Safety relays and light curtains: fault-finding safely

It's 3am. The robot cell on the end of line two won't start. The light curtain on the load bay shows red. Nothing is in the beam. The operator has wiped the lenses twice and pressed reset a dozen times.

Your fitter opens the panel. The safety relay has one channel LED on and one off. Someone says "just link it out for now, we'll sort it on days."

That's the moment that matters. A safety circuit fault is still a fault. But the fix that gets the line running fastest is the one that can put someone's arm in a robot's reach.

What the safety relay is telling you

Most safety relays are dual channel. Two contacts on the e-stop or the guard switch, or two outputs from the light curtain, both feeding the relay. The relay only closes its safety outputs when both channels agree and the reset has been pressed.

So read the LEDs before you reach for the meter. On most relays you'll see the supply, channel 1, channel 2 and the outputs. The manual will tell you what each one means. Print it and keep it in the panel.

The common patterns:

• No supply LED: start with the 24V. Check the fuse and the supply to the relay. Our guide to 24V DC control supply faults covers that.
• One channel on, one off: the two channels disagree. Most relays treat that as a fault and lock out until both channels have opened and closed again. Look for one contact in the e-stop or switch that's stuck, a loose wire, or a broken core in a flexing cable.
• Both channels on, no outputs: the relay is waiting for a reset, or the feedback loop isn't made.
• Everything on, machine still won't run: the safety side is happy. The fault is in the normal control logic. That's a job for the PLC inputs and outputs guide.

The feedback loop catches people out

Most safety relays watch the contactors they switch. An auxiliary contact on each contactor runs back to the relay. If a contactor hasn't dropped out, the relay won't reset.

HSE's guidance on light curtains wants a contactor used for this job to have a way to monitor whether its main contacts are on or off. It adds that this "can sometimes be achieved by using an auxiliary contact on the same former as the main power contacts" (HSG180, paragraph 56).

So if the relay won't reset, check that feedback. A contactor with welded main contacts will hold its auxiliary open, and the relay is doing exactly what it should. That's a contactor fault, and the contactor needs changing.

Light curtains: the usual suspects

Before anyone touches the safety relay, rule out the simple stuff on the curtain itself.

• Dirt, condensation or a scratched lens. Clean both sides.
• Alignment. A knock from a pallet or a forklift can move a column a few millimetres. Most curtains have alignment LEDs. Use them.
• Something in the beam that nobody notices: a strap end, a label stuck to the guard, a cable tie.
• A connector that's been crushed or has water in it.
• A reflective surface next to the beam, such as new stainless cladding.

Our guide to photoelectric sensors has more on alignment and reflections. The same physics applies.

Muting and reset are part of the safety system

Many curtains on pallet lines are muted so the load can pass. If muting has stopped working, every pallet trips the curtain.

HSG180 says that where muting isn't needed, "the facility should not be available". Where it is fitted, selecting mute should need a locked switch or some other secure means, so only trained people can do it (paragraphs 104 and 108). So a muting sensor that's been knocked out of line is a real fault to fix. Adding your own mute is a modification, not a repair.

Reset works the same way. When a light curtain trips, HSE expects the machine to stay stopped until the curtain is clear and someone operates the controls again (HSG180, paragraph 85). PUWER guidance says operators shouldn't be able to restart just by resetting a protection device. The start control should also be needed (L22, paragraph 165). If a cell restarts the moment the beam clears, something has been changed. Find out what.

Never link it out to get running

Here's our view. Bridging a safety relay or a light curtain to get through the shift is the worst fix on site. You've turned a stopped machine into a dangerous one, and on paper it looks fine.

PUWER says a control system isn't safe unless a fault in it can't result in "additional or increased risk to health or safety" (regulation 18). A link in the safety circuit does exactly that, on purpose.

HSE looked at a set of incidents where control systems went wrong. In that sample, the biggest single cause was the specification, at 44%. "Changes after commissioning" accounted for 20%. HSE warns the sample was small (HSG238). Even so, one in five is a lot of incidents that started with someone changing a working system.

If you have to fault-find with the curtain overridden, that's a planned job. Isolate the hazard, keep everyone out, and put the link on a tag with a name on it. Then take it off and test before the shift starts. Don't fault-find on the live robot. Remember too that a tripped curtain or an e-stop is not isolation (L22, paragraph 195). Our safe isolation guide covers the full routine.

Test it before you hand it back

After any repair, test the curtain the way HSE describes. With the machine at rest, pass the right size test piece slowly down the curtain in three places: near each column and in the middle. The indicator should change as the piece goes in and out, and stay put while it's in the beam (HSG180, paragraph 128). Then check the machine actually stops when the beam is broken.

Write it down. HSG180 says results of tests after repair or modification that could affect the curtain's safety should be recorded and kept (paragraph 121). The periodic inspection is a separate job. HSE's recommended maximum gap is six months* for type 4 curtains and 12 months* for type 2, depending on the machine and the risk (paragraph 124).

The cost of the wrong person on it

Most mechanical fitters can clean a lens. Fewer can read a dual channel relay, trace a feedback loop and prove the curtain stops the machine. When that skill isn't on shift, one of two things happens. The cell sits until days, and you lose a shift of output. Or someone links it out, and you carry a risk nobody signed up to.

Both cost more than getting the right engineer in at 3am. Be honest about who on each shift can do this work, and have a breakdown callout lined up for when nobody can.

What to do on Monday

1. Print the safety relay manual and the LED table, and keep them in every panel with a light curtain.
2. Walk the site and look for links across safety relay terminals. Remove any you find and test.
3. Check every light curtain has its periodic inspection date and test piece recorded.
4. Write down who on each shift can fault-find a safety circuit. Fill the gaps.

* Inspection intervals and paragraph numbers correct as of 3 October 2026. Check HSG180 on HSE's website for the latest. Paragraph numbers refer to HSG180, first edition (1999), and L22, fourth edition (2014, amended 2018).