Support of cable to BS5839-1

[Graeme]

I know that the standards asks for support i.e metal clips that can withstand the same temperature as the cable itself when the cable relies on them as sole means of support

but if an installer uses metal clips onto a brick wall does the fire rating of the support extend to the plastic wall plugs used to put the screws into for the metal clips as these plugs are supporting the cable as well?

[kurnal]

Yes its a good point in theory but sense and reason must come into it as well.
Usuallt the load on a clip is perpendicular to the screw, it would be more critical if in line with the screw shank.

And in the real world for how long do we  the resilience of the alarm cable to be maintained? Surely once the alarm has actuated and people evacuated then it doesnt really matter if the smoke detector falls off the ceiling and the sounders melt- its done its job. I would suggest the plastic plugs drilled into brick are much better protected than the rest of the exposed system notwithstanding the screw.

[Graeme]

That was my thinking that the wall plug is protected inside the wall but i am just curious.
The cable is also supposed to stay on the wall after the alarm has evacuated the building.

[Wiz]

Has anyone done any tests to see if the fixings for a cable being used for the connection of sounders, and clipped to a ceiling, does not suffer from the rawlplug melting by the heat from a fire below and the cable breaking as it falls down?.
A building with phased evacuation may require the alarm sounders to be able to operate for long periods after a fire may be detected ( This must surely be for almost an hour if we are talking about a system with more than four phases of evacuation thereby requiring 'enhanced' grade cable.
Whilst I can understand professor K's points when we are talking about cable fixed to walls, I think that Graeme's question is possibly more relevant to fixings on ceilings - I could imagine rawlplug style fixings failing pretty quickly.

[John Webb]

An even bigger problem is when there is a false ceiling to whose tiles sounders and visual beacons are fitted. As the ceiling is damaged by a developing fire and starts to collapse the wiring can be mechanically disrupted with consequential loss of alarm. This may have happened at the 1993 Littlewoods store fire at Chesterfield within four minutes of the deliberate ignition of clothing on display. There was little warning of the fire on the floor of origin due probably to this damage. (See the Derbyshire Fire & Rescue Service's Fire Investigation Report page 47, paras 2.5.7,8 & 9)

[kurnal]

John do you have access to that FI report??

I should have remembered that incident and have been looking for that report for other purposes without success.

The conclusions to be drawn so far are therefore that :
1- fire alarm cables should never rely on any structure for their support that does not itself have guaranteed integrity in a fire for a suitable duration (which may be significant period depending on the evacuation strategy of the building)
2- Cable fixings need to be considered in their entirety including the ancillary screws and rawl plugs or similar.

3- In newly built IT suites I have seen fire alarm cables fastened to cable trays intended for network cables with plastic tie wraps- the trays having been installed at first fix-  and havn't thought about questioning their integrity before now.

Is there anything in BS7671 or similar that covers this Dr Wiz?
Starting to regret my flippant initial response now Graeme!!

[John Webb]

Kurnal,
I have a photo-copy, not an original, so the photos aren't very good. Penny Morgan (who occasionally contributes to the forum) and myself were the Fire Research Station team (with a photographer) who assisted Derbyshire F&RS with the investigation, so I saw the damage in person.

What was it you were particularly interested in?  Do e-mail me direct if you wish.

[Thebeardedyorkshireman]

Kurnal
Without going off on one, it was the good doctor and myself who did the cable tests at BICC warrington which added to the issue surrounding the cable changes to BS 5839, 2002.
We were angling for enhanced to be pyro only, for a variety of reasons including the cable clip issue. Pyro can be sufficiently supported by resting on support walls and ceiling grid. In a high temp fire test, soft skin cables rapidly expand in diameter to resemble cylindrical cigar ash. When unsupported (in our tests, just to protect the innocent) they failed to perform. When firmly clipped with metal clips this whole fragile structure remains for a duration of time, acting as an insulator both inter-conductor and to earth (if present) If the clip failed or was not present, the 'cigar ash' fell away and you had a short circuit ( the cable does not break which is the point of all this claptrap ) please dont add water or mechnical movement children or we will be here forever.
 This is a very short 'jist' of some very long and detailed testing and represents my personal views & recollection only.

In short, if you lay the cable 'on' tray it is supported along its entire length and plastic clips are fine. If you clip it to a vertical tray or to the underside of a metal tray you should use metal clips spaced in accordance with the manufacturers instructions. This may answer your IT room situation. You could allways use pyro, which is a cable designed to do a job, not a cable designed to pass a test
Dave

[Wiz]

Dave (TBY), any experience / knowledge of plastic plug fixings melting in a fire within 30 / 60 mins?

