Wiring 16 — Dual-Channel Safety Relay with Monitored Reset

Dual-Channel Safety Relay with Monitored Reset

Prerequisite: complete Wiring 13 (E-Stop Chain) first. That lab teaches the single-channel NC E-stop loop. This lab extends it to the Cat 3 / PLd safety architecture required by IEC 62061 and ISO 13849-1 for higher-risk machinery (robots, presses, high-speed conveyors).

Why single-channel is not enough

A single NC wire can fail safe by going open (the machine stops) — but it can also fail dangerous: a wiring fault that bridges the NC contact can hold the control circuit permanently energised so that the E-stop cannot stop the machine. A single channel cannot detect its own dangerous failure mode.

Dual channel + safety relay solves both failure modes

The EStopButton component has two independent NC contact pairs (NC1 and NC2). Each channel is wired to its own input on the safety relay (S11/S12 for channel 1; S21/S22 for channel 2). The safety relay only enables when both channels are closed simultaneously. A fault that bridges one channel to +24 V while the other opens is detected as a cross-channel discrepancy — the relay locks out and refuses to reset.

Monitored reset prevents automatic restart

After the E-stop is un-latched (mushroom head released), the machine must not restart automatically. An operator must walk to the panel and press the reset button to confirm the hazardous area is clear. The safety relay's S33/S34 (EDM/reset feedback) terminals monitor the reset button: the relay only re-enables after seeing the button pressed and released. This is the monitored reset requirement.

Safety contacts break the load path

The safety relay's output contacts (13/14) sit in series with the contactor coil circuit. The safety relay grants or withholds permission to run — it does not drive the motor directly.

What to wire

1. Seed +24 V and 0 V buses from the PSU.
2. Power the safety relay: tb-24v → ksr-1.A1; tb-0v → ksr-1.A2.
3. Channel 1: +24V → estop-1.NC1-IN; estop-1.NC1-OUT → ksr-1.S12; +24V reference → ksr-1.S11.
4. Channel 2: +24V → estop-1.NC2-IN; estop-1.NC2-OUT → ksr-1.S22; +24V reference → ksr-1.S21.
5. Monitored reset: +24V → reset-1.NO-IN; reset-1.NO-OUT → ksr-1.S33; ksr-1.S34 → 0V bus.
6. Safety contact → coil path: +24V → ksr-1.13; ksr-1.14 → tb-coil.t0; tb-coil.t1 → relay-1.A1; relay-1.A2 → 0V bus.
7. Healthy lamp: ksr-1.23 → lamp-1.+; ksr-1.24 → 0V bus.
8. PE bonds: PSU PE and 0V to ground bar.