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Wiring Matters

Wiring Matters - Autumn Issue 2015

Fire performance of cable supports

Recently, BRE undertook research concerning the fire integrity of cable supports and fixings. The aim of the experimental programme was to assess the ability of a selection of commercially available cable supports for electrical installations to maintain their integrity and hold electrical cables in place when exposed to temperatures typically encountered in compartment fires.  A second scoping study was carried out to assess the performance of a range of commercially available fixings for cable supports in concrete substrates when exposed to elevated temperatures. 

With the permission of BRE Global Limited (BRE), Wiring Matters provides you with that report here.

Gary Gundry, electrical safety specialist, technical consultant, and prospective owner of Redford Charles Training Centre, takes us through the report and provides a summary of what the findings mean.

The research on which this article is based was commissioned by the Department for Communities and Local Government (DCLG) and carried out by BRE.  Any views expressed are not necessarily those of DCLG, with whose permission the article is published.
 

In recent years, some firefighters have tragically lost their lives as a result of becoming trapped in ‘collapsed’ cables within burning buildings ― but a new regulation, introduced by Amendment No. 3 into BS 7671:2008, seeks to address that. The regulation came in to effect on 1 July 2015.

Photograph of the fire burning during the experiment

The regulation ― which now requires all new cables installed in escape routes to be supported in such a way that prevents them from collapse in the event of a fire ― follows the deaths of two firefighters at Harrow Court, Hertfordshire, in 2005; four firefighters at a highly insulated warehouse at Atherstone-on-Stour, Warwickshire, in 2007; and another two firefighters at Shirley Towers, Hampshire, in 2010.

In each incident, being trapped in fallen cables was reported as a contributing factor in the deaths in at least two of these incidents where the firefighters became entangled in cables that had fallen across doorways, in and along corridors of escape routes. This was most likely due to the failure of the supports used to secure the cables, such as non-metallic cable trunking, conduit, and/or plastic clips, when exposed to heat or direct flame.

Consequently, post-incident investigations left coroners with no choice but to make recommendations to a number of individuals, authorities, and UK government for safety improvements, stating that “action had be taken to prevent the recurrence of fatalities”.  One such recommendation was for BS 7671 to be amended so that all [emphasis editor's own] cables, not just fire alarm cables, are supported by fire resistant cable supports’.  Hence, Regulation 521.11.201 was introduced:

‘Wiring systems in escape routes shall be supported such that they will not be liable to premature collapse in the event of fire.  The requirements of Regulation 422.2.1 shall also apply, irrespective of the classification of the conditions for evacuation in an emergency.

NOTE 1: Non-metallic cable trunking or other non-metallic means of support can fail when subject to either direct flame or hot products of combustion.  This may lead to wiring systems hanging across access or egress routes such that they hinder evacuation and firefighting activities.

NOTE 2: This precludes the use of non-metallic cable clips, cable ties or cable trunking as the sole means of support.  For example, where non-metallic trunking is used, a suitable fire-resistant means of support/retention must be provided to prevent cables falling out in the event of fire.’

 

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What does all of that mean in practical terms?

In 2012, the Department of Communities and Local Government (DCLG) commissioned the Building Research Establishment (BRE) to do some experimental work in this area as part of the Investigation of Real Fires project, and to report on that work to the fire community and other stakeholders, as appropriate.

That work has since been completed and the report is now available.  BRE advises that the work was never supposed to be a comprehensive testing programme of individual components or products, but more a ‘proof of concept’.  So, the findings from the research could be used to demonstrate a possible simple solution to the issues that have raised concerns.

As the 12-page report is too comprehensive to reproduce in this article, the detail and findings have been summarised below.

Cable support experiments

Stage one of the research was to design and construct a fire test room (ISO room), sized 3.6 m x 2.4 m x 2.4 m, with an attached timber-framed and plasterboard (single layer 12.5 mm Type F) lined corridor of dimensions 3.5 m x 1.5 m x 2.4 m (see Figure 1).  The walls, floor and ceiling of the ISO room were lined with two layers of 12.5 mm Type F plasterboard to contain the crib fire and, with the structure being on wheels, it was slightly raised from the ground; hence the corridor was not in line with the top of the ISO room (see Figures 1 and 2).

