Think in terms of ?Protection Techniques? [Section 500.7] versus ?Wiring Methods? [Sec 501.10]. Recognize that they are related subjects but not the same.
At the moment, we have essentially dealt with two different protection techniques (Explosionproof Apparatus [500.7(A)] and Purged and Pressurized [500.7(D)]. We have also been discussing two different wiring methods that are acceptable in Division 2; i.e., various threaded metal raceways or Type TC cables. Both are referenced either directly or indirectly in 501.10(B), the Division 2 Section.
If the enclosure were required to be explosionproof in Division 2, enclosure seals are necessary to maintain the integrity of the ?Explosionproof Apparatus? protection technique and the wiring method is relevant only as to the specific method of the seal?s installation.
If the Type Z ?purged and pressurized? protection technique is used, enclosure seals are generally unnecessary no matter what the wiring method. There is a moderate exception to this general statement- seals may be needed to maintain the pressure in the enclosure. This will depend on the overall design of the purging/pressurizing system. However, since there is no explosionproof integrity to maintain, the seals are not required to be explosionproof either.
Setting the enclosure requirements aside for the moment, we then review the needs of seals as they apply to wiring methods at ?boundaries.?
For Type TC cable only systems it?s fairly simple ? they don?t need them. UL states Type TC cables have a ?gas/vaportight continuous sheath.? Subsection 501.105(E)(4) is the only place that mentions a ?boundary? as it applies to cable seals and it doesn?t apply to Type TC.
For raceway systems the situation is significantly more complicated. Each case has to be evaluated under 501.105(B)(2) and its exceptions. Even if one is required, it does not need to be explosionproof.
