A 277 GF caused by a bad ballast within 'older' lighting fixture caused a 2000 amp main to go out on GF. Now, that was one very unhappy facility mgr. Tracking it down was brutal; no tripped lighting CB. We went thru all the feeders initially. On-site the next day, the fried ballast was discovered. BTW, none of the 20 amp lighting circuits were tripped, and they all tested OK. The ballast was 'open' on the line feeds, and open on the output.
