Control System Migrations | Part 7 | Best Practices for Installation, Testing, & Commissioning

Introduction | Control System Migrations | Part 7 | Cutover, Commissioning, and the Final Push

August 2025 — by Tom McGreevy, PE, PMP, CFSE — Welcome to part 7 of our Control Systems Migration blog series. In this installment, we’ll be covering the cutover phase, which is where it all comes together. This is the point where months or even years of preparation culminate in the actual replacement of the old control system with the new. It’s a high-stakes, high-pressure moment, and one where success is determined by how well you’ve planned, documented, and executed.

The term “cutover” covers everything from physical equipment replacement to software commissioning and testing. It’s not just about wiring panels; it’s about making sure every step, from demo drawings to site acceptance testing, is aligned and accounted for.

Do I Need to Begin with a Full System Backup?

The short answer: Absolutely.

Before any equipment is touched, every element of the current system must be backed up. That includes program logic, Human Machine Interface (HMI) configurations, and current “as-found” drawings. Photos of panel internals and field installations can also be valuable, not just as references in case you need to troubleshoot, but as a last-resort rollback option if something unexpected forces you to pause or reset the transition.

In a rip-and-replace scenario, rolling back may not be feasible, but having a complete picture of the system you’re decommissioning can still help solve problems when they arise during construction or testing.

What Should I Include in a Cutover Execution Plan?

Your cutover execution plan should be specific and clearly documented. It must describe step by step how the cutover will proceed and clarify who’s responsible for each task. It should also detail what tools, drawings, resources, and timing are required for each stage.

This plan should leave no room for ambiguity. What’s happening to each wire? Which devices stay, which go? Are there mystery components, the purpose and disposition of which is not 100% understood? Those need to be resolved before the first wire is lifted, or if not, at least addressed as part of your early cutover activities.

Most importantly, there is significant value in making sure this plan is in the hands of the right people. Having a perfectly crafted set of work packages and drawings means nothing if the team in the field doesn’t have them. This kind of breakdown in communication is surprisingly common, but fortunately, it is also completely avoidable.

What Pre-Shutdown Work Should Be Done Before a Control System Migration?

Any construction or staging work that can be done before the shutdown should already be complete. This includes routing and tagging cables, installing panels where possible, staging materials, and setting up temporary facilities like backup power in accordance with OSHA safety guidelines. If it can be done early, do it early. This will reduce the pressure during actual outage windows and create space to address the unexpected.

The Details Matter — Down to the Wire

One of the most critical aspects of a successful cutover is understanding where every single wire goes and what it does. If wires aren’t clearly labeled, properly documented, or tied to an understood function, you risk losing control over the tactical situation very quickly.

Similarly, you must know the purpose and disposition of every field device. Is it being reused, replaced, or removed? Has it been tagged and labeled correctly? These details feed directly into the accuracy of your demo drawings and revised documentation, which in turn drives construction confidence and efficiency.

Even the basics, like wire sizes, must be documented. Tasks like these may seem like a small detail, but mismatched or unlabeled wire sizes can lead to serious setbacks during installation.

Construction Documents vs. Loop Sheets

It’s also worth noting that loop sheets, while useful for function testing and configuration, are not construction documents. Teams need full demo drawings, updated termination diagrams, and accurate cable schedules to perform field work efficiently. Relying on loop sheets for installation will almost certainly slow the progress and may invite error and confusion.

Mechanical Completion: Knowing When You’re Ready

Before applying power to the new system, everyone involved must agree on what defines mechanical completion. At this point, all installation work should be finished, verified, and supported by construction assurance documentation. It’s a formal milestone that marks the transition from building the system to bringing it to life. Assurance activities in support of demonstration of Mechanical Completion include visual inspections, comparison to approved drawings, wiring continuity checks, and proper ground measurements (of both safety and signal ground).

Site Acceptance Testing, Commissioning, and Function Checks

Once mechanically complete, the system undergoes site acceptance testing (SAT) the first time it’s powered on in its new environment. This phase confirms that nothing was damaged during shipping or installation, and that devices are behaving as expected at a basic level.

From there, teams move into loop checks, verifying that inputs and outputs are correctly wired and responsive. These checks ensure that transmitters, control valves, and I/O points communicate properly with the system and that grounding is correct. This may also include bumping of motors for those motors controlled by the system, and verification of good communications to any and all third-party devices. It is critical that EVERY I/O device that had its wiring touched during the cutover be checked, to give high confidence in wiring integrity and to enable efficient functional testing.

Finally, functional testing begins. Depending on the system, this could include “water runs,” simulation of Safety Instrumented Functions (SIFs), and validation of interlocks. Every step should follow a documented test plan, not just for consistency, but to ensure accountability and traceability.

The temptation to rush through these tests can be strong, especially during time-constrained shutdowns. But skipping steps here can have serious consequences, ranging from costly mistakes to safety hazards and legal liabilities.

The Takeaway

The cutover process is considered the most visible and intense phase of a control system migration. It’s where all the planning, documentation, and collaboration either pay off or fall short. When executed well, the cutover is a moment of accomplishment, the grand finale of your migration efforts. But without discipline, rigor, and proper preparation, it can quickly become chaotic, stressful, and, worst of all, dangerous to equipment and people

This phase rewards diligence, not improvisation. Success lies in backing up thoroughly, planning clearly, assessing and addressing risk, labeling accurately, executing deliberately, and testing without compromise. If all of that is in place, your team can move forward with confidence, and your process can start up on a solid, resilient foundation.