Clause 4.4 of IEC950 (and clones) describes how to design ITE for resistance to fire. The clause is clear that the construction must minimize the risk of spread of flame outside of the equipment. Clause 4.4.5 describes what components require a fire enclosure, and clause 4.4.6 describes the construction of the fire enclosure. The latter clause is very specific in the design of the bottom and sides of the fire enclosure to prevent the spread of fire by dripping. However, it does not specify how to construct a fire enclosure to prevent the spread of flames. One might believe that clause 4.3.14 describes the construction of the sides and top of a fire enclosure. However, a closer review reveals that this clause describes the construction of openings to prevent an electrical hazard and does not address the spread of flames.
So, how is a fire enclosure to be designed to prevent the spread of flame outside of the equipment? It appears that 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. Does the standard need more work in this area? 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? Your thoughts would be appreciated. Richard Woods Sensormatic Electronics
