Essentially these circuits function by controlling the supply of pressure fluid to the machine operating devices. The control can be direct (power interlocking) or indirect (control interlocking).Actuation of the control or interposed valves is by fluid from an interlocking valve or by a solenoid energized from an electrical interlocking switch
Increasing degrees of protection are provided by the use of dual media and opposed mode interlocking devices plus cross-monitoring of the state of the valves. Examples of typical circuit arrangements are referred to in Table 4.8