What is the difference between proof and burst pressure?

Proof pressure is the maximum pressure which can be applied to a pressure switch without causing damage. While burst pressure is the maximum pressure that can be applied to a switch without causing leakage or rupture. A switch that has been subject to proof pressure should still be operational, but may require a recalibration of the setpoint. However, a switch that has been subject to the pressures near or at the burst pressure can be permanently damaged.