Normally Closed Contact (NC)
It is the contact that is closed while the relay or switch is at rest and opens when triggered. NC is preferred on safety and limit lines, because when a cable breaks or the contact fails, the circuit opens by itself and the machine behaves in the direction of stopping.
The safety advantage of the NC contact comes from its failure behaviour. In a closed circuit, a broken conductor, a loose terminal or a burnt contact cuts the current; the system interprets this as a "stop" command. On a limit line built with NO, the same break goes unnoticed and the limit protection is silently disabled.
The typical places for NC in a crane installation are these:
- Limit switch: the NC contact of the upper and lower limit switches goes in series between the remote control relay output and the contactor coil. That way limit protection works independently of the radio link.
- Electrical interlocking: the NC auxiliary contact of the forward contactor goes into the path of the reverse coil, and the NC contact of the reverse contactor into the path of the forward coil.
- Emergency stop chain: the machine's hard-wired emergency stop circuit is built with NC logic.
Include NC lines in periodic function testing: if a closed contact fails to open at the moment it should, you will only notice it by triggering it deliberately.
The context in which this term is used: Wireless Crane Remote Control Selection Guide