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CU, head and meter enclosures fire resistance

Discussion in 'Electrics UK' started by frank999, 29 Jul 2020.

  1. JohnW2

    JohnW2

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    I don't really understand that diagram. I would normally expect the vertical drop to the socket to all be buried in a wall, in which case it would be 'supported' by it's entrance into the top of that wall (and also where it emerges from somewhere at the top right of the room), and therefore presumably could not do what your diagram is suggesting

    .... or are you assuming that the wall, as well as the ceiling, has 'collapsed'? (if so, I suspect that the firefighters would have more to worry about than cables!)

    Kind Regards, John
     
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  3. AndyPRK

    AndyPRK

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    I think you just need to appreciate fire fighters risk their lives going into buildings to saves lives.

    They really don’t want to be electrocuted and die in the process as their are enough dangers
     
  4. JohnW2

    JohnW2

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    We know that, and very much admire, and are grateful for, what they do. As a matter of detail, I don't think that they should get electrocuted, if they're competent, since one of the first things they would (should) do, before moving around in a burning building, is to kill the power.

    The issue is not with those who risk their lives going into buildings to save lives - it is with those who've grown out of that and spend most of their time sitting at desks. As I said, one would like to be able to trust the opinions and advice of the fire brigades about these matters, but after the 'con-combustible' CU business, I personally am not at all sure that we can.

    I would personally like to see some facts, figures and proper risk assessments. As For example, as I've said in the past, it's only a matter of time (if it hasn't already happened) before someone dies as a result of 'fiddling' in a CU which is in an earthed metal enclosure, and I would like to see some concrete information which enabled me to form an opinion as to whether there is enough benefit 'in the other direction' to balance those deaths.

    Kind Regards, John
     
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  5. bernardgreen

    bernardgreen

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    If it was buried under plaster and the plaster failed due to the heat of the fire then the cable could easily "rip" itself out of the wall.

    The room may be free of fire ( extinguished or all fuel burnt ) and could be the only access route to persons trapped elsewhere. Making progress through a room filled with smoke, debris and zero visibility is very hazardous.
     
  6. JohnW2

    JohnW2

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    I would probably be inclined to regard that as 'barrel scraping', requiring multiple failures (albeit that may happen). Furthermore, I imagine that many would feel that they had satisfied regulations by clip(s) which were largely (or totally) reliant on being fixed to a plastered wall - so if the plaster 'failed', one could easily be back to situation you envisage.

    I would love to see some (ideally a good few) photos of situations in which these 'dangling cables' have arisen, so that I could try to develop some understanding of the sort of ways in which it is happening (not to mention some data on how often it happens).

    Kind Regards, John
     
  7. flameport

    flameport

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  8. JohnW2

    JohnW2

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    Thanks. I'll have a look.

    I would imagine that surface-mounted mini-trunking is probably the most common situation in which this could be an issue.

    Kind Regards, John
     
  9. AndyPRK

    AndyPRK

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    You can even get metal clips which fit inside trucking now.
     
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  11. SimonH2

    SimonH2

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    I vaguely recall that the main incident that triggered this requirement, things had been hot enough for the concrete to start spalling. It's certainly not hard to consider plaster failing under that sort of heat. Even a cable that's free at one end can be a significant problem if you get caught up in it.
     
  12. JohnW2

    JohnW2

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    Is it therefore your belief that all cables buried in plaster should be clipped within in the chase, using metal fixings which are somehow secured to the underlying masonry?

    Kind Regards, John
     
  13. EFLImpudence

    EFLImpudence

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    Are people forgetting the the regulation only requires prevention of premature cable collapse?
     
  14. JohnW2

    JohnW2

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    They might to be forgetting but, rather, simply don't have access to a clairvoyant who could tell them what it's meant to mean.

    Most dictionary definitions of "premature" talk about 'earlier than usual' and/or 'earlier than expected'. Hence, if, with knowledge of the way in which a cable has been installed, one 'expects' it to collapse within 5 minutes of the start of a fire, one presumably has to do nothing to prevent it collapsing that quickly?

    Maybe written by the same person who brought 'non-combustible' into the equation? :)

    Kind regards, John
     
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  15. EFLImpudence

    EFLImpudence

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    That is true but some seem to think the cabling should remain in place even after the building has collapsed.
     
  16. JohnW2

    JohnW2

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    They do. However, whatever "premature collapse" of cables is meant to mean, it certainly doesn't mean that.

    As I said, unless someone can produce a better explanation, the requirement appears to be to take steps to prevent the cable collapsing any earlier 'than expected'!

    Kind Regards, John
     
  17. SimonH2

    SimonH2

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    Far from it.
    But in the case I can remember, and which I suspect triggered the change in regs, firefighters were working in a building (yes, another tower) where the concrete was already spalling from the heat - i.e. bits of concrete were falling off the underside of the floorslab above them. That also implies that the ceiling (probably plasterboard fixed to timber battens) had already fallen. Two unfortunate firefighters found themselves with fallen cables tangled round the valvegear of their breathing apparatus, were unable to escape, and sadly died. IIRC they were engaged in a sweep of the building to ensure that no-one was left in there - long after a normal evacuation should have finished. Part of the report's recommendations was to change the design of the BA to make it less susceptible to entanglement.
    It's not unreasonable to apply "reasonable" measures to avoid that happening again - but there is some debate as to where that "reasonable" line should be drawn.
    What appears clear (to me at least) is that we cannot assume that "anything above the ceiling" is automatically catered for by the ceiling lasting as long as anyone will be in the building. I also think that we may not assume integrity of the plaster holding cables in the wall. But both of these would need to be done on a case-by-case risk assessment - in a two story block of 4 flats I think it would be reasonable to assume the ceiling is still up and the plaster intact by when the building is declared "clear". In a tall tower block, that is highly unlikely to be the case. Also, in the corners of a room it may be less of an issue, but on a route likely to be used during/after a fire it would be more of a concern.
    Then we might need to consider which is worse amongst different options. Take a typical flat with concrete slab floor slabs ... Cabling to the power sockets is likely to run across the ceiling and down the walls to the sockets. When the ceiling goes, it's unlikely that the cables will stay out of harms way - that could only happen if no slack AND they take a straight line route across the ceiling. So is it better that they stay dangling at random heights ready to entangle the BA of an unlucky firefighter - or is it better for them to fall to the floor and add to the general detritus they would already be expecting to have to negotiate ? One thing is for certain, unless the cables are fixed to the floor slab with fire-resisting fixings, they are not going to stay out of harms way when the ceiling comes down - so that perhaps outlaws the practice of clipping cables to the timber battens before the plasterboard is put up ?
    EDIT: Also, these days, if the cables are clipped to the timber with fire-resistant fastenings, then as the ceiling comes down, it's going to try and pull the cabling down with it - possibly ripping it out of the walls before they would have fallen due to their own weight.

    Or perhaps we rethink the fire resistance of typical ceilings - perhaps making them 1 hour (or more ?) fire resistant, which would imply additional construction costs (and maintenance restrictions).

    BTW - separate thread coming along when I find time, I'm now had a look at BS EN 61439 (I have access to many BSs online at work), and it doesn't say a number of things that people seem to think it does ;)
     
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