Newer ones come with the switch soldered to the cable and they are shrink-wrapped. If you installed one of the older supplies and attached the cable to the switch, the most likely cause of the problem is that the wires were not plugged onto the correct lugs on the power switch.
Most power supplies have wiring diagram on a label stuck to the top of the supply.
If the power supply was one of the newer models, the most likely cause of the problem is that the power cables going from the new power supply to the motherboard were plugged-in backwards. Unfortunately, if that is the case, the motherboard may now have been damaged. It may have been dead to start with, but other things could have caused the same symptoms. If the switch and motherboard connectors appear correct, try to isolate the problem. See if the circuit breaker still trips if all power supply connectors are disconnected.