Consider the pumping system shown below. In order to operate the pumping system, the reset switch S1 is first closed and then immediately opened. This process allows the current to flow in the control branch circuit, activating relay coils K1 and K2; relay K1 contacts are closed and latched while K2 contacts closed and start the pump motor.

In order to shut down the system (about 20 seconds later), pressure switch will detect the excess pressure and the pressure switch contact will open, deactivating the control circuit, de-energizing the K2 coil, opening the K2 contacts, and thus shutting the motor off.

In the emergency shutdown, the timer relay contacts should open after 60 seconds, de-energizing the K1 coil, which in turn will de-energizes the K2 coil and thus shutting the pump off.

For the system failure, pressure tank explodes.

Draw a simple Fault Tree and calculate the system failure rate with following failure rates given to us (probability of power failure is 0):

Probability of failure for pressure switch is 0.001 Probability of reset switch failure is 0.002 Probability of failure of pump is 0.0015 Probability of failure for tank is 0.005 Probability of failure for relays is 0.004. Probability of timer failure is 0.001 Probability of failure for relay spark is 0.001. Probability of failure for leak of flammable fluid from tank is 0.002.