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Does BS7671:2018 521:10.202 apply to a bungalow floor void? (Securing cables against collapse in a fire)

Former Community Member
Former Community Member

Good afternoon, I hope you are all well.



Firstly, I am not an electrician, but I am trying to save a lot of disruption all at once if I can.



Here’s the reason for the question. A couple of years ago I had a new Hager consumer unit installed by an electrician and had it fully populated with RCBO’s at my request. one RCBO for each circuit. It also has one main isolator switch. I have recently decided that I now want to re-organise the ring final throughout the bungalow. To achieve this I would like to do most of the installation myself one room at a time to avoid major disruption all at once. The bungalow is a small 2 bed with wooden floorboards covering 78sq/m approx. Under the floorboards there is an an 18inch deep (approx) void.



My initial thoughts are that I could run all the cables required for an entirely separate and new ring final, (not connected to anything) so an electrician can come in and connect all the new sockets and the new cables to the consumer unit when ready, and at the same time disconnect the old ring final. Thus, swapping old circuit to new.



As these cables will be run under the floor and cannot collapse on to anyone during a fire, are they still required to follow the new regulations and be secured to the underfloor joists with metal cable clips, or can they run free on the ground under the wooden floor as the existing cables do now.



My personal opinion is that a fireman would prefer to potentially step on resting cables, rather than step through clipped suspended cables if his or her foot were to go through the floor in this situation. Also from what I have read it is more desirable, thermally, to have cables in free air as opposed to being fixed or covered  However, I would be grateful for your expert guidance in these matters.



Other information which may be relevant:



Cable to be used will be 2.5mm twin and earth( 2x2,5 and 1x1.5cpc)



The new ring final will have 12 double sockets in total, inclusive of 2 spurs. 1 spur will be terminated at 1 socket in a bedroom, and one spur to be terminated at the RCBO in the consumer unit to ensure that there will be no junction boxes under the floor.



I know it’s not pretty, but the new cables will come up through the floorboards into short lengths of 38x25 plastic trunking (approx 8inches long) and then into surface mounted 25mm deep click mode pattresses and sockets. For fire considerations, the trunking will be fixed to the wall with screws into brick (no plastic rawlplugs). A neat cut-out will be made in the skirting board to ensure trunking meets the floorboards. Also for fire considerations, the cables in the trunking will be restrained by a safe-d metal cable clip.



The pattresses will be attached to the wall with 3 screws. Two with plastic rawlplugs and one directly into brick for fire considerations.



I hope I have given you all enough information to go on, and wish you all well.



As always, many thanks in advance.



Simon


  • 0
    I see no need for  fire resistant fixings under a raised ground floor with limited space thereunder. It is most unlikely that fire men would enter such a space.

    I also doubt the need for fire resistant fixings for very short lengths of vertical plastic trunking. Under what circumstances would it present an entanglement hazard ?


    Of greater concern is the proposed circuit layout. If I have understood correctly, you are proposing a single cable into the circuit from the consumer unit. That would make it not a ring final circuit and would not be permissible unless a larger cable is used.


