|reply to nunya |
said by nunya:Generally I spec ground wires in every type of raceway. Often, the building owner makes a decision not to have the ground wires to save costs. Depends on the type of project and system. Some systems (IT, modern elevators, UPS, VFDs, etc) call out for having a separate ground wire to lower the impedance to ground, over that of steel conduit. I hadn't thought about it, but absolutely requiring a separate ground in EMT is a good idea.
I normally run a EGC in EMT, but I rarely run one in IMC or heavywall. If it's spec'd, I'll pull it.The conduit is a fine ground. A good engineer will calculate the conduit size, length, and type compared to the over-current protection. This can save customers thousands of dollars in wire and labor, with zero additional risk.
As to calculating whether or not the steel conduit provides an adequate fault path to ground (the impedance of hte conduit), its a rare engineer that actually does it. Tables are available from conduit manufacturers, as to what is the maximum distance of different size conduits, before the ground ability of the conduit becomes unsuitable. These should be used on any job where the steel conduit is being used as the ground path.
said by nunya:Agreed as to how many times I've seen EMT separated at set-screw connectors. I haven't seen compression couplings fall apart; and in my opinion, this only happens if the coupling wasn't wrench-tightened. Its best to literally spec 'wrench-tightening', and prohibit set-screw couplings, IMHO. I believe EMT compression couplings offer quite a few advantages over set-screw couplings, including use in wet environments, and providing better protection against radiated noise via the concentric shielding around the conductors from end to end. Of course, it pays to check if the contractors left the fittings loose, or actually tightened them. I have seen plenty of times where they did not, and were either forgetful or hoped someone wouldn't notice.
The only reason I pull one in EMT is that it tends to be abused over the years. Some places use it as a long storage hanger. "Handymen" mess with it and leave it in a fubar state.
I can't tell you how many times I've happened into a piece of EMT that's separated at a joint. The "compression" type couplings and connectors actually fall apart way more than the set screw.
said by jack b:Nice photo, jack. I've never seen anything like that. Its a good reason, IMHO, not to use set-screw fittings on EMT. The coupling is literally melted.
Maybe the current draw was less than the over-current device rating.