The light is not causing the main breaker to trip.
The main breaker might be bad or there might be a power short to ground.
Maybe the power to ground is a damaged power wire that is very close to ground but not actually touching.
Only when you have a lot of current does the current jump across the gap with a spark.
It is difficult to try to diagnose an old complex system in person and even more difficult to try to do it remotely.
Sometimes you just have to take things apart and check each thing individually and clean up anything that is suspect.
At 40 years old or older, any part can potentially have corrosion or other issues.
