New shower pump keeps tripping out the circuit breaker

I wonder if anyone can give some clues as to the problem here. New shower pump required as the old one which had worked well for a number of years packed up. Shower is normal mira cold/hot feed type, and pump is required to get a good amount of pressure.
Hot water in the cylinder is supplied by either immersion heater or central heating. Since new pump fitted, circuit breaker on the electric keeps tripping out. It was suggested that the cause was the water was too hot and I needed to turn down the temperature. In the shower, the lever for hot /cold supply is turned right over to cold ( can't go further) so it's as though no cold water is coming into the shower. If you move the lever in the shower across to get warmer water the circuit breaker trips.