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sunilji (Electrical) (OP)
28 Jan 03 20:08
Dear All,
My problem relates to the method of laying and fixing single core armoured LV cables of 630 sq.mm. cross section in a trench over a distance of 20 meters. The cables are laid from the secondary of the distribution transformer to the LV breaker. All guidelines from cable manufacturers only point out the use of trefoil clamps. However I do not find the mention of the neutral conductor anywhere. How is one to fix the neutral cable ? Does it need to be bundled with the phase conductors ( as in a 4 core cable ) ? Do you get 'quadrafoil clamps' ? Any ideas anyone ?
jbartos (Electrical)
28 Jan 03 21:04
Suggestion: Low voltage cables installation layouts are less strict than medium voltage cable installation. The neutral can be installed as an additional run beside the trefoil, e.g. on quatrefoil supports.
Also, the cables could be bundled. However, the planar layout is frequently seen on walls installed by the Utilities. Apparently, the bundling means an extra work that is not necessary if there is enough room.
Helpful Member!(2)  bigamp (Electrical)
29 Jan 03 14:16
Sunilji,

What are you doing with armoured single core?? Hope it is not SWA.

When you say trench, I assume you are direct burying the cables??  If that is the case, you do not need trifoil calmps for the phase conductors, double cable ties every 300mm would do.  You can run the neutral conductor spaced from the trifoil, say 300mm.

If your phase conductors are two trifoils, run the neutral between the two trifoils (equidistant from each trifoil).

Quadfoil clamps are not a good idea.

If your cables were running above ground on ladder then you would use trifoil clamps for the phase conductors and single cable clamps for the neutral clamping every second rung on the ladder.

Regards

sunilji (Electrical) (OP)
29 Jan 03 19:27
Thanx jbartos & bigamp,
The cable is being run in an underground cable trench which is to partly outdoor and partly indoor. there are 6 runs ( 4 core per run )of the cable to be run between the transformer and LV panel and we propose to run it in two layers of 3 trefoils each. We propose to fix the cable on aluminium rungs to form a sort of under ground ladder structure. The trench is later to be filled with sand and fitted with removable trench covers. We are going in for armoured cables for mechanical strength. We shall be glanding the cable with brass glands fixed on aluminium gland plates. Do you think the armouring could give rise to eddy currents ?
Unfortunately the route is too twisted to fix a busduct which would have been much simpler.
bigamp (Electrical)
29 Jan 03 21:25
Sunilji,

I have yet to see single core SWA cable, I do not think that anybody made it (or would want to make it).  Normally single core cables are unarmoured.  In my distant past I recall aluminium wire armoured (AWA) single core cables being used on one site.  AWA struck as being next to useless.  You may well have eddy current problems and maybe circulating current problems in the SWA.  Remember, your gland plates would need to be earthed and if you use SWA glands, these will bond the SWA to earth via the gland.  Glands normally are brass or aluminium.  

Your method of installation (in the trench with removable lids) sounds fine.

Make sure that you select the conductor size based on the worst case part of your run (which sounds to be the bit in the trench that will be covered with sand).  Six trefoils of 630mmsq, you must have a big transformer.  How many neutrals are you putting in?  Some wiring codes will let you have a neutral as small as one third the size of the phase conductors, but you would be well advised to consider a larger neutral if there is unbalanced load or harmonic load.

It is always nice to have a symmetrical arrangement with the neutrals, so T for a trefoil, N for a neutral if you were to have four neutrals then on each layer you would have:

T N T N T

Sounds a bit explosive!

Regards    
jbartos (Electrical)
29 Jan 03 22:00
Suggestion: The installation with armored cables will resemble a segregated-phase bus with all advantages and disadvantages of the segregated-phase bus.
Advantages:
1. Faults only in the form of ground faults
2. Protection against fault spreading to more than one phase
3. Field forces, static and dynamic, only between enclosure and conductor, not between phases
4. Protection against moisture and contamination
5. No losses in surrounding conducting materials
Disadvantages:
1. May get more expensive
2. More grounding is required
sunilji (Electrical) (OP)
1 Feb 03 20:19
Tnx to jbartos & bigamp for their learned and prompt response.
The transformer rating is 1600 KVA, 22kV/433V i.e. a full load current of 2250Amps. We propose to lay 6 runs per phase of 630 sq.mm. aluminium conductored, armoured cable i.e. 370 Amps per cable. Another precaution we are taking is to cut each length exactly equal.
We are also going a fully rated ( 100%) neutral as at least 25% of the load is non-linear( variable speed drives ) and we expect harmonics to be high.
We do not anticipate significant imbalance as the single phase loads are less then 2% of the total load.
The cables are proposed to be terminated on aluminium gland plates with flange type brass cable glands which ensure that the armouring can be securely earthed. Can eddy currents / ciculating currents be significant as long as the armoring is properly grounded  ? I was under the impression that trefoil formation would help to mitigate eddy currents and circulating currents.
My main reason for preferring armoured cable is because of point 4 mentioned by jbartos i.e. protection against moisture and contamination.
bigamp (Electrical)
2 Feb 03 13:54
Sunilji,

SWA will not provide protection against moisture or contamination, it is there only to provide a bit of protection against impact.  If you wanted to protect against moisture and contamination then you would use lead sheath (not suggesting that you do!).

Circulating currents can be significant and will lead to cable derating.  Check out MV cables.  In these the conductors have copper wire or tape screens.  The current ratings for these cables are on the basis that the screens are earthed at each end and are always less than the ratings for LV cables of the same CSA, due to added heating caused by the circulating current in the screens.  

Personally, I do not think you require a full sized neutral for your application.  As I understand it, third harmonic current is the one that causes problems, and this is typical of lots of single phase switched mode power supply loads.  VSD's (6-pulse, 12-pulse) are not supposed to generate third or triplen harmonic.

Regards   

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