Don - this resistance -
(someone step in here and shove my feet in my mouth if I'm misguided)
RE: Old metal boxes being 'grounded'....I've found that most of these old metal boxes are not truly 'grounded'...they are bonded. There is this thin, small gauge copper wire running from box-to-box and connecting here and there to metal pipes...thus showing a 'ground' when tested...but this is not the equipment grounding conductor called for when required to ground devices...and this situation would give you a pretty high resistance reading also....i.e. this 'bond' is not going to clear a fault very effectively....OK, go ahead and shoot.
