I had similar symptoms with my upstairs lighting circuit. I had added different lamps and fittings at various times over the years. Basically the junction box below my upper hall floor was overloaded and had overheated and eventually shorted the live, neutral and earth wires together. I was very lucky not to have a major fire (thanks to the sensitive circuit breaker). The previous advice is good and will hopefully guide you to the source of the problem. I can't offer much more advice as I don't know your competence at electrics, but opening up ceiling roses and switches with the power OFF could give you an indication of cooked cables (colour and smell). Lamps usually fail open circuit so if your breaker lasts for a short time before tripping I would suggest a wiring problem that takes that time to heat what remains of the insulation to melt and cause the extra current to flow to the wrong place and cause the trip. Hope this helps. Andy