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Topic Title: Unusual use of 3 core Armoured cable
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Created On: 21 September 2017 03:47 PM
Status: Read Only
Linear : Threading : Single : Branch
 Unusual use of 3 core Armoured cable   - stevesmith01 - 21 September 2017 03:47 PM  
 Unusual use of 3 core Armoured cable   - broadgage - 21 September 2017 05:35 PM  
 Unusual use of 3 core Armoured cable   - AJJewsbury - 21 September 2017 06:38 PM  
 Unusual use of 3 core Armoured cable   - broadgage - 21 September 2017 07:58 PM  
 Unusual use of 3 core Armoured cable   - davezawadi - 21 September 2017 08:57 PM  
 Unusual use of 3 core Armoured cable   - broadgage - 21 September 2017 09:14 PM  
 Unusual use of 3 core Armoured cable   - mapj1 - 21 September 2017 10:36 PM  
 Unusual use of 3 core Armoured cable   - statter - 22 September 2017 07:13 AM  
 Unusual use of 3 core Armoured cable   - JohnRRussell - 22 September 2017 08:36 AM  
 Unusual use of 3 core Armoured cable   - stevesmith01 - 22 September 2017 10:52 AM  
 Unusual use of 3 core Armoured cable   - broadgage - 22 September 2017 02:47 PM  
 Unusual use of 3 core Armoured cable   - cptspaulding - 22 September 2017 01:56 PM  
 Unusual use of 3 core Armoured cable   - stevesmith01 - 25 September 2017 01:13 PM  
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 21 September 2017 03:47 PM
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stevesmith01

Posts: 14
Joined: 03 July 2013

Recently did an large office refurb retaining the existing distribution gear and cabling, the building having a dedicated transformer about 40 M away from the main LV panel.
The LV cables from the tranny to the panel are 2 x 3 core 185mm, Al conductors with Copper armour, per phase.

What I found unusual was that the 3 individual internal conductors of each cable were terminated together via a brass clamp having 555mm stamped on the side (3x185=555).
Seems a bit unconventional to me, can anyone expand on this method?
 21 September 2017 05:35 PM
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broadgage

Posts: 2380
Joined: 07 August 2007

To whom do the cables belong ?

If they are part of the customers installation, then I believe that 4 core cables are required for three phases and neutral, plus the copper "armouring" for earth.

If the cables belong to the DNO, then 3 core with a combined neutral and earth is fine.

I do not like lugs or terminations that accept multiple conductors as I feel these to be less reliable than terminating each conductor in its own lug.
That is however my preference and not AFAIK a requirement.

If I have understood correctly, each cable is carrying only a single phase, all 3 conductors for the same phase.
That is prohibited with STEEL armouring due to magnetic effects, but should be fine with copper.
 21 September 2017 06:38 PM
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AJJewsbury

Posts: 15811
Joined: 13 August 2003

Hang on, we've got 2off 3-core cables, with all three cores of each cable crimped together? So just two live conductors over all? So either this is single phase (L+N and armour for earth) or split phase with the armour as N (possibly CNE, depending on where the N-PE link is).

It sounds to me like the arrangement does fall foul of one of the recent changes to BS 7671 though - 521.8.1
521.8.1 Each part of a circuit shall be arranged such that the conductors are not distributed over different multicore cables, conduits, ducting systems, trunking systems or tray or ladder systems.

This requirement need not be met where a number of multicore cables, forming one circuit, are installed in parallel.

Where multicore cables are installed in parallel each cable shall contain one conductor of each line.


although I can't quite see what the problem with alternative arrangements like yours would be.

- Andy.
 21 September 2017 07:58 PM
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broadgage

Posts: 2380
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I understood the arrangement to be a pair of 3 core cables for each phase, i.e. six cables in total.
With no mention of any neutral, presuming that the copper armour is serving as both neutral and earth.
OK if it belongs to the DNO but not if it belongs to the customer.
 21 September 2017 08:57 PM
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davezawadi

Posts: 3843
Joined: 26 June 2002

I read this as 2 cables of 555mm2 from the TX, single phase, with normal copper shields as earth. Whilst a little odd it is certainly not a particular problem. Can you explain further? A dedicated TX with only a single phase supply is certainly unusual.

