Abstract: A foil coaxial shunt with a risetime of 2ns has been developed to measurethe diode currents. Based on the principle of dynamics, the mechanical strength ofthe shunts for high impulse currents has been analysed. A new strength formulafor the maximum allowable current of the shunt is derived. The formula showsthat the maximum allowable current of a foil shunt depends not only on the para-meters of shunt, but also on the duration of the measured impulse current. Thecalculated values with this formula are in better agreement with the experimentalresults than Park's ones.