Transition to the Safe State (Fail-Safe Behaviour)
The system behaving in the direction of stopping rather than continuing motion in the event of a fault, interruption or uncertainty. The relay dropping out when power is lost, the outputs dropping when the signal is lost, and the use of normally closed contacts are concrete applications of this principle.
The essence of the principle is that safety rests on something not working rather than on something working. Continuous power and a continuously valid signal are needed to sustain motion; when either is lost the system naturally stops.
- Power side: relay and contactor coils pull in as they are energised; if the supply is lost the contact opens of its own accord.
- Signal side: when the watchdog time expires the outputs are released.
- Wiring side: normally closed contacts are used on safety and limit lines; if the cable breaks the circuit opens and motion stops.
The opposite design is to build the stop with a normally open contact: if the cable breaks the stop command never arrives and the fault stays hidden. Apply this principle at three points in a radio-controlled crane panel: let the normally closed contact of the mushroom button carry the control voltage, wire the limit switches in series with the contactor coil as normally closed, and make all outputs drop when the receiver supply is cut.
Context in which this term is used: Wireless Crane Remote Control Selection Guide