Machinery modifications: when you change it, you own it

It's a Friday afternoon. The production manager wants faster changeovers on the case packer. The operators lose ten minutes every time they open the fixed fencing at the back.

Your best fitter has an idea. Take the fencing off, fit a light curtain, wire it into the safety relay. Two days' work, parts on the shelf. By Monday the changeover's half the time and everyone's happy.

Nobody's broken anything. But somebody may have just become the manufacturer of a new machine, without a technical file, a declaration of conformity or a risk assessment for what they built.

Where the line is

Most changes are fine. HSE is clear that a rebuild to the original spec, even with newer parts, "will not usually attract" the supply rules. Repainting, servicing, changing motors and fitting identical guards are fine too. Even swapping old safety parts for better ones, such as modern interlocks wired in the same way, "does not amount to a substantial change" (HSE, refurbished and modified machinery).

The trouble starts when the change is big. The same page gives two tests. The first is when "significant new hazards and risks are introduced". The second is when new methods of control replace the old ones, like computer control on a machine that used to be manual. A change like that "may amount to being considered a 'new' machine". Then you must do the conformity assessment and meet the Supply of Machinery (Safety) Regulations 2008* in full.

That means a technical file, a declaration of conformity, and the right UKCA or CE marking and labels. In effect, you've built a new machine.

The light curtain trap

Here's the example that should worry every maintenance team. HSE says swapping fixed fencing and interlocked doors for photoelectric or laser scanning systems, with the extra logic units they need, "will probably require re-compliance" before the machine goes back into service (HSE).

That's our case packer. A sensible fix, done by a good fitter, that changes the whole safety strategy of the machine.

It works the same on a line. If you add or swap a machine in an assembly, and that change has a big effect on how the whole line runs or how safe it is, the assembly may need to re-comply. Machines in the line that the change doesn't touch need no reassessment.

On a big machine there's some relief. If you only change one part, you only need to reassess the parts and systems that change could affect. The rest needs no new conformity assessment, but your reasons for leaving it alone belong in the technical file.

Who counts as the manufacturer

The supply regulations define the manufacturer as a person who designs or makes the machine, either to sell it "with a view to its being placed on the market under that person's own name or trademark; or (ii) for that person's own use" (regulation 2).

Read the second half again. You don't have to sell anything. If your team designs and builds a new machine for your own site, that's you.

HSE says the duty to work safely applies "whether the machine owner, user or contractors working for the owner or user" do the work (HSE). Bringing in a contractor to do the wiring doesn't hand the problem to them. HSE's page on powered gates makes the same point. The "manufacturer" can be "even the premises owner or occupier" if they design the system and get a contractor to build it (HSE, in situ manufacture).

Small changes still have rules

Below that line you're still under PUWER. If you adapt a machine, it must still be suitable for its job (L22, paragraph 53). Major modifications, refurbishment or repair work count as "exceptional circumstances" that call for an inspection before the kit goes back to work (L22, paragraph 102).

Where a machine carries a specific risk, PUWER limits who can work on it. Repairs and modifications must be done by people "specifically designated to perform operations of that description" (regulation 7).

And your risk assessment must be reviewed when "there has been a significant change in the matters to which it relates" (Management Regulations 1999, regulation 3). We'd treat a new light curtain as a significant change. The same goes for a new infeed, a faster drive or a changed PLC sequence.

Breakdowns are where it happens

Most big modifications are planned. The risky ones get done at 3am.

It's 3am and the photo-eye on the infeed has died. There's no spare. Someone moves the eye from the outfeed and changes a timer in the PLC to make up for it. The line runs. Nobody writes it down. Six months later the machine is doing something its maker never designed, and nobody on days knows why.

We've written about how temporary fixes become permanent. A changed safety function is the worst version of it. The fix might be safe. But nobody's checked, and nobody owns it.

This is why the engineer you get at 3am matters. A good one fixes the fault and leaves the design alone. If they can't, they tell you straight that it's a stopgap, they write down what they changed, and they make sure days know before the shift ends. The cost of the wrong engineer is a quick fix that quietly becomes a new machine.

What we'd do

Draw the line before anyone picks up a spanner. We'd sort changes into three groups.

1. Like for like. Same part, same function, same wiring. Fitters can just do it.
2. Changes to function or controls that don't touch safety. A manager signs them off, the risk assessment is checked, and the machine is inspected before it runs.
3. Anything that changes the safety strategy, the controls method or the hazards. Stop. Talk to the maker or a competent machinery safety adviser first, and plan for a technical file.

Ask the maker before you change anything big. If a change was "foreseen or agreed by the manufacturer" and covered by their paperwork, the original marking stays valid (HSE). One email can save you a lot of work. If you're buying kit someone else has changed, our guide to buying second-hand machinery has the checks.

Keep the guarding rules in mind too. The order of guards under regulation 11 still applies when you swap fencing for a light curtain.

What to do on Monday

1. Walk your three busiest lines. Ask the shift fitters what's been changed since each machine arrived. Write down every answer.
2. Check the PLC programs on those machines against your last backup. Anything that's different needs a name and a reason.
3. Put the three groups in writing and brief every fitter, including contractors, before their next shift.
4. For any change in group three, find the technical file. If there isn't one, call the maker.

* Supply of Machinery (Safety) Regulations 2008 and HSE guidance on modified machinery correct as of 30 October 2026. Check HSE's page on refurbished and modified machinery for the latest. Paragraph numbers refer to L22, fourth edition (2014, amended 2018).