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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

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  • 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

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  • 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

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