For details on thermal withstand and I2t, see I2t Thermal Withstand. Energy limiting devices reduce the I2t let-through in the event of a fault. The values are typically specified by the manufacturer:
Typical MCB Energy Limiting Characteristic
Using typical data given above, for a 10A, 10kA fault rated MCB would have an Ip/A of 1000 (103), give an energy let-through of approximates 2,000 A2s.