A:
Yes, in general, it is safer to use normally closed (NC) contacts than normally open (NO) contacts for safety light curtains for the following reasons:
✅ Why is normally closed safer?
1. Power failure causes immediate shutdown, default safe state:
Normal closed contacts are closed in normal state. Once power is lost, the line is loose, the light curtain fails or the cable is damaged, the circuit will automatically disconnect, causing the system to shut down or enter a safe state.This is the basic principle of "fail-safe design".
2. Easier to monitor disconnection or failure:
The controller or safety relay can detect whether the circuit is closed in real time. If the contact opens abnormally, an alarm will be immediately issued or the output will be cut off.
Normal open contacts cannot be disconnected naturally in the event of a fault, and may be mistakenly believed that the equipment is still operating normally, which poses a risk.
3. Safety standard recommendation:
International standards (such as ISO 13849-1, IEC 61508) and related mechanical safety specifications also recommend the use of normally closed contacts as safety signal interfaces.
