Hi Terry:
You ask a bunch of questions about material use and
flammability ratings for such materials.
Some of the questions ask about applications of the
standards.
Some of the questions ask whether the requirements
truly reduce the possibility or consequences of fire.
1. Does small size exempt it from a burn rating?
Yes.
Sub-clause 4.4.3.1, second paragraph, implies that
"small parts" may not need to be rated for
flammability.
Sub-clause 4.4.3.3, 5th dashed paragraph, 1st dot
paragraph exempts "...other small parts which would
contribute negligible fuel to a fire."
I have found that most certifiers will exempt "small
parts" where "small" is defined as not exceeding 13
mm maximum dimension.
2. mounted in a metal panel
Such mounting provides a good heat-sink for the
material. If the material wall is relatively thin,
this means the metal must approach material ignition
temperature before the material will burn. Usually,
this means much more thermal energy is required to
ignite the material than if it was tested by itself
(as is done in the traditional material ignition
tests.)
However, such performance is not considered by the
standard.
3. Is there some consideration to abrasion?
No, not explicitly. But, see Sub-clauses 3.1.2 and
3.1.3.
4. "Where it is not practical to protect components against
overheating under fault conditions, the components shall
be mounted on materials of FLAMMABILITY CLASS V-1 or
better, and shall be separated from less fire-resistant
material by at least 13 mm of air."
This statement is taken as applying to electrical
components (e.g., resistors, semiconductors, coils) which
themselves generate heat and will overheat under fault
conditions. It is not taken as applying to a grommet
through which wires pass.
4. Also what about gasket material inches away from source
of energy?
As a general rule, materials more than 13 mm (0.5 inch)
from an electrical part need only be rated HB. See
Sub-clause 4.4.3.3, 5th dashed paragraph. While this
sub-clause applies to specific parts, most certifiers
will apply it to other materials under the first dot
paragraph, "other small parts."
5. What is the `letter' and `spirit' of the standard on this?
I've already given the 'letter' of the standard. Further
explanation of the 'letter' is given in Sub-clause 4.4.1.
The 'spirit' of the standard is given in the Introduction,
Principles of Safety, Fire.
As in electric shock requirements, fire requirements
include a principal safeguard (prevention of overheating)
and one or more mitigating safeguard alternatives (e.g.,
prevention of spread of fire by limiting fuel to the
extent practical, or containing a fire by means of a
'fire enclosure').
6. It seems to me if the part can't contribute to the start
or spread of fire the flame rating is just another form of
over bureaucrating things (if that's a word).
Specifically (and if I understand your descriptions
correctly), the grommet and gasket you mentioned are
indeed insignificant and don't contribute to the start or
spread of fire. Hence, no requirements if you apply the
"small part" and other criteria.
However, all plastic materials -- except insignificant
plastic parts as agreed upon between you and your certifier --
must be rated HB minimum, and up to V-1 depending on
application.
If you've ever sat in on a standards committee meeting,
you will understand the several forces operating on the
standard:
* manufacturers, who, protecting their turf, want the
least impact on product design and cost;
* certifiers, who want to maximize their revenues through
material, component, and product "safety" certifications;
* flame-retardant additive manufacturers, who want all
plastics to be flame-retardant;
* occasionally, government regulators who have a very
specific agenda to change or add a requirement.
So, there are "bureaucrating things" in our safety
standards. Occasionally, such "bureaucrating things" are
just that, and contribute nothing to the safety of the
product.
Safety standards are never tested to determine if the
requirements do what the committee members intend for
them to do. Safety standards are developed through the
"BOGSAT" process.
BOGSAT = Bunch Of Guys Sitting Around Talking.
There is little or no engineering that goes into safety
standards. As one wag said,
"Safety standards are the collective inverse of bad
experiences."
Of course, we have no engineering discipline called
product safety engineering to support any requirement in
the standard. So, whoever has the loudest voice, or the
most experience, or the most authoritative position, or
who represents the most powerful certification house
usually gets what he wants in the safety standard.
Best regards,
Rich
---------
This message is coming from the emc-pstc discussion list.
To cancel your subscription, send mail to majord...@ieee.org
with the single line: "unsubscribe emc-pstc" (without the
quotes). For help, send mail to ed.pr...@cubic.com,
j...@gwmail.monarch.com, ri...@sdd.hp.com, or
roger.volgst...@compaq.com (the list administrators).
