Amendment 3 BS 7671: 2008
Amendment 3 was created in response to the increased number of consumer unit fire caused by HRC and the danger posed by melting plastic from the consumer unit. After consultation with various bodies and organisations BEAMA came up with a “solution” that was introduced as Amendment 3 BS 7671: 2008.
Awareness of consumer unit fires had been increasing in professional circles for a number of years. But when London Fire Brigade publicly stated that there was an average of 5 consumer unit fires per week in the capital the subject was taken far more seriously.
In a video interview with IET Chief Engineer Geoff Cronshaw, London Fire Brigade Investigator Charlie Pugsley said that the main fault that they had come across leading to ignition of consumer units was loose connections.
The purpose of Amendment 3 is that “any fire is contained within the enclosure or cabinet and the escape of flames to the surroundings of the cabinet or enclosure or into conduits trunking or ducting is minimised, as intended by Regulation 421.1.201.”
Amendment 3 states that:
421.1.201 Within domestic (household) premises, consumer units and similar switchgear assemblies shall comply with BS EN 61439-3 and shall:
i. have their enclosure manufactured from non-combustible material, or
ii. be enclosed in a cabinet or enclosure constructed of non-combustible material and complying with Regulation 132.12.
NOTE 1: Ferrous metal e.g. steel is deemed to be an example of a non-combustible material
Amendment 3 also states:
There is no specific requirement in regulation 421.1.201 for fire-rated cable glands or intumescent sealant to be used however, this does not preclude the manufacturer /installer using these or other methods, should they be considered necessary.
There can be little argument that the progression from consumer units constructed from a flammable material like plastic to a non-combustible material such as steel is a welcome move.The main reason recorded for the issue of consumer unit fires (HRC) is poor workmanship including:
- Failure to tighten neutral conductor connections to the same torque as line conductor connections, due to a misconception that they carry less current
- Failure to check factory installed connections for tightness where required to do so by the manufacturer
- Tightening connections to an incorrect torque and/or with inappropriate tools
The introduction of a metal enclosure is undoubtedly a positive step. But the purpose of Amendment 3 is that “any fire is contained within the enclosure or cabinet and the escape of flames to the surroundings of the cabinet or enclosure or into conduits trunking or ducting is minimised”. How is this going to be accomplished without the requirement and enforcement to use intumescent sealant and fire proof glands? And the original problem Amendment 3 was that of poor workmanship. How does Amendment 3 improve workmanship and how effective is amendment 3 going to be if either intumescent sealant or glands aren’t installed?
The example given in Amendment 3 of a non-combustible material for the consumer units is steel. Steel has been embraced by all manufacturers as the preferred manufacturing material for compliant consumer units.
The melting point of steel is approximately 1300°C whereas the melting point of plastic is 170°C. What is more dangerous? With a steel enclosure not only will there be the potential for temperatures to reach a higher level in a metal consumer unit but you also have the effects of the increasing temperature. If the box is in an area containing plastic material these will have melted and contributed to further ignition sources before reaching 1300°C, so if the box has plastic trunking encasing the cables this could melt allowing greater exposure to the material in the box, allowing greater ventilation and allowing the fire to grow further to reach the higher temperatures.
Also the areas in which these boxes are usually located have an increase fire loading, this can have a number of effects including insulation allowing the fire to grow.
Domestic consumer unit locations are usually located in areas that have an increase fire loading such as beneath coat hooks or under stair cupboards and are “insulated” by a wide range of items manufactured from various materials. This “insulation” may obscure the temperature increase from being seen but then become fuel for the fire to spread as the temperature rises in the consumer unit and potentially ignite materials stored in close proximity.
The table below includes details on the melting and ignition temperatures of materials that might be hung or stored in close proximity to the consumer unit.
|Melting points and ignition temperatures|
|Plastic||Melting Point Range||Ignition Temperature ℃|
|ABS||88 – 125||416|
|Acrylics||91 – 125||560|
|Cellulosics||49 – 121||475 – 540|
|Nylons||160 – 275||424 – 532|
|Polycarbonate||140 – 150||580|
|Polyesters||220 – 268||432 – 488|
|Polyethene ld||107 – 124||349|
|Polyethene hd||122 – 137||349|
|Polypropylene||158 – 168||570|
|Polystyrene||100 – 120||488 – 496|
|Polyurethanes||85 – 121||416|
|PVC||75 – 110||435 – 557|
|Wool||228 – 230|
So whilst the introduction of non-combustible consumer is welcomed it has not removed the cause of the temperature increase in the first place – High Resistance Connections.
Surely it would be better to warn of the presence of HRC so the situation can be managed? By installing ThePIP pre-ignition device you can either isolate the supply, provide localised visual/audible warning or be alerted by third party monitored systems @85°C ± 5° pre smoke or fire.
People | Property | Protected