SiteIt's 6am. The hygiene team has just finished the washdown on the slicer line. The line starts, runs for a minute, and the control fuse blows. You fit another. It holds for ten minutes, then goes again.
By 7am it's dried out and runs all day. Tomorrow morning, same again.
That's an earth fault. Water is getting into something on the control circuit and giving the 24V a path to earth. When it dries, the fault goes away. It will be back after the next washdown, and one day it won't dry out.
Earthed or floating: know which you've got
Before you look for the fault, find out how the control circuit is set up. Look at the drawings, or look in the panel for a link or a wire from the 0V terminal to the earth bar.
Most machine control circuits are one of two kinds:
- Earthed 0V. The 0V side of the control supply is bonded to earth. An earth fault on the +24V side is a short, so the fuse or breaker opens. You know about it straight away.
- Floating. Neither side is connected to earth. A single earth fault does nothing you can see. Some panels have an insulation monitor or an earth fault lamp to warn you.
The second one is the trap. With a floating circuit, the first earth fault is silent. A second one, somewhere else, can join two parts of the circuit through the metalwork. That can bridge a stop contact, pull in a relay that should be off, or hold one in that should drop out. That's why the warning lamp matters. If the earth fault lamp has been on for months and everyone ignores it, the machine is one fault away from doing something nobody told it to.
The earth itself is a legal duty
The Electricity at Work Regulations require precautions, "either by earthing or by other suitable means", for metal that could become charged in a fault (regulation 8). HSE's guidance makes clear that covers the metal casings of equipment that "may become live under fault conditions" (HSR25, paragraph 125).
So a missing earth wire on a sensor bracket, a guard or a panel door is a fault in its own right. Fix it while you're there.
Mind the 0V to earth link
Many panels with an earthed 0V have a removable link between the 0V bar and earth. It's there so you can take it out and find earth faults.
HSE's guidance on regulation 9 says a removable link in a referenced conductor is allowed. That's only when "suitable precautions are adopted" so it isn't removed or operated in a way that causes danger (HSR25, paragraph 154). In practice that means the link goes back in before you hand the machine back. Label it, and note it on the job sheet. A link left out turns an earthed circuit into a floating one, and nobody knows.
Finding it
With the machine isolated and proved dead, take out the 0V to earth link if there is one. Then measure the resistance from the +24V and the 0V to earth.
A healthy circuit reads very high. A low reading tells you there's a path to earth somewhere on that side. Now split it:
- Open the fuses or breakers for each 24V group one at a time.
- Measure again each time. When the reading goes back up, you've found the group.
- Work along that group, disconnecting field devices or opening terminals until it clears.
Our half-split guide covers the method. On a long run, start at a junction box halfway along.
Be careful with an insulation resistance tester here. At 500V it can damage PLC cards, sensors and power supplies. Disconnect the electronics first, or use a low test voltage. Our guide to insulation resistance testing on motors covers the basics.
If the fault only shows when wet, test when it's wet. Do the measurement straight after the washdown, before the line starts and the heat dries it out.
Where they usually are
In our experience, control earth faults come from a short list:
- Sensor and solenoid connectors on washdown lines, where a seal has gone or a cap is missing.
- Cables in drag chains or moving arms, where the insulation has worn through.
- Glands that were never tightened, letting water run down a cable into a junction box.
- A cable pinched under a guard or a cover plate after maintenance.
- Solenoid coils that have broken down to the valve body.
Look for the cheap fix too. A fuse uprated to stop it blowing, or an earth fault lamp with the bulb taken out. Both happen.
Why it's more than a nuisance
A control earth fault feels like a minor job. On a safety circuit, it can be a serious one. PUWER says a control system isn't safe unless a fault in it "cannot result in additional or increased risk" (regulation 18). A second earth fault that bridges an e-stop contact is exactly that kind of fault. Dual channel safety relays are built to spot faults like this, but only if they're wired and set up as designed. Our guide to safety relays and light curtains covers how to check.
Drives have their own version. A motor or cable earth fault will trip an inverter, and resetting it over and over can finish off the drive. See inverter trips for that one.
The fault that waits for nights
Washdown earth faults have a habit of turning up at shift change. Days have gone home. Nights has a hygiene team, a production manager and maybe one fitter. If that fitter isn't an electrician, the choice is to keep fitting fuses or wait for days.
Neither is cheap. Every lost shift on a food line is product at risk, and every fuse is a guess. Be honest about which faults your night cover can find, and plan for the ones it can't. Our piece on fitters, electricians and controls engineers can help you map the gap, and shift cover can fill it.
What to do on Monday
- Find out which of your control circuits are earthed and which are floating. Write it on the panel door.
- Check every panel with an earth fault lamp. If one is lit, raise a job today.
- Check the 0V to earth link is in, and labelled, in every panel that has one.
- After the next washdown, measure the control circuit to earth on your wettest line before it starts.
Paragraph numbers refer to HSR25, third edition (2015). Check HSE's HSR25 page for the latest.