How to Wire a Guard Locking Safety Switch to a Safety Relay

A guard locking interlock has more terminals than most panel builders expect — four to six contacts, a solenoid coil and an indicator circuit. Wire them in the wrong roles and you either lose the safety function entirely or spend a day chasing a relay that will not reset.

This is the practical wiring sequence, the terminal logic behind it, and the mistakes that cause commissioning failures.

Note: this article is general guidance. Always work from the wiring label fitted to the specific switch and the machine's own safety design documentation. Installation and commissioning must be carried out by competent personnel.

Part of our guard locking series: the complete buyer's guide  •  spring lock vs solenoid lock  •  IS 16812 / EN ISO 14119 explained.

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Understand the four circuit groups first

Every solenoid guard-locking interlock separates into four functional groups:

Terminal numbering varies by model. Confirm against the label before you strip a single core.

Step 1 — Decide your architecture

Single channel (Category 1 class): put one door NC and one lock NC in series into a single safety relay input. The relay only sees a closed circuit when the guard is both closed and locked. Simple, but a single fault can defeat it.

Dual channel (Category 3 / 4 class): you need two independent “closed and locked” channels feeding the two inputs of a dual-channel safety relay with cross-fault detection. Two ways to achieve this:

• Four-contact device with independent circuits: wire door NC into channel 1 and lock NC into channel 2. This evaluates the two functions separately, which is what you want when the control system must distinguish “closed” from “locked” independently — but check your safety design, since each channel then monitors only one function.
• Six-contact device with internal series links: some compact devices cross-link the blocks internally (e.g. terminal 12 to 41, and 22 to 51), so each pair forms a complete series “door closed and locked” channel available at 11-42 and 21-52. Both relay inputs then see the full guard condition, giving you dual-channel architecture from a single device.

Where independent door and lock circuits are required by the safety design, choose the slim-body four-contact format; where you want two complete safety channels from one unit, choose the six-contact cross-linked format.

Step 2 — Wire the safety channels

1. Take the two NC safety channels to the dual-channel inputs of the safety relay (typically S11/S12 and S21/S22 — check your relay).
2. Keep the two channels in separate cores of the same multicore, or in separate looms where the risk assessment calls for it, so a single crush cannot short both.
3. Do not fuse or switch anything into the safety channels other than the interlock contacts.
4. Use the relay's cross-fault monitoring if it has it — that is what buys you Category 3.

Step 3 — Wire the NO status contacts to the PLC

The NO contacts close when the actuator is withdrawn (guard open) and when the lock bolt retracts (unlock confirmation). Take them to standard PLC digital inputs. Use them for:

• HMI status display — “guard open”, “lock released”.
• Interlock plausibility checks (flag impossible combinations such as bolt extended with actuator withdrawn).
• Production logging and cycle interlocks.

They must never be wired into the safety chain. They are not forced-disengagement contacts.

Step 4 — Wire the solenoid correctly

• Observe polarity. On most devices E1 is positive, E2 negative. Reversed polarity will not release the bolt.
• Drive E1/E2 from a safety relay output or safety PLC output, not from a plain PLC output. The unlock command is itself a safety-relevant signal.
• Gate the unlock command behind the run-down or standstill condition: stop request → drives disabled → standstill monitor satisfied → relay energises E1/E2 → bolt releases.
• Fit a suppression diode if your relay contact requires it, and check the coil's rated current (commonly around 200 mA / 4.8 W at 24 V DC).
• Do not leave the solenoid energised while the guard is manually unlocked.

Step 5 — Wire the indicator

The LED terminals are independent of the safety circuits. On wide-range executions they accept 10–115 V AC/DC, which lets you drive the lamp from a convenient panel supply. On standard executions they expect 24 V DC at low current.

Step 6 — Function test before release to production

Verify each state on the machine, not on the bench:

Confirm that the machine cannot be started in any state except the first row, and that opening the guard mid-cycle produces a stop.

Five wiring mistakes that fail commissioning

1. NO contacts in the safety circuit. The relay will appear to work and the machine will run with the guard open.
2. Solenoid driven from a standard PLC output. A software fault or output failure can release the guard.
3. Unlock command not gated by standstill. The bolt releases while the machine is still running down.
4. Metal cable glands on a plastic-bodied switch. Compromises the insulation class and the IP67 seal.
5. Actuator entry gap out of tolerance. Most devices want a ready position of 1.0–3.5 mm and alignment within ±1 mm of the slot centre. Outside that, you get intermittent contacts and a relay that will not reset.

Devices and matching safety relay

The Voxintech VXT-SS series provides both wiring approaches, plus a matching 24 V DC dual-channel safety relay module.

• VXT-SS-W2-D-NCNO-L-NCNO — independent door and lock circuits, three M20 entries for left / right / bottom cable routing.
• VXT-SS-W5-D-2NC1NO-L-2NC1NO — internally cross-linked, two complete series safety channels at 11-42 and 21-52.
• VXT-SR-24 — dual-channel safety relay module for guard monitoring and unlock control.

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