---------- Forwarded Message ----------
From: Chris Dupres, 100014,3703
TO: "Kendall Wilcox", INTERNET:kwil...@fcpa.fujitsu.com
DATE: 4/27/97 6:13 AM
RE: Copy of: Fire Enclosures
Hi Kendall.
You wrote:
< No matter what the size of the openings on the top of the unit, if you
can pass the abnormal tests above without flame spreading from the
unit, can a regulatory agency deny approving the construction ?
(assuming of course the enclosure meets all other requirements for
enclosing electrical/mechanical hazards etc.) I don't think so. Rich
Nute mentions that he has received different interpretations from
various regulatory agencies regarding top enclosure openings. Rich,
do you think there would be differing interpretations if the abnormal
test passes ?>
I've always worried about the nature of the 'fire' in relation to these
tests.
Some fires are simply fuseholders etc. getting very hot, i.e. forming a
local hot spot, maybe an arc or something, and the local flammable material
forming a fairly low energy fire that is primarily the effects of this
local hot spot. These fires are characterised by flames which lick around
a bit and are easily contained by a simple enclosure, and heat is spread by
convection (goes upwards). These fires often go out when the power is
removed.
Some fires however are extremely aggresive, and may be caused by something
like a short circuit transformer which produces very large amounts of
flammable gas which 'jets' all over the place like gas burners. Or highly
flammable material may burn, such as Perspex, Polystyrene etc., producing
highly exothermic reactions which fuel themselves. In these cases heat
doesn't just go upwards, but comes out of any hole it can find. These
fires don't go out when the power is removed.
It seems that the 'rules' don't differentiate between these types of fire,
I think this puts the onus back on the designer to anticipate the nature of
a fire and show that it has been taken into account when the enclosure was
specified.
Another thing I worry about is the nature of the gas given off. Some of
these plastics are much more dangerous in the gas they give off while
burning, than any risk of fire. This is covered in general terms in IEC
1010, but materials are not specifically listed, but that's outside the
scope of this thread.
Chris Dupres
Surrey, UK.
