I realize a possible answer is that the code says so.
On a branch circuit, where all the cabling is metal clad, and all the boxes are metal, what purpose is served by the green ground wire? Once an outlet or switch is installed in a box, there will be continuity between the wire and the metal, so what does the wire add?
If there were plastic boxes, then you need the ground wire. Old BX wiring used the metal as the ground. What changed, and why?
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Replies
It was found the ground was not always continuous via the metal jacket and end clamps.
Probably because someone can come along and insert a plastic box in the middle of a run and not bother to keep the ground continuous. Or otherwise screw up the continuity.
Plus it is a lot easier for the inspector to see an independent ground.
There are also issues with some of the end clamps not producing reliable ground connections.
If your view never changes you're following the wrong leader
I got a real bad shock once at the factory. A fork lift had hit the conduit and pulled it apart, also causing the hot wire to short to the ungrounded end. I'm standing in water on the loading dock and standing on the metal dock plate when I touched the conduit. Almost lights out for me.
None of the metal clad cable circuits in my house read positive for a ground, even with all metal connectors and metal boxes.
...that's not a mistake, it's rustic
Edited 10/18/2007 8:17 am ET by BungalowJeff
USAnigel touched on it. Think about the effect of anti-short bushings with BX cable to box connections. (You do use anti-short bushings, right?) You can't count on the armour to provide a complete equipment ground path.
Scott.
Edited 10/20/2007 1:03 am by Scott