SiteIt's 2am. The bottling line has gone strange. Half the sensors on the HMI show off. The PLC dropped out and came back on its own. A valve bank chattered and stopped. Your fitter has been chasing sensors for an hour, one at a time.
None of them are faulty. They're all on the same 24V supply, and the supply is sagging.
When lots of unrelated things fail at once, stop looking at the things. Look at what they share. On most machines that's the 24V DC control supply.
The signs that point at the supply
These are the patterns that make us check the 24V before anything else:
- Several sensors or inputs dropping out together, on different parts of the machine.
- The PLC or HMI rebooting, or a remote I/O station losing comms.
- Contactors or relays chattering, often all at once. Our guide to contactor chatter covers why.
- Faults that come and go with load, such as when a valve bank fires or a motor brake releases.
- The PSU "DC OK" LED flickering, or the supply clicking as it cuts out and retries.
Measure at the supply, then at the far end
Start at the power supply output terminals. Then measure at a terminal block out on the machine, as far from the supply as you can get. Do both with the machine running, or trying to run. A supply that reads 24.1V with everything idle can drop well below that when the valves fire.
What you're looking for:
- Low at the supply: the supply is overloaded, failing, or losing its own input.
- Fine at the supply, low at the far end: volt drop in the cable, or a bad connection on the way. Check the 0V as well as the +24V. A loose 0V common does just as much damage.
- Swinging up and down: the supply is in current limit or hiccup mode. Something is drawing too much.
If you have a scope or a meter with min/max, use it. Dips that last a few milliseconds won't show on a normal meter, but they're long enough to reset a PLC.
Something is pulling it down
The most common cause, in our experience, is a short or overload out on the machine. A sensor cable crushed under a guard. A connector full of water on a washdown line. A solenoid coil that's gone short. One bad device can pull the whole supply down, and everything else on it goes with it.
So split it. Most panels feed the 24V out through fuses, MCBs or electronic circuit breakers, one per group. Open them one at a time and watch the supply voltage. When it comes back up, you've found the group. Then work along that group. Our half-split fault-finding guide explains how to do it in the fewest steps.
That's also why the fusing matters. The Electricity at Work Regulations require "efficient means, suitably located" to protect every part of a system from excess current where needed to prevent danger (regulation 11). On a control supply, good fusing also gives you something to split at. A panel with one fuse feeding 60 sensors gives you nowhere to start.
Watch for the cheap fix here. Fitting a bigger fuse or a bigger supply to "stop it tripping" hides the short. It also means the cable that's shorting can now carry more current before anything opens.
When the supply itself is failing
Power supplies wear out. Heat is the usual cause. A supply mounted at the top of a hot panel, above the drives, with blocked filters, has a hard life.
The signs are a supply that runs hot to the touch, an output that's low or noisy under load, or one that won't start up after a power cut. If it's been in 15 years, and the panel runs warm, it doesn't owe you anything.
Before you swap it, check what's on it. Add up the loads if you can. Machines get extra sensors, a new valve bank or a camera over the years, all hung off the original supply. If it was sized for the machine as built, it may now be running at its limit.
Loss of supply should be safe
A 24V fault should stop the machine safely. PUWER says a control system isn't safe unless a fault or "the loss of supply of any source of energy" can't add to the risk (regulation 18). The guidance calls this "minimised failure to danger" (L22, paragraph 193).
On a well-designed machine, a sagging 24V drops the safety relay and stops things. But check what happens on yours. Does a valve hold or drop when its coil loses power? Does a brake engage? A machine that does something unexpected on loss of 24V is worth a proper look on days.
Low voltage is still live work
24V DC is well under the extra-low voltage limit. HSE's guidance on the regulations gives "at or below 50 V ac or 120 V dc"* as an example of an extra-low voltage system (HSR25, paragraph 143). The supply itself won't hurt you.
What's next to it will. The 230V input to the power supply sits on the same DIN rail. The drives and contactors a few centimetres away are at 400V. Testing live exposed conductors with a meter counts as live work (HSR25, paragraph 50). Shroud what you can, and use GS38 leads. Our guide to earth faults in control circuits covers the other trap: a 0V that's earthed, or one that isn't.
The cost of chasing sensors
A supply fault is quick to find if you know to look at the supply first. If you don't, it can eat a whole shift. Sensors get swapped, cables get changed, and none of it fixes anything. By morning you have a stores bill, a backlog, and the same fault.
That's the price of not having an electrician on shift who thinks in systems. If your nights are covered by mechanical fitters, be honest about which faults they can't chase. Agree when they call for help. Our 30-minute rule is a good place to start.
What to do on Monday
- Hold a spare 24V power supply for each size used on your critical machines.
- Measure the 24V at the far end of your worst machine with it running. Write the number down.
- Check the 24V on each machine is split into fused groups. Plan a fix where it isn't.
- Look at where your power supplies are mounted. Move the ones cooking above the drives.
* The extra-low voltage example and paragraph numbers correct as of 14 October 2026. Check HSE's HSR25 page for the latest. Paragraph numbers refer to HSR25, third edition (2015), and L22, fourth edition (2014, amended 2018).