A circuit for fencing input signals to circuits in a voltage island when switching between a normal and a standby power supply is disclosed. A voltage detector detects the switch over in power source and generates a power switch signal. The power switch signal is synchronized to a standby clock and a normal clock. The synchronized standby clock signal is delayed by a counter to allow circuit stabilization. The normal and standby clock signals are logically combined and used to fence input signals to the circuits on the voltage islands.