(c) Copyright 2000, Richard Nute
Hi Richard:
You posted an interesting approach to fire. For me,
it triggered a number of comments that I hope you
will find useful.
Objective:
I suggest that the objective of the design in
respect of fire is:
No fire shall spread from the equipment to the
local environment.
This objective shall be accomplished by means
of both a principle safeguard and a supplmental
safeguard.
The principle safeguard is that of controlling
fault-condition power dissipation such that
temperatures and rate of energy transfer do not
exceed that necessary to ignite and sustain
ignition of nearby materials.
The supplemental safeguard is that of containing
a fire resulting from sustained ignition of any
internal material and from any consequent internal
fire spread.
Testing:
A fire is the result of an abnormal (or fault)
condition. A priori.
Electrical overheating is the result of excessive
power density dissipation.
Only those faults that result in significant
overheating need be considered. For low-voltage
circuits, faults resulting in maximum continuous
current (not short-circuit) in a low-value
resistance (~1 ohm) can produce sufficient heat
energy for ignition (I*I*R).
Most common materials found in electronic
equipment must be heated to about 350 C or more
for spontaneous ignition to occur.
While blocking of ventilation openings results in
high internal temperatures, it is unlikely that
such temperatures will approach material ignition
temperatures. However, high internal temperatures
may trigger a failure that can result in ignition.
Few standards have component temperature limits for
abnormal conditions.
Cheesecloth is a good test for spread of fire from
the equipment.
Markings and instructions:
If installation clearances make the difference
between fire and no fire, then safety requires
installation exactly as prescribed the manufacturer.
For me, this leaves no margin. I would insist that
there be no fire with a worst-case installation with
zero clearances and zero ventilation.
Fire is a poorly-understood safety issue for electronic
products. The standards don't do as good a job as they
do with electric shock. We've done a poor job at fire
forensics analysis, so we don't have a thorough under-
standing of root causes.
Best regards,
Rich
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