Emergency stop circuits that won't reset

It's 11pm. The case conveyor in goods out has stopped. Someone hit an e-stop when a carton jammed. The carton's gone, the operator says they pulled the button back out, and the reset lamp is still flashing.

There are 22 e-stops and four pull-wire switches along that run. Your team leader has walked it once and says they're all out. The reset still won't take.

E-stop faults look simple and burn hours. The circuit is a long string of contacts in series, spread across the building. One bad contact anywhere stops the lot.

Walk the line again, properly

The first walk is always too fast. Do it again with a torch, and touch every button.

• Twist or pull every mushroom head. Some look out and are still latched.
• Check the pull-wire switches. Most have a flag or a window that shows tripped, and many need a separate reset on the switch itself. A slack wire trips them too, so look for a wire that's been knocked or has stretched.
• Look at the e-stops on the HMI stands, the operator panels and the back of the machine. The one nobody uses is often the one that's in.
• Check any e-stop on a pendant or a plug-in station. A pendant that's been unplugged without its blanking plug will hold the circuit open.

If the line has a safety PLC or a networked relay, the HMI may tell you which e-stop is open. Read it before you start walking.

Then go to the safety relay

If every stop is out and the reset won't take, open the panel and read the safety relay. Our guide to safety relays and light curtains covers the LEDs in more detail. For an e-stop string, three patterns matter.

• Both channels off: the circuit is open somewhere. Go and find the break.
• One channel on, one off: the two channels disagree. Many relays lock out on that until both channels open and close together. Press and release the e-stop nearest the panel firmly, then reset. If that clears it, someone pressed a button slowly and one contact moved before the other.
• Both channels on, no outputs: check the reset circuit and the contactor feedback loop. A contactor that hasn't dropped out will stop the relay resetting.

Half-split the string

When the circuit is open and you can't see why, don't test every button one at a time. Split it.

Find a junction box roughly halfway along the run. With the right precautions in place, measure each channel there. Voltage present means the break is downstream. No voltage means it's upstream. Split again. On a 22-button run you'll find the bad section in four or five readings. Our half-split fault-finding guide explains why it works.

This is live testing on a control circuit, and HSE counts "testing of live exposed conductors using a test instrument" as live work (HSR25, paragraph 50). Live work is only allowed when it's unreasonable for the kit to be dead, reasonable to work live, and suitable precautions are taken (Electricity at Work Regulations, regulation 14). A 24V string usually passes that test. The 400V next to it in the same box doesn't care.

What you'll usually find

In our experience, the fault is rarely the button itself. These come up again and again:

• A contact block that has come loose from the back of the head. The button moves, the contact doesn't.
• Water in an e-stop box on a cold dock door or a washdown line.
• A cable crushed by a forklift or a pallet where it runs low along the conveyor.
• A loose terminal in a junction box, often one somebody worked in last week.
• A pull wire that has stretched over time and now sits right on the trip point.

That first one is worth a second thought. A contact block hanging off the back of a mushroom head can fail open, which stops the line. It can also sit closed, so the button does nothing when someone hits it. Monitored contact blocks catch this, but not every machine has them. If you find one loose, check every other button on that run before you leave.

Don't bypass it to get running

Here's our view. Linking out an e-stop, or the whole string, to "get through to the end of the shift" is never a fix. It turns a stopped conveyor into one that can't be stopped.

PUWER requires e-stops where they're needed, and they must "operate in priority" to the normal stop controls (regulation 16). A control system isn't safe if it impedes an e-stop (regulation 18). A link does exactly that.

If one button is faulty and you can't get a spare, the honest options are a proper repair, a like-for-like part from another machine with that machine taken out of use, or a stopped line. A link with a tag on it is still a link.

While you're in there

Two things often sit behind a run of e-stop faults.

First, operators using the e-stop as a stop button. PUWER guidance says e-stops "should not be used to stop the equipment during normal operation" (L22, paragraph 177). A button pressed 40 times a shift wears out faster, and the people pressing it stop treating it as an emergency. If the normal stop is in the wrong place, move it.

Second, restart behaviour. Pulling out an e-stop should never start the machine. PUWER says starting, including restarting after a stoppage, must need "a deliberate action" on a control (regulation 14). If the conveyor moves the moment the button comes out, stop and find out why before you hand it back.

And remember an e-stop is not isolation. HSE guidance is clear that e-stops and interlocked guards "are not suitable as a means of isolation" (L22, paragraph 195). If you're going into the conveyor to free a jam or change a part, lock it off. Our guide to safe isolation covers how.

The hour nobody counts

An e-stop fault on a long conveyor can take a fitter who knows the circuit 20 minutes. It can take someone who doesn't know it three hours, walking and guessing. The difference is drawings, junction box labels, and someone on shift who can read a safety relay.

On a busy night in a warehouse, those three hours are missed trailers and a backlog for the day shift. Work out what an hour of that costs with our downtime cost guide. Then decide whether the next one waits for days or gets the right engineer on site.

What to do on Monday

1. Number every e-stop and pull-wire switch on site, and put the numbers on the drawings.
2. Label the junction boxes on each long run so a half-split doesn't start with a hunt.
3. Have someone press every e-stop on your worst line and check each one stops it. Note any that feel loose.
4. Ask your operators if they use the e-stop as a normal stop. If they do, find out why.

Paragraph numbers refer to L22, fourth edition (2014, amended 2018), and HSR25, third edition (2015).