MC tape/sheach/metallic wrap should be bonded to the system ground endlessly.
Toners can't pump grounded conductors -- they function as micro-radio emiters -- and the pumped conductor functions as a feeble antenna.
The receiver has a variable pick-up circuit so as to just barely pick up the primary emitter.
This is how it's possible to walk down the conductor.
This also explains why 9VDC batteries don't last hardly any time at all: they're being drained at full flow.
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Even though the sheath suppresses the signal, one can pick up the hot spots every time the conductor enters an (unshielded) junction box. This is often more than enough to dope out the situation.
All tested conductors have to be un-loaded, broken out of circuits.
ONLY the hot should be pumped, the neutral is VERY problematic in old work.
The Greenlee gadget will immediately indicate when its being drained into a short. It can't deal with 'bolted shorts.'
You can spot which leg has been ruined by isolating it from both ends.
It's extremely common for j-men to assume that the cable has been damaged -- when that's the last possibility.
It's far more likely that someone's mis-wired the cable, or that it's tied into a load that is internally faulted... even a flaky receptacle is enough to cross hots to neutrals -- and get to ground.
In old work, conductors are far more likely to fail in and around junction boxes -- it's the heat of utilisation and it's where bad make-up compounds into failure.
In my specific experience, I've only had one experience of drywall screw penetration. Even it only tagged the hot. That was enough to ground it: tin framing.
(When routing MC, stay away from drywall screw pattern zones.)
