What this switch does on a guarded gate
The D4SL-N4HDA-D4 is a guard lock safety-door switch from Omron — the kind that physically locks a machine guard closed until the control circuit tells the solenoid to let go. It uses a mechanical lock with a 24 VDC solenoid release, so when power is cut to the solenoid the lock stays engaged; that's a fail-safe hold for the guard. The built-in switch contacts are 3NC + 2NC, meaning five normally-closed circuits wired in series for the safety chain, which gives you redundancy and lets you monitor both the door position and the lock status independently. The metal head (D option) handles the mechanical abuse of a production line — guards get slammed, operators lean on them, and a plastic head won't last the route. The M20 conduit outlet (code 4) matches standard metric fittings, so you're not hunting for adapters at 2 AM. An orange LED on the 24 VDC indicator tells you the solenoid is powered and the lock is released; when it's out, the guard is locked. That's a quick visual for the guy walking the line — no meter needed to know the gate is secure.
Contact configuration and safety circuit fit
The 3NC + 2NC contact arrangement is specific: three contacts handle the door position monitoring, two handle the lock status. In a typical Category 3 or 4 safety circuit per ISO 13849-1, you need at least two NC contacts in series for each monitored function. This switch gives you that plus a spare on the door side, which simplifies wiring if you're feeding two separate safety relays or a dual-channel safety PLC input.