Hello from San Diego:
Richard Woods has done a very good analysis of the fire
enclosure requirements of IEC 950.
He finds:
1. For all circuits, there are limits on the sizes and
locations of bottom openings and limits for the bottom
material itself.
2. For low-voltage circuits, there are no limits on the
sizes of top and side openings.
For top and side openings, the standard is anomolous. If
there are no limits, the implication is that low-voltage
circuits do not require a fire enclosure on the top or side.
Richard concludes, "...we must revert to a subjective review
of the construction in determining if an opening in the side
or top of a fire enclosure will allow the escape of flames.
Of course, the problem with a subjective review is that
professionals can and will disagree on the whether or not an
appropriate solution has been applied."
I agree.
I have had such discussions with several certification houses,
each with its own opinion. One accepts no fire enclosure on
the top or side. Another does not, but cannot provide criteria
for the size of the openings.
Richard asks, "So, how is a fire enclosure to be designed to
prevent the spread of flame outside of the equipment?"
To answer this question, lets look at an existing construction
that is specifically designed to prevent spread of fire from a
box-like enclosure construction: The wood stove.
A wood stove has NO top or bottom openings. It has side openings
at the bottom to provide combustion air. It has a chimney or
flue for exhaust gasses.
(A wood cooking stove has user-manipulatable top openings. The
flame that escapes is a function of the size of the opening.)
If the side openings are closed, the combustion will cease due to
the lack of combustion air.
If the chimney or flue is closed, the stove will fill with exhaust
gasses, and the combustion will cease due to lack of combustion air.
As the stove fills with exhaust gasses, some exhaust gas will escape
through the combustion air openings. (This creates two hazards, one
is an external fire due to the hot gasses mixing with air, the other
because of the exhaust gasses filling the room and displacing the
room air.)
An electrically-caused fire in a product behaves similarly to a
wood stove fire.
If there are any top openings or side openings near the top, the
heat and flames will escape the enclosure. If the openings are
small enough and the enclosure wall thick enough, the flames will
be extinguished because of the cooling effect of the enclosure and
the starvation of oxygen from the flame during passage through the
opening.
To my knowledge, no one has done any research studying the
extinguishing effects of hole sizes and wall thicknesses.
"How do you determine the size and location of openings in the side
and top of the fire enclosure to prevent the spread of flames?"
There is a reasonably good test for the effectiveness of hole size
in preventing spread of fire. This is the cheesecloth test, where
a single layer of cheesecloth is placed over the openings in question,
and a bunsen burner with a 3/4-inch flame is placed at various
locations within the product where a flame is likely. If the
cheesecloth chars or burns, the hole size is not acceptable.
(The only problem with this test is that the allowable openings MIGHT
fail the test! We don't know.)
"Does the standard need more work in this area?"
Yes.
Best regards,
Rich
