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Cables exiting LV Switchpanel into concrete trough below

fiftyhertz

Hi All,

Ive inherited a job, where we have a large LV Switchpanel sitting ontop of an open concrete trough.

I have approximately 18No. 95mm/120mmm SWA 4 Core cables feeding panel boards exiting into the trough below from the switchpanel for about 3 meters before it exits the trough and below the building but still above ground level (the buildings on stilts) onto a tray.

I am just struggling to find the correct de-rating factor in BS7671, 117-120 doesn't seem to quite fit the bill, but 119 seems to be the closest. But cant see it working, especially as it recommends 75mm apart, witch is 18 x 75mm = 1350mm and a maximum of 6 cables

This must be a reasonably standard arrangement, but i am stumped.

Any help would be much appreciated.

Thanks

  • 0

    I wonder if 117 is the answer, a tray/ladder on either side of the wall with 9 cables on either side, and a louvre in the center of the trough to allow ventilation in

  • 0

    Without seeing the actual arrangement I can't be sure, but if, for example, you've a cable pit that's actually large enough to stand in with a decent amount of ventilation you might quite realistically be looking at free air, possibly on ladder / tray depending on mounting and with an associated adjustment for grouping including outgoing circuits on the same section of the rack... After all the void is at that stage as large as some rooms anyway.

    Most likely (but you are right to check) it's the buried section that's the constraint anyway.

    Re "it recommends": Don't forget that Appx 4 doesn't say you can only install it this way; it is saying the tables are based on those assumptions. You are free to use other methods if you can justify them, for example by reference to ERA 69-30.

  • 0 in reply to Jam

    Hi Jam,

    thanks for the reply, the problem is I don’t think I can say that the trough itself has ventilation, as the switch panel is sitting above it and effectively acting as a lid. The room itself has large louvred doors, so I’m thinking if I install a Louvre in the trough itself (which is paradoxically formed in concrete above ground)it should create some forced ventilation and thereafter, I might be able to use Figures for tray as you say.

    Edit: the trough is approx 2m h x 1m w x 5m l

  • 0

    i assume you know you can get cables packed rather closer sideways before you need to consider the neighbours, than you can vertically - as vertically cables are heated by the ones below.  it may not be mechanically practical, but if you can lay the cables side by side or even on a rack that is canted at an angle they will cool better than parallel to the trench wall.

    you will need some vertical stacking but if it can be offset so that they are not exacttly one below the other it will cool better.

    In reality nothing beats getting a bit of airflow, so do not put any cable  fully loaded  too close to the lid, ideally below the highest point that air can escape. The simple model assumes the wall is very high and airflow at the top is essentially unobstructed here it sounds like neither is true.

    If the trench is above ground do you need to consider the heating effect of direct sunlight ?  This can easily add 10-20C to the temperature compared to underground on a hot summers day.

    Mike

    PS that illustration  is from a paper that in turn refers to the ERA test results. in your case De is 40mm or 50mm approx. and presumably loading of something like 300/400 A per core top whack..
    Can you say if, perhaps from size of the available supply, that not all 18 cables will be going full chat at the same time ?

  • 0 in reply to mapj1

    Hi Mike, thanks for the reply very helpful. Yeah youre right, i would expect the cables to be around 60% capacity, so actually quite low. The trough although above ground, is actually inside the building, its very hard to visualize. so i dont think heating gains will apply. 

    I think ive actually found the solution, which is going to be doubling the trench size to a 2m wide trench, with the switchpanel covering 700mm or so, then over the other 1.3m with a GRP grated flooring to allow ventilation into the trough, the whole front of the LV Switchroom is essentially louvred so should allow in plenty of ventilation. 2 No. trays are going to be mounted in the centre of the trough suspended, each with 9 cables, and every 3 cables allow 2DE.  Also, going to upsize each of the cables to give some allowance for heating

    The GRP to the front of the switchpanel, also comes in handy that its non-conductive, and essentially acts as an earth matt to some extent.

  • 0

    well 60% of full current is (I squared R) 0.6* 0.6 of the heating so  about 0.36,  and  temp rise drops pro-rata more or less with dissipation.  You can have almost as many cables as you like horizontally spaced by two diameters or more, adding more runs to the edges does not really affect the middle ones. Not true vertically where the poor so and so at the top gets all the heat of all the cables below it so adding more keeps making it worse.

    Grid/ mesh floor /lid is good so is wider trench. Less clear on your insulated earth mat idea, that I may have mis understood

    Mike

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