Definition of non-combustiblity and limited combustibility

[Tom Sutton]

I received to following enquiry recently can you assist.

I’m currently working on my bachelor thesis in Fire Safety in an international comparison”. I’m in a company that is mainly planning projects in Nigeria, so often we have to use the British Standard. I came across these two Standards, since I wanted to define what is meant as “non combustible” and what “limited combustible”.

In the Approved Documents B it is written that non combustible products have to be tested to 476-4 or to 476-11.

In the Approved Documents Table A6: Use and definition of non-combustible materials.
a.Any material which when test to 476-11:1982 does not flame nor cause any rise in temperature on either the centre (specimen) or furnace thermocouples.
d. Products classified as non combustible under 476-4:1970

In 476-4 non combustible means
 
The material shall be deemed non-combustible if, during the test, none of the three specimens either
1) causes the temperature reading from either of the two thermocouples to rise by 50 degrees C or more above the initial furnace temperature, or
2) is observed to flame continuously for 10 s or more inside the furnace.

That is a huge difference, particularly when the Approved Documents define in Table A7 a limited combustible material as

[…] which when tested to 476-11:1982, does not flame and the rise in temperature on the furnace thermocouple is not more than 20°C.
 
There are differences between the two standards the specimens are different.
In 476-4 it’s a cuboid with dimensions 40*40*50mm and
In 476-11 it’s a cylinder with a diameter of 45mm and a height of 50mm.

This results in a higher proportion of surface/volume of the cuboid. The cylinder has a lesser proportion. What in turn means, that the cuboid is more reactive and therefore produces more heat than the cylinder. This would mean higher requirements for limited combustible to 476-11 than non combustible to 476-4. But I don’t know if this explains the difference of 50°.

What am I missing?
 

[wee brian]

Tom

That's abit accademic for us lot. Try Dr Sarah Colwell at BRE. She's good at this kind of stuff.

[Tom Sutton]

Thanks wee brian I agree but this is why I posted on here you never know and I was going to suggest he contacts one of the test centres to see if they can help but having a name makes it even better.

[Midland Retty]

Tom

That's abit accademic for us lot. Try Dr Sarah Colwell at BRE. She's good at this kind of stuff.

Speak for yourself Wee Brian

So anyway Tom... erm....speak to Dr Sarah Colwell or Warrington Fire as they should be able to help. 

[nearlythere]

Heck. I was going to answer that question but you lot just don't have faith in some forum members.
Not going to now, so there.

[kurnal]

To answer you would also need to study the characteristics of the calorimeter or whatever else the test rigs comprise of. I guess the thermocouples will be contained within a housing or reservoir of some sort- the thermal characteristics of this housing and the duration of the tests may also vary.

I havn't looked but I guess if you look at the scope statement at the front of each BS this may explain why we have two standards? Horses for courses?

[Tom Sutton]

Thanks guys but just thinking about it makes me brain hurt. Its well outside my league and I prefer to let those who know, deal with it. Good old Sarah.

[CivvyFSO]

It could be simply the position of the thermocouples.

One talks about the furnace thermocouple, the other talks about 'either of the two thermocouples'

476-4 has a thermocouple local to the specimen, and a furnace thermocouple. It makes sense that if we are taking a measurement much closer to the material being tested, then we would expect a higher reading.

I can't find BS476-11 to see where the measurements are taken so this could all be drivel.

[Fishy]

In Part 4, the delta-T (change in temperature) is calculated from the start temperature to the maximum temperature. For instance, a material having a start temperature of 748degC and the maximum of 800degC will be considered a failure, although it could be a practically inert product. However, in Part11 the delta-T is the difference between the maximum to end temperature and, hence, in my opinion, this is a less onerous test. Besides, there are differences between the furnaces such as the density of the packing materials.
 
In part 4, the duration of the flaming is limited to a maximum of 10s. On the face of it, this may seem less onerous, but if the product flames for a period of, say, 5s then the temperature will in all likelihood rise more than 50degC. In Part 11 the requirement is ‘no flaming’, but the definition is no sustained flaming for more than 5s. One could have flashes of fire throughout the test lasting less than 5 seconds, but it will not be recorded as ‘flaming’.