Normally Open Contact (NO)
It is the contact that stays open while the relay coil is de-energised and closes when the coil pulls in. On a crane remote control the start and direction commands are given with this type of contact: if there is no command the circuit stays open, so when power is lost or the receiver output drops, the movement stops by itself.
The behaviour of an NO contact can be summed up in two words: closed if there is a command, open if there is not. This is exactly why movement commands on a crane are given with NO contacts — the default state of the system is stopped. If the cable breaks, the receiver supply is cut, or the transmitter's broadcast stops, the contact stays open so the contactor coil is not energised.
Where the NO contact sits in practice:
- In series between the phase end of the contactor coil and the coil itself.
- In parallel with the wired start button (operation together with the existing installation).
- On the start/trigger input of a panel, as a pulse or a continuous command.
On the other side the situation is reversed: limit and safety lines are built with normally closed (NC) contacts so that when a cable breaks the circuit opens and the movement stops. In other words, within the same system, movement commands are designed as NO and stopping and limit functions as NC. During commissioning, test separately that both types give the expected behaviour with no load.
The context in which this term is used: Wireless Crane Remote Control Selection Guide