Schematic of the ISO and corridor

Figure 1. ISO room and attached corridor, showing timber sleepers and concrete lintels above corridor construction

The test rig was then completed by positioning two timber sleepers and two concrete lintels above the corridor, spaced approximately 750 mm apart, and alternate to each other, to which the cables and supports were then secured.  For ease of reporting, BRE numbered the lintels from 1 to 4, where lintel 1 was closest to the ISO room, and therefore closest to the fires.

Photograph of the room and corridor used in the experiment

Figure 2. Real world set-up

Close up of cable set fixed to timber sleeper with different supports

Figure 3. Close up of cable set fixed to timber sleeper with different supports

Cables and cable supports

In each experiment, five standard 1.5 mm2 flat twin and earth PVC sheathed cables were fixed to the lintels by means of commercially available ‘fit for purpose’ cable supports (see Figure 3), the detail of each is shown in Table 1 of this article.  However,  for ease of reference, four were of metal construction, with one type used to secure a cable oversleeved with a length 20 mm heavy gauge PVC conduit, and the fifth was moulded plastic cable clips.  To help simplify things going forward, wherever the term ‘support(s)’ is used, it refers to metal cable supports; ‘clip(s)’ refers to plastic cable clips; and ‘fixed’ or ‘fixing(s)’ refers to the means of securing the cable supports and clips to the structure.

All of the supports, except for clip 5 (as it was fixed using its own pin) were fixed to the lintels by means of zinc plated hardened steel 7 x 1¼ inch screws, but in the case of the concrete lintels, standard plastic wall plugs were also used.

Table 1 Cable supports used in the experiments

Number  Detail
Support 1 Zinc plated saddle support. These supports are used in the automotive industry for fixing cables.
Support 2 Galvanised steel spacer bar saddle for use with conduit.
Support 3 Passivated stainless steel cable support with fold over fastening tabs for use with 2 core 1.5 mm2 fire alarm cables. The support meets the requirements of BS 5839-1.
Support 4 Double cable saddle support for 2 core 1.5 mm2 fire alarm cable. This support was made of copper and coated with a polymeric coating. The support is understood to comply with BS 476 Part 6, BS 476 Part 7, UL 94 and meets the cable support requirements of BS 58391:2013.
Clip 5 Moulded plastic flat 1.5 mm2 clip with pre-fitted zinc-plated carbon steel pin.

 

Bench-scale experiments on fixings

A series of bench-scale experiments were also carried out using a range of commercially available fixings for cable supports for use in concrete substrates. Including screws with plastic plugs and self-tapping screws, the fixings were installed into standard aerated 3.6N concrete blocks of dimensions 440 mm by 215 mm by 100 mm deep. Each block was exposed to one set temperature - from the range 100 °C, 200 °C, 300 °C and 400 °C - for one hour.

Cable Supports - Fire 1 results

The first fire burned for approximately 34 minutes and throughout that period temperatures were monitored at various points across the rig (using instrumentation and thermocouples that are not discussed in this article).  The maximum recorded temperatures at ceiling height in the ISO room and the corridor were 397 °C and 302 °C, respectively. 

To visualise those measured temperatures, especially at each lintel, and see how each support performed, refer to Table 2 as it clearly shows which supports remained intact and those that did not.

After the fire, the rig was allowed to cool down before the condition of the cable supports were examined and documented.

Table 2 Summary of condition of cable supports after the first experiment with the average maximum temperatures recorded at each lintel

Lintel Type

Average maximum temperature at lintel (°C)

Average temperature when cable observed to drop (°C) Support 1 Support 2 Support 3 Support 4 Clip 5
1 Timber 264 258 All intact All intact All intact All intact Four failed
2 Concrete 294 286 All intact All intact All intact All intact All failed
3 Timber 255 243 All intact All intact All intact All intact All failed
4 Concrete 204 No drop All intact All intact All intact All intact Four failed

 

NOTE: The average maximum temperature was determined by averaging the values recorded by each pair of thermocouples located at the lintels.

