Kent, I'm trying not to lose sight of the forest for the trees here. I agree that the expansion joint is probably completely functional. It also appears to be a well intentioned effort by the installer to anticipate and compensate for the inevitable movement in the conduit run. The installer took the initiative, after all, since the design did not call for it.
My point is, even had the installer used the correct EMT type fitting, functional or not, the fault would still have happened. With the EMT restrained by overly tight straps, it would still have pulled apart at the loose coupling, the nearest, weakest link. But, had the installer followed through, using the appropriate RMC to match the choice of expansion coupling, the fault may well have been prevented. Even when very loose, threaded fittings are unlikely to pull apart completely. Of course, using 423' of 3" RMC instead of EMT, would have resulted in a more expensive job, though as a percentage of the overall cost, maybe it wouldn't have amounted to much. IMHO, Monday morning quarterbacking, EMT was a poor choice for this system, subject to nearly 4" of movement. On a job of this scale, there are simply too many opportunities for a loose coupling to occur, whether initially on installation, or over time after repeated thermal cycling. S... happens. Kind of analogous to why we pull an equipment grounding conductor in metal conduit systems, even though the conduit itself provides an equipment ground. RMC would provide for much better sleeping at night. PVC, nasty as it is in some respects, in this case, would have been significantly less likely to start a fire. It's harder to forget to glue a joint than to overlook wrenching an EMT coupling. If one does pull apart, since PVC is nonconductive, it's less likely to cause a fault. Better yet, as the report recommends, is to design a means to clear the fault, should one occur. In a perfect world, EMT might be fine. Dick Richard, I think you are right, the weakest link in the chain was a compression fitting. Maybe one that was not properly tightened. An EMT connector will thread right into a RMC coupling and it doesn't look like it would interfere with the expansion joint motion, so I still think the expansion joint may be functional. It should slip with modest hand pressure. Wouldn't the installer notice if an expansion joint wouldn't work? And then not use it. it just wouldn't make any sense to install it in if it didn't work. Kent Richard L Ratico wrote: Kent, I looked at the photo of that expansion coupling and initially thought, "That'll work". But.... if the conduit was RMC OR IMC all the joints would have been THREADED into couplings, and even if by mistake they were not wrench tight, they would not have pulled apart. Even if strapped too tightly, the roof blocks would probably pull over first. With EMT, contraction pulled apart the weakest link in the chain, a loose slip fitting. Thanks William. Dick Solarwind Electric --- You wrote: 2) The expansion joint fabricated from adapters for different conduit systems probably works fine and may not have been a contributing factor to this fire, but it is prominently visible in the report. --- end of quote --- _______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Options & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org
