Fire resistant electrical "cupboard"

[Tim Watts]

I will be wiring lighting soon (hooray) and I have determined the way I want to do it is bring all cables back to 4 sets of places in my dormer

- near the ouside where the cable trays run.

Each of these 4 locations will be a "cupboard" on the dormer uprights in some semi dead space (where the ceiling is low) but usefully accessible. I'm not a fan of junction boxes all over the place even if that's how it's usually done.

I will stick SELV transformers here too.

There'll be a 5th cupboard housing a small sub-CU for all external circuits (sheds, outside power, pond, whatever) fed by a distribution circuit from an MCB in the main CU.

So it seems sensible to make these cupboards, which are mounted on timber framing in a very wooden part of the house, a little more fire resistant - just in case a SELV transformer goes ape, or a JB gets hot from a fault.

I was thinking ply lined inside with 12mm plasterboard. Simple. All we are talking about is if something gets hot, as in ignition point of paper type hot and melts, that it is unlikely to set the enclosure alight.

Or is there a better/easier way? I've seen a reference to fire resistant ply:

What does that mean???

[Nightjar]

Wouldn't it be easier to use a ready made steel enclosure?

[Tim Watts]

No -

I'd prefer something I can screw and cable clip to.

Most people would probably just use ply. I was wondering what I could do for minimal extra effort to give it a bit of belt and braces :)

[harryagain]

If you are that worried by fire as nightjar says, the way to go is steel enclosure (and steel conduits and trunking)

Use all metalclad switch gear/switches/sockets etc

This is all properly designed stuff, unlike anything you cobble together which might even be counter productive.

[Nightjar]

A steel enclosure with a sheet of ply mounted inside?

The fire retardant ply you mentioned seems to limit the surface spread of flame, rather than being completely fire proof. So lining with plasterboard is probably better, as well as most likely cheaper, perhaps with an ordinary ply or MDF sheet at the back as a mounting board. For a one hour fire wall in one of my factories, we had to use two layers of

12mm plasterboard, with the joints not aligned. The doors might be a problem unless you make those from metal.

[Tim Watts]

Does sound like the way to go...

Doors - I was thinking to cover the inside in plasterboard too - at least a big enough bit that will mostly go into the hole when they close.

Am less worried about the doors as they will not be in direct contact with the equipment[1]. I was thinking more about the SELV PSUs going up in smoke.

Right now, the only one I have is on an open rafter and open rafters are really hard to set alight. But in an enclosed space with thinner ply, it becomes slightly more credible that a bit of really really bad luck could start a fire.

PB is cheap and quick so not much effort for a small but significant gain I feel...

[1] I have a main CU mounted on ply. But it's a small bit on a brick wall up against the ceiling - so even if the ply catches fire it cannot really set anything else alight. It was mounting lots of equipment especially with power electronics in against a load of structural timber in the roof that got me thinking...

[Andrew]

Any sheet material that is decribed as class O should be suitable.

Here is chapter and verse from the people who know :-

[John Rumm]

Probably just ply impregnated / treated with flame retardants. It does not make it fire proof as such (it will still burn if exposed to one), but ensures it won't in itself sustain a fire.

I would have though plasterboard would be a quicker and cheaper option. You can clad it in ply for easy "nailability" and cocoon it in fibreglass quilt if you are really annal ;-)

[Dave Plowman (News)]

Plasterboard is probably the easiest way of providing a degree of fire retardation. But being a cupboard, if you really think there is a danger of this some form of alarm inside would be a good idea?

[Tim Watts]

Thanks John (and Andrew, Dave) - all interesting :)

[Nightjar]

In the days of halon fire extinguishers, you used to be able to buy ones with a length of plastic tube attached for use in small enclosed spaces. If there was a fire, the tube would melt and release the gas, putting the fire out.

[meow2222]

Don't forget ventilation needed - vent through a metal tube to stop fire spread. Food cans wired together perhaps?

NT

[Tim Watts]

I don't think that will be a problem with maybe 1 PSU per cabinet - should be able to lose heat by conduction.

But if it were, and intumescent vent is probably what I'd go for (self seals in a fire).

Everything else is just wago terminals - this is just me being belt and braces with a "what-if" something got hot because of a bad contact (happens).

[Nightjar]

Use the Davy lamp principle and fit a fine mesh over the vent.

[Tim Watts]

That would also work - best not use ali mesh though :)

[meow2222]

Would need to be a bigger cabinet then.

How quickly after fire escapes do they block?

NT

[spuorgelgoog]

You could use meter cupboards. Fireproof metal box, f/r backing board for fixing things to, and a neat(ish) door and frame with a cam-lock.

[Tim Watts]

Too deep. No this is more of an in place fabrication.

It'll start as a bit of board over the studwork - then get sides and a door as renovation progresses.

I am settles with the idea of ply with a bit of PB stuck inside - all it needs to do is cope with an item running hot if it goes horribly wrong.

I'm convince PB will do that OK :)

[The Natural Philosopher]

Build with masterboard/multiboard instead of plasterboard, though even plasterboard has reasonable fire resistance

It skims and paints fine.

My experience of house fires is that hot gas and flame do the most damage, not just a high temperature.

Serious fireproof is double metal or ceramic (brick?) with insulation between. a bit OTT?

[Tim Watts]

Thank you - I like the look of that:

Indeed. One time in a million some PSU or JB will fry badly.

As I've taken the decision to keep my lighting JBs bundled in groups for accessibility, one in each corner of the roof, it would make sense to mount them on a bit of 18mm ply fixed to the dormer uprights.

From there it was a tiny step to make them part of a wall (the upper parts of the wall will have celotex, so we have some "spare" depth going anyway).

From there it was a tiny step to say "how can we take the 1:1000000 risk of a PSU gettind red hot and stop it being a risk to the timber that surrounds it.

That's the trouble with "brownfield" development.

Greenfield - you just say "meh - this is the perfect layout, make it so".

Already developed - well your stuffed and make do.

Brownfield - contrains everywhere but just enough freedom to say "how can we do this really nicely to be easy to maintain"...