Retrofit
Planning a PLC changeover around the shutdown window you can actually get
The window is the constraint. Everything else in a retrofit is arranged around it, including the decision to stop.
On a greenfield project, the risk is finishing late. On a retrofit, the risk is that the plant does not restart — and that risk is carried by production, not by the project. It changes how everything should be planned.
The first question is not what the new system will do. It is how many hours you can genuinely have, and what happens to the business if you overrun them by four.
Everything that should be finished before the window opens
- New panel built, wired and factory acceptance tested
- New program written, simulated and tested against the panel
- Screens developed and reviewed by the operators who will use them
- Field cabling pre-run and labelled at both ends wherever the route allows
- Point-to-point mapping from old I/O to new, printed and checked by two people
- Rollback route tested — physically re-connectable, not theoretically available
- An hour-by-hour plan agreed with production, with names against tasks
Set the abort point before you start
Agree in advance the hour at which, if you are not at a defined milestone, you stop and roll back rather than push on. Decided calmly a week beforehand, it is a sensible engineering control. Decided at 4am by tired people who have already invested six hours, it is not a decision at all.
A typical 14-hour window
- H+0
- Isolation, permits, lock-off. Photograph everything before disturbing it.
- H+1
- Disconnect and remove old controller; existing panel left intact and re-connectable.
- H+3
- New panel positioned and power terminated.
- H+5
- Field cores landed against the mapping sheet, checked by a second person.
- H+7
- Power up, I/O loop checks point by point. Abort point sits here.
- H+9
- Dry run — sequences and interlocks proved without product.
- H+11
- Wet run under supervision.
- H+13
- Hand to production, engineer stays on site.
Two things consistently take longer than planned: landing field cores when the existing labelling is wrong, and loop checking when a device that was working before the shutdown is not working after it. Both are best absorbed by finishing the earlier steps quickly, not by shortening the loop check.
And the changeover is not over when the plant restarts. An engineer should stay through the first full production runs. Hand-back is a decision somebody makes, not a time that arrives.
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Next step
Tell us what the plant is doing
Send the problem, not a specification. An engineer will come back to you — usually the one who would do the work.
An engineer responds within one business day.