How Do I Trace the Cause of my Central Heating Boiler Fusing My House Elect?

I have A Grant Combi 90 MKII Oil Burning Boiler. When it fires up to heat the water or for the heating to come on, it trips the fuse box. No other warning lights are on or cut out switches operating on the boiler. Could it be the Water Pump? or the pumps switching mechanism. If so how would I check it and repair it? Cheers .