Field Verification Versus Schedule Pressure on a PLC Replacement
A 10-week project replaces three legacy PLC-5 panels with a new control system: 180 discrete and analog I/O, four vendor skids, and a SCADA historian. The engineering firm issued drawings marked “Issued for Construction” based on P&IDs that are 12 years old and carry hand-drawn field changes. Two weeks into field checkout, technicians find 14 discrepancies: a solenoid wired to the wrong terminal block, missing surge suppressors on six 24 VDC circuits, and a motor starter interlock that is not shown on any drawing. Startup is in five weeks. The project manager proposes skipping further field verification to protect the schedule and capturing everything as redlines in the turnover package. The contingency is $40,000; reworking one panel is estimated at $6,000 and two days. The client's operations team will not accept a startup with unresolved safety interlocks. The project sponsor wants a decision this week.
1. Evaluate the project manager's proposal to skip field verification. What schedule, cost, safety, and contractual risks does it create?
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The proposal transfers unknown risk into startup, which is the most expensive place to find it. The missing motor starter interlock is a safety issue, not a documentation cosmetic; if it is not verified, the client may refuse acceptance or a start may occur with an unintended permissive. Wrong terminal wiring and missing surge suppressors can cause intermittent trips, damaged cards, or nuisance shutdowns after turnover. Redlines captured at the end are less reliable because the people who found the discrepancies may no longer be available, and the client is asked to accept a system that was never fully checked. The cost comparison is also misleading: one panel rework is $6,000 and two days, but discovering a missing interlock during startup could cost far more than the $40,000 contingency in delay, rework, and contractual penalties. The right decision is to verify safety-critical and power circuits now, then use a risk-based sample for the rest.
2. Propose a prioritized field verification and change-control plan that fits the five-week startup, including budget and schedule considerations.
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Freeze the drawing set and split verification into three tiers. Tier 1 is safety and shutdown interlocks, motor starters, emergency stops, and 24 VDC power; verify every one and correct the missing interlock through an RFI and MOC. Tier 2 is the 14 known discrepancies plus all analog loops and vendor-skid interfaces. Tier 3 is a 20% random sample of remaining discrete I/O. Assign two technicians with a QC checker, use a daily punch list, and track Category A items that block startup. Estimate two panels of rework at $12,000 and four days, leaving contingency for the third panel and schedule float. Parallel-path the panel rework with loop checks, and require the contractor to incorporate redlines before turnover. Do not accept “redlines later” for safety circuits; those must be corrected and retested before energization.
3. What documents must be updated, and what acceptance criteria should the client sign before turnover?
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Update the as-built P&IDs, wiring schematics, panel layouts, I/O list, loop drawings, network drawings, and alarm and interlock setpoint lists. Incorporate all redlines and RFI answers, and issue a controlled as-built revision. The turnover package should include loop-check sheets, interlock test records, panel inspection records, calibration certificates, MOC records, the RFI log, spare-parts list, O&M manuals, and training records. Acceptance criteria should require all safety interlocks tested and signed, all Category A punch items closed, no temporary jumpers or forces left in the PLC, and a documented configuration backup. The client should sign a turnover certificate that lists any remaining Category B items with owners and dates. A redline package alone is not acceptance evidence for a safety interlock.