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 21 September 2017 09:14 PM
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broadgage

Posts: 2380
Joined: 07 August 2007

Originally posted by: davezawadi

I read this as 2 cables of 555mm2 from the TX, single phase, with normal copper shields as earth. Whilst a little odd it is certainly not a particular problem. Can you explain further? A dedicated TX with only a single phase supply is certainly unusual.



I presume 3 phase, indeed a single phase supply so large as to need the cable sizes mentioned is virtually unknown.

Single phase supplies of more than 125 amps are rare, and for that size a single much smaller cable would suffice.
 21 September 2017 10:36 PM
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mapj1

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Joined: 22 July 2004

The thinking behind 521.8.1 is to minimise the formation of large loops of current, and the consequent magnetic fields that induce problems in neighbouring cables and metalwork generally. By arranging 'flow and return' to be close, and therefore almost cancelling, the far field is greatly reduced, and as a bonus the supply inductance is lowered. Given that the same number of cores are needed in either case, there are not that many cases when the use of consolidated cores like this is better, although the termination is more complex.

-------------------------
regards Mike
 22 September 2017 07:13 AM
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statter

Posts: 145
Joined: 06 February 2013

IIRC this was quite a common practice in 1980's. I recall seeing analysis that demonstrated that using bunched conductors increased capacity of the cables compared to either single cable of the same overall CSA and materials. I have seen this on both DNO and customer installations (under 15th edition). I think these were all single point bonded and aluminium sheathed/armour.
regards
 22 September 2017 08:36 AM
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JohnRRussell

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Joined: 02 December 2003

It's still common practice now. Several of our small hydros, up to 1500kVA, have a DNO connection at LV. The connection is made in 'Bunched Wavecon' - ie all cores of a wavecon cable clamped together and treated as a single core cable. Up to 11 such cables (3 per phase plus 2 for neutral).
 22 September 2017 10:52 AM
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stevesmith01

Posts: 14
Joined: 03 July 2013

To clarify

2 cables per phase plus 1 Neutral all 3 core 185 Al conductors.
Separate 70mm single core Earth from TX.
Copper "Armour" not connected at the panel end, no glands used, cables simply pass through individual holes and the gap made up with putty compound.
All 3 conductors per cable are clamped together so in effect 2x 555mm csa per phase.
I'm not sure who owns the cables, I did assume it was the DNO as I wasn't aware that contractors would use this method.
 22 September 2017 02:47 PM
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broadgage

Posts: 2380
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Originally posted by: stevesmith01

To clarify
2 cables per phase plus 1 Neutral all 3 core 185 Al conductors.
Separate 70mm single core Earth from TX.
Copper "Armour" not connected at the panel end, no glands used, cables simply pass through individual holes and the gap made up with putty compound.
All 3 conductors per cable are clamped together so in effect 2x 555mm csa per phase.
I'm not sure who owns the cables, I did assume it was the DNO as I wasn't aware that contractors would use this method.


Sounds OK to me as a separated earth and neutral are provided, presuming that the earth is sufficient, 70mm sounds a bit small for a presumably large supply.
Was the 70mm earth intended to SUPPLEMENT the copper armouring, rather than being the sole earth ?

"cables pass through individual holes" NOT holes in a steel plate I hope, due to eddy current heating.
On small supplies like domestic services, the risks of eddy current heating are more theoretical than actual, but eddy currents are a real concern on heavy supplies.
 22 September 2017 01:56 PM
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cptspaulding

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Joined: 08 August 2017

I had an installation in Orkney where the DNO proposal seemed to suggest they would be using a 3 core Al waveform cable for each phase from their substation to the main switchboard.

I tend to assume they know what they're doing. I never got back to the site to see how it was actually installed in the end.
 25 September 2017 01:13 PM
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stevesmith01

Posts: 14
Joined: 03 July 2013

The 70mm Earth conductor must meet the adiabatic as the copper "armouring" isn't connected at the LV panel end.
The Cables pass straight through an Ally plate, the site transformer is 1MVA with the 1600 A ACB set at 1200A.
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