I had some problems a few months ago with a circuit breaker tripping too. It was the Earth Leakage Circuit Breaker or whatever electricians call the things (Is it Residual Current Device?). Anyhow, it was tripping as one of the case fans in a PC had worn out and the effort to turn the fan was drawing a huge load. After I replaced the fan - no more power outages!

PS. Thank heavens for UPSs. Who invented them? They deserve a medal.