Wiring 13 — E-Stop Chain with Healthy Indicator Lamp

E-Stop Chain with Healthy Indicator Lamp

An emergency stop (E-stop) button is the physical switch a machine operator uses to immediately halt a dangerous machine. Pressing the mushroom-head button should always stop the machine — even if a wire breaks. This requires a specific wiring pattern.

Why NC (normally closed)?

The E-stop contact is wired NC — closed at rest. The control circuit runs through the NC contact. While the button is not pressed, the contact is closed and current can flow to the load (contactor coil, safety relay, or PLC input). When the button is pressed — or when the wire breaks — the contact opens and current stops.

The failsafe property: a broken wire to the E-stop button looks the same as the button being pressed. The circuit opens either way, and the machine stops. A wiring fault can never leave the machine running with a broken E-stop — unlike a NO-wired E-stop, which would leave the machine running and oblivious to the fault.

The indicator lamp

A healthy indicator lamp shows whether the safety loop is closed. The lamp must be wired after the E-stop NC contact — on the output side. That way:

• Lamp ON = E-stop not pressed, NC contact closed, circuit healthy, machine is permitted to run.
• Lamp OFF = E-stop pressed, or a wiring fault exists — the loop is open and the machine cannot run.

Connecting the lamp directly from +24 V (before the E-stop) would mean the lamp stays on even when the E-stop is pressed — that would give a false "healthy" reading. The lamp belongs on the output node.

What to wire

1. Seed +24V and 0V distribution buses from the PSU.
2. Safety loop: tb-24v.t1 → E-stop NC1-IN; E-stop NC1-OUT → relay A1 (the relay coil represents any downstream load); relay A2 → 0V bus.
3. Indicator lamp: connect lamp+ to the E-stop NC1-OUT node (same node as relay A1), so the lamp reflects the state of the NC contact. Connect lamp- to the 0V bus.
4. PE bonds: PSU PE and 0V to the ground bar.