps,

john all the tested circuits ok and all aceesories had flying leads connected, will when possible post pics on sparky resurection when possible of the stripped cable with the cpcs in the trunking cut really short.

once again chaps cheers.


Regards Daren

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..... Dont pee in my pocket and tell me its raining ......


www.quest-electrical-sw.co.uk
 03 April 2013 11:24 PM
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sparky89

Posts: 1
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this may be of some use to you but may not in my opinion if the system installed is conduit based weather it be black or steel provided you have good readings there is no problems


PG NUMBER'S: 163-164 of bs7671

REGS NUMBERS:-

543.2.2 (vi) : A METAL CONDIUT, METALIC CABLE MANAGEMENT SYSTEM OR OTHER ENCLOSURE OR ELECTRICALLY CONTINOUS SUPPORT SYSTEM FOR CONDUCTORS

543.2.7 : WHERE THE PROTECTIVE CONDUCTOR IS FORMED BY A METAL CONDUIT, TRUNKING OR DUCTING OR THE METAL SHEATH AND / OR ARMOUR OF A CABLE , THE EARTHING TERMINAL OF EACH ACCESSORY SHALL BE CONNECTED BY A SEPRATE PROTECTIVE CONDUCTOR TO AN EARTHING TERMINAL INCORPORATED IN THE ASSOCIATED BOX OR OTHER ENCLOSURE.

543.3.6 : EVERY JOINT IN METALLIC CONDUIT SHALL BE MECHANICALLY AND ELECTRICALLY CONTINUOUS.
 04 April 2013 11:35 AM
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OMS

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OK - if you want to do this properly, you need to recognise that metal conduit exhibits totally different impedances when exposed to different fault currents.

Conduit will actually exhibit a maximum impedance when the fault current is about 60A and this will be roughly 3 times the measured DC resistance.

Also, metal conduit enclosing faulted cables has high reactance - the impedance angle would exceed 30 degrees and be very variable based on the magnetic properties of the particular steel used in the conduit.

A lot of this is theoretical and it's usually sufficent to add conduit impedance to circuit resistance and check against theoretical impedance when fault curents are less than 100A or greater than 100A.

So for example - if you have If <100A based on your intial estimate, for 20mm Heavy gauge conduit, then impedance of the conduit will be around 4.5milliOhms/m in your calculation.

If the fault current then proves to be gretaer than 100A, then you would use a conduit impedance of about 2.5 milliOhms/m for the calculation.

So, measure R1 - and then the conduit using a DC ohmeter. Multiply your measured R2 value by 3, evaluate the fault current and see where that is in terms of the 100A "divider" - if the fault current is low, your practical Zs will be R1 plus 3 x R2

If the fault current is well above 100A then the Zs will be closer to your measured R1 plus about 1.5 x R2

If you have nice low Zs readings you are proving earth continuity. In fact if you are using a loop impedance tester on a high current range this is a more demanding test than a continuity test using an ohmeter at 12V at 200mA.


True John - but even on a high range, yo can't get anything like enough current flow to influence the impedance angle. That's why conduit was tested by very high current tests to prove continuity and to record "impedance" - of course we don't do that kind of thing anymore.

A loop test will tend to underestimate Zs - and grossly underestimate on low fault current circuits

Simples - - although I've always thought GN3 to be a bit light on information regarding this matter

regards

OMS

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