I disconnected a 277 volt light whip the other day and when the bare end of the neutral wire brushed up against the metal box, it had a little arc on it. Being on the end of the run, why would there be any spark there at all? Would you consider this common and acceptable?
There's always a chance of a difference in potential between a neutral (which is a current-carrying conductor) and the ground. Remember that the ground is generally either steel conduit or flex, which doesn't conduct electricity as well as copper. A volt or two difference between them is certainly possible but doesn't cause any harm.
Now if the hot conductor is still connected and the neutral causes a spark when it touches ground, then of course it will. The loose neutral is seeking a completed circuit and <can> do this via a grounding conductor or conduit.
If it sparked when you grounded the neutral, that means it actually became the *middle* of the run with that electrical connection, and was carrying up to half the unbalanced neutral current of the system right through the box and ground wire
Well, at least of that run. Maybe more like 1/4 once all the line resistances are factored in. (Doesn't take current much to spark, even an AA battery will spark.) Point being, current on the neutral is normal, as is the end of the neutral to be a few volts higher than ground, since you're going to get a little voltage drop over the length of the run- anything up to about 2.5% or so is OK for the end of a heavily loaded & unbalanced circuit; beyond that, you're probably into exessive voltage drop range.
Common and acceptable? Yes = and a 'wake up' call for you.
First of all, do not consider that "0 volts" means, by itself, that a wire is 'dead.' Every neutral, and many grounds, will read '0', yet still be carrying plenty of current. Open that neutral or ground, and, suddenly, one end becomes 'hot' - and will zap you bad. Look at it this way ... the electricity leaves the panel, does it's work at the light, then returns to the panel. You don't want to get across that path .... anywhere in the circuit.
Next, remember that electricity takes ALL paths home. It cares not if something is at the 'end of the run' or not. So, if a circuit is hot, and has more than one light on it ... and you touch the neutral at 'the end' to ground ... some of the current powering the remaining light will return via the ground path you've just created.
Finally, it's possible that the spark was caused by a capacitor, or other electronics, discharging.
Now - here's the "nanny statement:" 277 can easily become a 'can't let go' situation. TURN IT OFF. Sure, that fiberglass ladder might offer you some protection .... but if the shock don't get you, the fall might.
I disconnected it from the light. The switch leg wasn't energized and I took out the whip end that went to the light. The only wires under the wire nut for the neutral were the one in the light whip and the one for the light ballast.
Thanx. I had a thought but you eliminated it for me with your answer. My guess it is nothing more then the arcing when you paralled the neutral to the ground. The arcing is likely due to the load and the ground was obvious different thus you had potential. When grounding the wire, it was like closing a switch hence the arcing.