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?