If any ballast shows leakage of more that say a drop of tar where the wires enter, or if it appears that the label is burnt (not just yellowed) then it should be considered bad. My experience has been that ignoring those signs will eventually result in a total meltdown with lots of smelly smoke!! I've seen this happen even with "P" rated ballasts!
For those not familiar, "P" rated ballasts have a built-in thermal protector which is supposed to kill the input power before it completely burns up. But a fire is still possible if any of the following happen:
1) The fixture is left on and cycles on and off from the protector. It will still build up lots of heat, and possibly causing:
2) Failure of the thermal protector [contacts weld closed] or:
3) On 240v ballasts, some thermal protectors open only one of the incoming lines. If the ballast has a short to ground, current can still flow from the other hot leg and smoke it. (Haven't seen 277 ballasts fail like this, IIRC 277 is a phase-to-grounded[neutral] voltage.)
In short, if it looks bad, change it!! Cheap insurance against possible violent failure.
[edited for spelling]
[This message has been edited by mxslick (edited 12-01-2004).]
Stupid should be painful.