During final inspection for a new 500kVA UPS installation we discovered the installer had pulled all the cable through 4" EMT but had used screw-set couplings instead of the compression couplings we consider standard practice. We recalled seeing that in writing somewhere, couldn't recall in which mil-hdbk it was specified. Regardless, the mil-hdbks are guidelines only, which gives us the final call and we couldn't think of anything that justified taking all the wiring out to replace the fittings. All the cables were run with a ground wire, and the cable running from the bypass switch to the main switchgear was in an unprotected cable tray, so grounding and EMI are out as concerns, which were the only reasons we could think of that compression couplings would be better than screw set. This is in an earthquake zone, but I can't see how that would make a difference.
Do any of you know why compression couplings would be favored/required over screw set? Is it just for grounding which is moot with a decidated ground wire?
Iv'e seen compression steel fittings on alot of print spects for schools. I really don't see any benefit in compression over set screw indoors. On large conduit it's almost impossible to get the couplings so tight that they don't turn when tighting the next coupling. They also have a tendancy to strip out the threads of the fitting on smaller conduit when over tightened.
Interesting, I'd not suspected the arguments were for longevity! Sounds like properly installed compression fittings are the way to go, but difficult to achieve- I'll have to remember to closely inspect them next time around; I'm the customer as well as the engineer. This installation is expected to last the lifetime of the UPS- 15 years. They used compression fittings on all the small conduit, but not the big ones, so cost and installation difficulty might have both come into play. I'm at least glad to hear there really isn't all that much difference between the two; if compression fittings really are that hard to install and always get done sloppily, we might just go back to screw-set, I'll research the specs some more and see exactly how they word it.
Steve, in my experience it is not that it is "easy" to make "sloppy" connections with compression type connectors; nor is it a matter of "difficulty" very often.
What seems to hapen is that the connection is tightened, then made loose by the pipe, as the pipe gets moved about during installation. Then, after assembly, it's not so easy to get a wrench on all the fittings.
A set-screw, in contrast, is not affected by these rotational forces, and makes the pipe slightly out-of-round, which further resists twisting.
I would say that the biggest contributing factor was probably cost. In bigger sizes like 4 inch, compression couplings can run as much as 3 times more expensive than set-screw couplings. If the installation is indoors and has no need for weather-proofing, I can't say that I see a reason for compression couplings except asthetic value or ease of sucking a pull string through as was mentioned earlier. If it's above the cieling or otherwise hidden, a wrap of "jap-wrap" at each end of the set-screw connector will greatly improve the runs jet-lining ability. As for getting large compression couplings to tighten properly, the trick I've found is to use two strap or pipe wrenches that you think are completely too big for the job. In regards to the grounding aspect, I've never had a problem with set-screw connectors as long as you make sure to tighten the screws all the way.
I wonder if anyone has applied a meter to a long run of EMT to determine the impedance when set screws are used, and compression. I've heard third hand that a company did a test and found compression to offer a lower impedance at fault current levels. Of course I always require a dedicated EGC to be installed within the EMT anyway, but the redundancy of the raceway is helpful IMHO.