Difference Between Insulation Rated Voltage and Rated Impulse Voltage?

Rated Insulation Withstand Voltage (Ui) is the voltage at Power Frequency (50 Hz or 60Hz) that might appear across the phases of the MCB, due to voltage fluctuations in the system (Supply Fluctuations and Transformer Tap Actions). This might prevail for a longer duration till corrective actin is taken. MCB has to withstand this POWER FREQUENCY OVER VOLTAGE. The capacity of the MCB to withstnad this voltage is called its Rated Inulation Withstand Voltage (Ui). Typically it is 690V for a 415V System

On the other hand, Rated Impulse Withstand Voltage (Uimp) is a non-power-frequency over voltage of very high magnitude, but appearing for a very short duration (typically, in micro seconds); they are also called surges or transients. the main cause for such transients are Lightning Surges & Switching Surges. The magnitude can be of the order of 8kA (in a 415V System). The MCBalso has to withstand this transient overvoltage. The capacity of the MCB to withstand this TRANSIENT OVER VOLATGE is called its Rated Impulse Withstand Volatge (Uimp). It is generally 8kV for a 415V System.