    I would suggest involving an electrician an electrician at an early stage to approve your proposed design. I see no reason why you should not do most of the installation your self, but suggest a chat in advance regarding design, followed by inspection and testing.
  • 0
    You do not need any special precautions Simon. Collapse is not any little 8" length up a skirting board, or something under a floor which had collapsed, but cables where a significant length could tie someone up enough to prevent escape. A cable for a vertical switch drop is similarly quite safe in a fire, but ones across a ceiling or along a wall where a significant length could hang down are not. If you use any longer lengths of mini-trunking then a metal clip occasionally through the back is a good idea, and rather more often across a ceiling etc. The accidents in the past were usually very large groups of SWA cables hanging down from failure of plastic fixings, but reasonable care to prevent T&E hanging down is required. Once behind plasterboard in a ceiling for example (which needs a serious fire to collapse) cables need no additional fixings. Cables are often run loose under suspended floors, it is quite safe, and firemen are used to taking care when the floor may have become damaged.
  • 0
    I understand he is going to replace an old ring with a new one Broadgauge. 2.5,1.5 T&E is fine.
  • Former Community Member
    0 Former Community Member
    Thank you for your replies, most helpful. With specific reference to Broadgage reply, I knew I would miss out something important however hard I tried. To clarify, there would be three cables terminating at the 32A RCBO in the consumer unit. Two for the ring final and another for one spur cable to one socket only. Also the other spur is one spur cable to one socket only. As a thought, is there any limit to the length of a spur wired directly into the 32A RCBO in the consumer unit?
  • 0
    in the under floor void, I would try to avoid the cables resting on the soil, just to try and prevent rodent damage. Clipped to lower half of the side or the bottom of floor joists would be better.
  • 0
    If you you have ever tried this in an 18" OM you would realise it is very difficult to clip cables under floors like this! As long as the oversite concrete is OK and the air vents have grilles it should be OK on the ground, unless it is a very old property, then you will hear and smell the rats.
  • 0

    simon201:
    Thank you for your replies, most helpful. With specific reference to Broadgage reply, I knew I would miss out something important however hard I tried. To clarify, there would be three cables terminating at the 32A RCBO in the consumer unit. Two for the ring final and another for one spur cable to one socket only. Also the other spur is one spur cable to one socket only. As a thought, is there any limit to the length of a spur wired directly into the 32A RCBO in the consumer unit?



    That should be fine. I misunderstood your post.

    Nothing prohibits connection of a spur at the origin of the circuit.

    There is not any length limitation that is specific to a spur from a ring final circuit. However ALL electrical circuits have a limit as to length in order to avoid excessive voltage drop.

    Most unlikely to be a problem in a normal size bungalow though.
  • 0

    davezawadi:

    If you you have ever tried this in an 18" OM you would realise it is very difficult to clip cables under floors like this! As long as the oversite concrete is OK and the air vents have grilles it should be OK on the ground, unless it is a very old property, then you will hear and smell the rats.



     Didn't say it would be easy Dave, just offering it up as a consideration. I have the same sort of void in my ground floor flat. I put in wet underfloor heating between the joists and wooden/tiled flooring on top, left the cables on the subfloor as they were. Got visits from furry friends and have lost ring continuity. It happens.

     
     

  • 0
    if you are going under the floor, then while you are there do verify the airbricks actually allow good airflow, but do not admit mice and rats.

    (at one extreme damp rotting joists, and the other a risk of a rats nest, neither is what you want)

    If it is an earth over-site then as others have said, tying the cables up is good, but difficult unless most or all of the whole floor is coming up -  otherwise, cables laying on the concrete sub-floor is fine  I do caution against having any sort of joint resting on the floor or dangling in free space - try to get all joints above ground and  in places you can get to, like inside  socket boxes (and put more sockets in than you need now -something will use them ).  Mind you, if the floor is coming up, then take the chance to insulate between the joists - in terms of heating it is worth it.

    Personally, I try and avoid too many spurs on a new ring if I can, even if it means 'out and back' cables are side by side, but regs are fine with it.

    As noted - make sure your tame electrician is OK with this - they may want photos of what you have done to prove it is OK. Understandably, some are nervous of adopting the resposibility for the work of others they cannot see.

    Equally, if the old circuit was slowly extended and then later pruned so that only the new wiring remained, then strictly in England at least there would be nothing that needed notification at any step in the process- though getting it overseen is still sensible, and it still ought to be tested for continuity and insulation integrity.

  • 0

    The pattresses will be attached to the wall with 3 screws. Two with plastic rawlplugs and one directly into brick for fire considerations



    There's no requirement for that at all. The requirement for stopping premature falling in case of a fire is only for wiring systems - not accessories. Besides most trunking-compatible back boxes are PVC so would have melted long before the rawlplugs.

        - Andy.

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