Guard door interlocks and two-hand control

Fixed guards keep people out of a machine. When an operator must open the guard or put their hands near the tool, interlocks and two-hand controls make sure the machine can only move when it is safe.

Guard door interlock switches

An interlock switch tells the safety circuit whether the guard door is closed. Main types:

• Tongue (key) switch: a separate actuator on the door enters the switch head. Removing it forces the NC contacts open (positive opening).
• Hinge switch: built into the door hinge.
• Coded magnetic / RFID switch: non-contact, hard to defeat with a simple magnet, tolerant of door misalignment.
• With guard locking: a solenoid locks the door shut until the machine has stopped (for machines that run down slowly).

How it is wired

Like an E-stop, the interlock's NC contacts go to a safety relay (often two channels). Opening the door drops the safety relay, which removes power from the dangerous motion. Closing the door must not restart the machine: a separate start or reset is required.

Interlocks are a favourite thing to bypass with a spare actuator or a magnet. Coded switches, concealed mounting and a design that does not make the operator's job harder all help prevent that.

Two-hand control

On presses and similar machines the operator starts the stroke with two buttons placed so that one hand cannot reach both. The rules (ISO 13851, type III):

• Both buttons must be pressed within 0.5 s of each other (synchronous actuation).
• Both must be held; releasing either one stops the motion.
• Both must be released before a new cycle can start.

The two-hand station is evaluated by a dedicated safety relay or a safety PLC, never by ordinary PLC logic alone.

Distance

As with light curtains, the buttons must be far enough from the hazard that a hand released from a button cannot reach the tool before the motion has stopped (ISO 13855).