Cable supports - Fire 2 results

The second fire was more intense than the first, and lasted approximately 20 minutes.  The maximum temperature recorded in the ISO room, 0.5 m from the ceiling, was 820 °C, and in the corridor, approximately 1.8 m from the ISO room, the maximum temperature recorded at the ceiling was 690 °C.

To visualise those measured temperatures, especially at each lintel, and to see how each support performed in the second fire, refer to Table 3, where, just like in the first fire, all of the plastic clips failed again but something else was observed too.  All types of supports fixed at lintel 2 also failed, but, on closer inspection, it was not the supports that had failed, but rather the plastic wall plug(s) used to fix the supports that had melted, so they ‘fell’ from the ceiling. 

Once the fire had cooled, all of the metal supports from lintel 2 were recovered and were found to be intact.

Table 3 – Summary of condition of cable supports after the second experiment

Lintel Type Average maximum temperature at lintel (°C) Average temperature when cable observed to drop (°C) Support 1 Support 2 Support 3 Support 4 Clip 5
1 Timber 578 481 All intact All intact All intact All intact All failed
2 Concrete 557 212 (Clip 5) and 221 (Clip 2) Three failed* Three failed* Three failed* Three failed* All failed
3 Timber 570 486 All intact All intact All intact All intact All failed
4 Concrete 388 310 All intact All intact All intact All intact All failed

NOTE: The average maximum temperature was determined by averaging the values recorded by each pair of thermocouples located at the lintels.
* Each fixing failed rather than the support because the plastic wall plugs melted.

Bench scale experimental results

The findings from the bench-scale fixings study showed that combustible wall plugs in concrete substrates demonstrated signs of weakening of mechanical strength from 300 °C and above. Additionally, these fixings can fail at 400 °C after up to a one hour exposure in controlled conditions. 

Non-combustible wall anchors and concrete screws were capable of retaining mechanical strength after exposure to 400 °C for one hour in the same controlled conditions. This allowed the conclusion that there are commercially available products which can maintain their mechanical strength at that temperature.

Conclusions

In this article we have tried to summarise the findings of BRE’s experiments, to demonstrate the effects of fire on a selection of cable supports and fixings.  However, it seems only right to report that the actual performance of any support and its fixing in a fire will always be dependent on a number of factors, including the combination of the cable supports or clips, their fixings, the weight of the cable, the distance from the fire and the duration of exposure.

BRE’s experimental work does, however, indicate that selecting and erecting supports that are capable of maintaining their mechanical strength when exposed to temperatures of, at least, 600 °C should prevent cables from falling, when exposed to typical compartment fires.

Looking now at the new requirement introduced into BS 7671, which calls for all cables within escape routes to be supported such that they will not be liable to premature collapse, whether they are fixed to a wall, or to the underside of a ceiling regardless of being clipped direct to the structure, or enclosed within or on a wiring system, does literally mean all cables. So, anyone who installs cables for door entry systems, digital TV and data-cables, for example, must comply with this new requirement.  Installers of such cables may however contain them within a non-metallic conduit or cable management system provided either is fixed to the building structure with non-combustible supports that would maintain mechanical strength when exposed to temperatures greater than, say, 600 °C, to prevent any of the cables from falling out and/or dropping. 

Following BRE’s results that demonstrated failure of fixings can result in failure of the support, it would follow that consideration should also be given to the fixings used for the supports.  With their research showing that there are simple non-combustible commercially available fixings that can maintain their mechanical strength up to 400 °C.

Finally, if there is only one thing that you should take away this, it is this: the new requirement in Regulation 521.11.201 precludes the use of non-metallic cable clips, cable ties, and conduit or cable trunking as the sole means of support for cables in escape routes. 

A full copy of the report is available for you to download.

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