A three-phase power system consists of a synchronous machine connected through a lossless transmission line to an infinite bus. A fault occurs on the transmission line. The maximum power transfer of this system when there is no fault is 5 p.u. and the power transfer is 2.5 p.u. The power angle curves during the fault and post fault conditions have peak values of 2 pu and 4 pu respectively. What is the permissible increase in the angular displacement (in degrees) between the voltages at the two ends of the system beyond which the circuit breakers could not clear the fault in time for the systems to remain in synchronism.