You'll have to grip it with an insulated plier and tug on each cable a
bit to see which one is loose, (**sometimes poking the lugs with an
insulated screwdriver works too**)...when you get sparks or the lights
to flicker from this procedure you've found your problem - careful
though - this is a live wire test & extreme caution must be used.
(**) I found a loose leg on a three phase this way - the lights would
come on & off randomly - the screw on one of the lugs seemed tight but
wasn't holding the cable secure in place - a few turns of a lug screw &
the problem was solved (**)
Roy Q.T.
Urban Technician
[I don't make em, I just fix em]