SiteIt's 11pm. The wrapper's stopped with the turntable half way round. Your night fitter checks the chain, the sprockets, the gearbox. All fine. They try the motor by hand. It turns.
An hour later they ring the day electrician at home. Five minutes on the phone and the answer comes back. The inverter's tripped on an earth fault. The cable's chafed where it goes through the guard.
Nobody did anything wrong. The fitter checked everything a fitter checks. It was just the wrong trade for the fault. And that hour came straight out of your night's output.
Three trades, three kinds of fault
Most sites lump all three under "maintenance". They do different jobs.
A fitter owns the things that move. Bearings, gearboxes, chains, belts, couplings, hydraulics, pneumatics, guarding. They read wear. They'll tell you a bearing's on its way out from the noise before it fails. On a site with old, heavy kit, most of your breakdowns will land here.
An electrician owns the power and the wiring. Motors, starters, contactors, overloads, inverters, control circuits, sensors and the cables between them. On a modern line, a big share of stoppages are electrical, and a lot of them are small: a prox knocked out of line, a loose terminal, a contactor coil gone, an inverter tripped on earth fault. Those are quick fixes for the right person and long nights for the wrong one.
A controls engineer owns the logic. PLCs, HMIs, servo set-up, networks, safety controllers. They're the ones who can go online, watch the inputs change, and tell you which signal never arrived. You may not need one every day. When you do, nobody else will do.
There's a lot of overlap at the edges. A good electrician can go online and read a ladder. A good controls engineer can wire a panel. But each trade has a centre, and faults tend to sit in one of them.
Why "just send whoever's on" costs you
Sending the wrong trade doesn't usually make the fault worse. It makes it slower.
The fitter on the wrapper did the right thing for a fitter. They worked through everything mechanical first. That's an hour. Then the call. Then the wait for someone who can deal with the drive. On a line that earns its keep by the hour, that's real money. Work out your own figure with our guide to what an hour of downtime really costs. Then multiply it by the number of times this happens in a month.
There's also a safety line. HSE's guidance on the Electricity at Work Regulations says the operation, maintenance and testing of electrical systems "must only be carried out by those people who are competent for that work" (HSR25, paragraph 74). Under regulation 16, anyone without the knowledge or experience has to be supervised. A fitter alone on nights with a drive fault is exactly the person that rule is about. They may be keen to have a go. That's when someone opens a panel they shouldn't.
Controls work has its own trap. A change to a PLC program, even a small one, can change what a safety function does. If it changes how the machine behaves, treat it as a modification, with all that brings. Our post on machinery modifications covers what you take on when you change a machine.
Let your fault log decide
You don't need a consultant to work out the mix. You need six months of breakdowns.
Go through your log and put each stoppage into one of four piles: mechanical, electrical, controls, and "operator or process". Be honest with the last one. Plenty of breakdowns turn out to be a product change or a jam. Then look at three things:
- how many faults land in each pile
- how long each pile took to fix, on average
- what time of day they happened
The first tells you where the volume is. The second tells you where the time goes. Often the electrical pile is bigger in count but shorter per fault, while mechanical faults are fewer but longer. The third tells you who you need on which shift.
If your log won't give you this, that's a problem worth fixing first. Our post on a breakdown log that actually tells you something shows what to record.
What the answer usually looks like
Every site is different, so treat this as a starting point, not a rule.
Older, heavy plant with few drives tends to need more fitters. A modern packing or logistics site with conveyors, sorters and servo kit tends to need more electricians, because sensors and drives are where it stops. Few sites need a full-time controls engineer. Most need one they can get hold of when the logic goes wrong, plus electricians who can at least go online to the PLC and look.
Then cover the shifts. If most electrical faults happen on nights, the electrician shouldn't be on days only. That sounds obvious. It's also how a lot of rotas are built, because days is what people asked for.
Where you can't justify a full-time trade, plan how you'll get one when it's needed. A known engineer you can call, an agreement with the machine builder, or cover booked for the risky weeks. What you want to avoid is working it out at 2am. Our breakdown callouts page is one route. Your own list of trusted contacts is another.
Our view
Ask "which trade?" before "who's free?". Build the team from your fault log, cover the shift where the faults happen, and have a plan for the trade you don't hold. Two hours of the wrong person is one of the most common, and most avoidable, costs in maintenance.
What to do on Monday
- Sort the last six months of breakdowns into mechanical, electrical, controls and operator piles.
- Check which shift each pile lands on, and compare it with who's on that shift.
- Write down who you call for a drive or PLC fault on nights, and put the number on the board.
* Paragraph numbers refer to HSR25, third edition (2015). Correct as of 15 September 2026. Check HSE's guidance on the Electricity at Work Regulations for the latest.