GA: 
 

Evening, I'm seeking some advise pls. I have a 4core 25mm2 SWA and was going to use this for two single phase submains running from and to a common point, where the circuits will then go into separate consumer units. limited loads a garage and stable. The cable will have DP isolation and RCD protection on each circuit (2cores L+N per sub main) at the supply point. I'm also looking to run a separate earth cable of 25mm2 which would be common to both circuits and armouring of the SWA cable is also earthed and common. My question is I believe this to be complaint with 17th Ed; As I can't see anything saying its not. Second question, could I common the neutral for both submains and therefore use the fourth conductor in the SWA as an earth which would save installing the separate earth cable? Again, I can't see anything incorrect as limited loads to a couple of sockets and a few lights.  thanks for assistance. GA

Hello G A, We are using the 18th edition nowadays. Many times the armour is adequate to use as a C.P.C.

Use SWA as CPC, a guide to the acceptability of steel wire armour use as CPC. (gadsolutions.biz).

Z.

Forget the 17th Ed - we're on AMD 1 of the 18th now…

Two cores each for two circuits - a bit unconventional, but I'm pretty sure entirely compliant. It's rather line running two circuits in the same steel conduit and using the conduit for the c.p.c.

Sharing a N though is probably a no-no. Firstly there's the lack of overload protection - as it's single phase the N will have to carry the sum total of the two circuits current - which will exceed the N core's capacity if the L cores are used to anything like their potential. Then there is the general requirement for isolation and for the wiring of circuits to be separate - if one of the two circuits were isolated and the N disconnected, the load end of the N conductor would likely be pulled up to full mains voltage by any connected load on the other circuit. Admittedly the main regulations for this (314.4 and 521.8.2) only specifically mention final circuits - but it would seem rather risky not to consider similar risks in distribution circuits, especially if it's not a well controlled environment.

   - Andy.

thanks, I agree a bit unconventional and the N point is well made in terms load. many thanks

If the distribution circuits start and finish in the same places, why not have just one circuit?

I am not clear how you propose to terminate one cable in two CUs.

25 sqmm seems big for “limited loads”, but perhaps the cable is very long and voltage drop is potentially an issue?

It seems unlikely that such a large CPC will be required.

What is the earthing system please?

Quite often in farm type installations a 300mA R.C.D. is installed at the origin of a sub-main. This then allows earth faults to disconnect without the need for large “long duration” fault currents to open protective devices. This offers fire protection. An “S” type or time delayed R.C.D. can be used at the origin, and instant 30mA types for say sockets for the final circuits.

705.422.7

Z.

I would advise against use of a single core as a common neutral, for the reasons already given by others.

My view is that when a 4 core cable is used as is proposed, that the published current carrying capacity should be reduced by about 15%. I presume that the published ratings are based on three phase, with only 3 cores carrying significant load. Loading all 4 cores produces more heat.

If however the loading is modest if compared to the cable size, then this is a non issue.

Will the stable occupiers be content if their installation is connected to PME?

Regards

BOD

perspicacious: 
Will the stable occupiers be content if their installation is connected to PME?

The horses??? ?

It would be cleaner if the 4 cores were one high current circuit at the origin, and then split into two at the load end, perhaps with an adaptable box and some line taps. You do not really want a situation where two switches are needed to make one cable dead.

Check the armour resistance, but you should be OK with LL NN and armour as CPC. perhaps some decent glands and pirahana nuts are in order.

Mike.

Zoomup: 
Hello G A, We are using the 18th edition nowadays. Many times the armour is adequate to use as a C.P.C.

Use SWA as CPC, a guide to the acceptability of steel wire armour use as CPC. (gadsolutions.biz).

Z.

Just to note that the calculation method for loop impedance shown in Table 4 on this site, whilst still demonstrated in the latest (2018) IET Electrical Installation Design Guide, should really be replaced for cables with line conductor csa 16 sq mm and above (where reactance comes into play more) by the method shown in Annex NA of PD IEC/TR 50480:2011 - specifically sections NA 4.4 (armour alone used as cpc) and NA 4.5 (external cpc). 

For the case where the armour is used as cpc, the total loop impedance contribution of the SWA cable for a line to earth fault is given by:
 

R1 is the DC resistance milliohms per metre) of the line conductor, R2 is the DC resistance (milliohms per metre) of the SWA.
 

Don't forget the above is in milliohms per metre, so multiply by length, divide by 1000. 

This leads to a slightly higher overall DC resistance per metre but a slightly lower overall reactance per metre.