A schematic diagram of a typical electric motor-driven emergency power system is shown in figure 7-40. Individual components included in the system are a reservoir, a motor-driven pump, an accumulator, a relief valve, a pressure switch, a snubber, and a control switch in the cockpit. The main difference in a system of this type and a normal (engine-driven) system is that instead of operating continuously, the pump operates only when pressure is needed in the system. For example, if the normal power system is inoperative, the pilot turns on the emergency system switch in the cockpit. Turning this switch on energizes a pressure switch that is connected into the emergency hydraulic system pressure line. The pressure switch is actuated automatically by hydraulic pressure. 

For example, when emergency system pressure drops below a predetermined point, the pressure switch turns the pump motor on. When the pressure builds up to the designed operating psi, the pressure switch turns the pump motor off.

The system is protected from excessive pressures by a relief valve, which is set to open at a pressure slightly above system operating pressure. Emergency power systems of this type are generally equipped with an accumulator for storing a reservoir supply of fluid under pressure. This prevents the pump motor from having to cut in repeatedly to maintain operating pressure in the system.