SiteIt's 5.50am, ten minutes before day shift. Low air alarms start popping up on one HMI after another. The case erector stops. Then the wrappers. Then the palletiser. Within five minutes every line on site is stood.
Your shift fitter runs to the compressor house. The compressor is showing a high temperature trip. There's one compressor. There's no standby.
Most sites plan for a machine breakdown. Very few plan for the air going, even though it stops everything at once. A single compressor is often the biggest single point of failure on the site, and it sits in a room nobody visits.
Supply or demand?
The first question is whether you've lost the air or lost the pressure. They look the same on the HMI.
Go to the compressor house first. Is the compressor running, loading and making air? If it has tripped, note the alarm before anyone resets it. High temperature, motor overload, low oil and a failed drain are all common. Our guide to compressor faults covers the fault-finding order.
If the compressor is running flat out and the pressure is still falling, the air is going somewhere. Listen. A burst hose or a blown fitting on a ring main is loud. Check the dryer and the main filters too. A blocked filter or a dryer that has failed can starve the whole site while the compressor sounds fine.
Then there's the slow version. A site full of small leaks, a compressor that's been working harder for months, and one hot morning that tips it over. Our post on compressed air leaks is the one to read after you're running again.
Losing air is a safety problem too
When the air drops, machines don't all stop neatly. Cylinders sag. Grippers open. Clamps let go. A pick and place head can drop onto whatever is under it. Some machines will hold position on trapped air, and some won't.
So get people out of the machines before anyone tries to fix anything. Treat every pneumatic machine as if it might move.
HSE's code of practice for PUWER says where equipment relies on pressure, "hydraulic or pneumatic", you should provide a means of isolation, and where possible a way to dissipate stored energy (L22, paragraph 199). Anyone working inside a machine during the outage still needs it isolated and locked off, air included. A machine with no air now can have full air in ten minutes. Our post on safe isolation covers the full routine.
The restart is where people get hurt
The air coming back is more dangerous than the air going. Every cylinder that sagged will try to return to position, all at once, the moment pressure builds.
We'd restart in stages:
- Close the isolation valves on each line or area before the compressor comes back.
- Bring the main up to pressure and check for leaks at the compressor house.
- Open one area at a time, with someone at each machine and everyone stood clear.
- Check each machine homes properly before the operators restart it.
It takes longer. It's also how you avoid a crushed hand at 6.30am.
Hiring a compressor
If the compressor is going to be down for hours, a hired unit can get you running. Check a few things before it arrives.
Compressed air systems fall under the Pressure Systems Safety Regulations. HSE says a relevant fluid includes compressed air at more than 0.5 bar* above atmospheric pressure. Before any qualifying pressure equipment is used, a written scheme of examination must be in place and an examination done (HSE, PSSR). For a mobile compressor, the owner has duties. HSE adds that the user of hired or leased equipment should make sure the written scheme is in place and the certificate of examination is current. So ask the hire firm for both before the low loader leaves their yard.
Then plan where it connects. You need a tie-in point on the main, the right hoses and fittings, a power supply or fuel, and somewhere safe to put it. If you've never worked any of that out, the hire unit can sit in the yard for two hours while you do.
Back running is not the same as fixed
Once the line is up, find out why it failed. A high temperature trip might be a blocked cooler, a failed fan or a hot room with no ventilation. A tripped motor might be a bearing on the way out.
If the repair touched the receiver, the pressure pipework or the protective devices, talk to your competent person before you call it done. HSE warns that after a major repair or modification, you may need the whole system re-examined before it goes back into use (HSE, pressure equipment). Our post on air receivers and the pressure regulations explains what the written scheme should cover.
And if a receiver, pipe or vessel actually burst, stop and think about RIDDOR. The failure of a pressure system with the potential to cause the death of any person is a dangerous occurrence, even if nobody was hurt (HSE, dangerous occurrences).
What it really costs
Here's our view. A compressor failure is the most expensive breakdown most sites never price up. When one machine stops, you lose one line. When the air stops, you lose all of them, plus the restart time on each.
Take your cost per hour of downtime for one line and multiply it by every line that runs on air. Then compare that number with the price of a standby compressor, a service contract with a real response time, or an engineer on call who knows compressors. For most sites, the answer is obvious once the number is on paper.
The other gap is people. Plenty of good maintenance teams have nobody who really knows the compressor, because it "just runs". When it stops at 5.50am, that's the skill you need on site within the hour.
What to do on Monday
- Write down what happens if your compressor stops: which lines go, and what it costs per hour.
- Mark the isolation valve for each area on the air main, and write a staged restart for the shift.
- Find a compressor hire firm who can deliver at night, and check where a hired unit would connect.
- Check who on your team, or on call, can fault-find the compressor and dryer.
* The 0.5 bar threshold correct as of 17 October 2026. Check HSE's PSSR page for the latest. Paragraph numbers refer to L22, fourth edition (2014, amended 2018).