[kurnal]

Thanks Dave. Nice insight as well into your and Dr Wiz' past.

So you have both Been Incarcerated in Corrective Care before have you?

No wonder Wiz is such a wet lettuce when it it comes to escaping from this home for the bewildered. Too much exposure to plastics fumes has addled his brain - you should have made sure he did not inhale when smoking those cigars. Now he cant tell the difference between a manikin  and a mannequin

[Thebeardedyorkshireman]

Wiz
we carried out no tests on the plastic plugs in a fire condition.
We can make a few observations if they will be of help though. The makers of FP200 (used to be called Pirreli) make a fixing gun which fires a stud through a metal clip to support the cable. This may be a better fire rated fixing,but you would need to talk to their techies.
A metal tray fixed to the ceiling with rawl bolts or para anchors and the cable laid on the top and fixed with steel clips is probably as good as it gets for an enhanced cable run.
If the enhanced run includes detectors and sounders along its length and electrically connected to the cable then the weakest link would be when the head melts and falls out, thus severing the continuity of the conductors. It may be that this would occur before the plastic plug melts. The base of the detector would normally be installed with screws into same plugs therefore providing another potential weakness. If you make the judgement that plastic plugs are bad news then you would need to eradicate all similar low temperature weaknesses from your enhanced circuit.
Thus in say a hospital basement duct you would have your enhanced circuits running through the area with detection on a separate sub loop with isolators which could be classed as sacrificial after raising the alarm.
This was the point of our tests in that cables running through an area on fire still have to function to provide facilities to other areas. In this context function means an ability to pass electronic data not just an electrical circuit which illuminates a bulb. I hope this is not an egg sucking situation but I will stop rattling on now.
Regards
Dave
ps what happened to the matron in the black stockings?

[Wiz]

Thanks for the above Dave.

It seems the answer might be as I thought it might be;

1) It looks like no-one has specifically looked at the possible problems of the use plastic plug fixings for fire resistant cable.

2) It is probable that the heat needed to seriously detrimentally affect a plastic plug fixing located within a wall is likely to be so great, that it will have seriously detrimentally affected lots of other things before it affects the fixing!

Is the above a fair summary?

For your info, I already have experience of the special fixing gun that is available. It works o.k., but is so powerful it often blows away all the plaster around each fixing! Also the cost of fixings is very high.

Wiz

p.s. Most of us in this home for the constantly bewildered are being dosed with drugs to stop us thinking about matron's black stockings. The drugs are obviously not working on you and I suspect you will soon be getting a visit from Professor Kurnal to find out what is going on.

[Fishy]

I wouldn't agree at all that "It is probable that the heat needed to seriously detrimentally affect a plastic plug fixing located within a wall is likely to be so great, that it will have seriously detrimentally affected lots of other things before it affects the fixing".  I recall that these are typically polyethylene or nylon.  In either case they the screws would quickly transfer enough heat through to the plugs to melt them - the melt zone only needs to reach the depth of the threads and they'll just drop out - especially on a ceiling.

Interestingly, I cannot find any BS or manufacturers' instructions that deals with this in any detail.  Pirelli just recommend their proprietary fixing system, for example.

[kurnal]

You are right fishy and so often surface mounted cables are simply installed using standard plastic wiring clips rather than the recommended  plastic coated metal P clips. And till Graeme raised it I for one have always observed it and not passed comment.

[Thebeardedyorkshireman]

I have just spoken to a technical chap from 'Rawlplug' who confirms that they have never conducted tests of a type we are refering to on a plastic plug. He concured that the wall mount may be preferable to a ceiling mount and that the dia of the hole, length of the screw,dia of screw and amount of compaction of the plug material would all be variables in the final outcome. The temperature would be the major consideration ( but we all know that).
If you use pyro and the clips fail the probability is that the cable will continue to work up to allmost the melting point of the copper. I do not believe that the new cables, including enhanced, will continue to function.
Perhaps this calls for additional fixing methods above and beyond that which is normally accepted if the full benefit of the cable integrity is necessary to ensure a working fire stratergy. In short, clip it on a metal tray bolted to the structure.
It would be necessary for somebody to specify this at the time of tender or you will get plastic plugs and screws.
With hindsight I wish we had tested the fixings when we had the furnace available to us. Its brookside now.
Maybe we need a note in the BS on the next revision?
Dave