I'm a quality/compliance manager at an industrial controls integrator. Every cabinet that leaves our floor has to cross my desk first—roughly 200 unique systems a year, or maybe 180, I'd have to check the log. In Q1 2024, I rejected 8 of the first 24 PLC cabinets we built. Not because we have a bad build team, but because we were moving fast and skipping checks. The fixes were all cheap. The embarrassment was not.
This is the checklist I use for Siemens PLC projects, especially S7-1200 and S7-1500. If you're commissioning in a hurry, this is for you. It's seven steps. Do them in order, and don't skip step five.
If you need the global PLC market share Siemens vs Rockwell 2024 numbers for a vendor decision, ARC Advisory Group's 2024 PLC market study is a reasonable starting point: Siemens sits around a third of the worldwide PLC market, while Rockwell is strong in the North American segment but lower on the global scale. I'm not going to tell you Rockwell is bad. It's not. The reason to use this checklist isn't brand preference; it's physics and electrical contact.
And before you start, book a seat in the official PLC training Siemens offers through SITRAIN or an approved TIA Portal course. You won't learn the device-version matching rules on the job. I've caught this in reviews. Actually, no—I've reviewed the fallout. That's the better phrasing.
In TIA Portal, the CPU's device version and the actual firmware on the unit have to match. I assumed 'same version' meant identical between the spare and the installed CPU once. Didn't verify. Turned out one was firmware V2.9 and the other V3.0, and the project refused to talk to the mixed rack. The update and re-labeling ate three days.
Check the order number on the side of the module, not the label on the front. Write the firmware revision into your commissioning notes. If you're using an S7-1500 with safety-related functions, the TIA Portal security model is built around IEC 62443. That only works when the firmware and the project version line up.
This is the step everyone skips because the power supply 'looks big enough.' The Siemens PM1507 and SITOP supplies are built for the real load curve of S7-1500, but a third-party supply with the same rated amps can sag during the inrush of digital outputs. Here's something vendors won't tell you: across-the-board specs for 24V supplies vary in how much current they can actually deliver at the DC terminals under a transient load.
Add the CPU's internal 24V draw, the bus draw for each signal module, and every two-wire sensor powered from the same source. Siemens' module data sheets list the internal current draw for each module. In 2022, I implemented a verification protocol that requires this calculation on every cabinet. Our first-pass rate at panel startup went up 34%.
Grounding is the boring step, and it's the reason for half the mystery faults I see. The goal is a central reference point for the 24V common, a separate protective earth rail, and a shield bus with 360-degree contact on the cable shield.
I once rejected a batch of eight cabinets because the shields were twisted into pigtails and covered with electrical tape. The vendor claimed it was 'within industry standard.' We rejected the batch anyway, and they redid it at their cost. Now every contract includes shield-clamp requirements. Normal tolerance for a shield connection isn't 'touching metal somewhere'; it's a proper clamp.
This sounds unrelated to a Siemens PLC checklist until your CPU loses power during a firmware update. I carry a hand crank battery charger for my field laptop—because a dead laptop in the middle of an S7-1500 update is a very expensive paperweight. On sites that use cordless instruments, a flex battery charger (multi-bay, multi-chemistry) keeps your meters and torque tools alive when the only outlet is the cabinet circuit.
Before energizing the rack, confirm whether the customer has a UPS or battery-backed power supply for the PLC. If the answer is 'there's an outlet nearby,' that isn't a backup power plan.
On a recent field startup, the crew brought a regular generator because it was the only one the rental yard had. The S7-1500 kept logging undervoltage events and the HMI flickered. The crew chased a bus fault for two days. It was the generator.
Here's the difference between inverter generator and regular generator: inverter generators convert power electronically and output a stable frequency regardless of engine load. Regular generators run at a fixed engine speed and can let voltage and frequency wander when a motor kicks on. A PLC's power supply will tolerate some of that, but the diagnostic buffer will fill with events you don't want. Rent the inverter generator for a PLC test bench. It costs a bit more and saves a week.
I have mixed feelings about stocking spare PLCs. On one hand, the cost adds up. On the other, a plant with no spare S7-1500 CPU creates downtime that's five times the part price. I settle this by keeping one spare CPU, one spare power supply, and one spare input module for each critical rack type. But check the spare's firmware revision before you need it. A 'same parts' list with different firmware versions is not a spare.
Before you close the cabinet and hit the main breaker, walk the rail one last time. I skip this when I'm rushing. One time I skipped it and a loose ferrule dropped onto a 24V terminal, shorting a whole sensor loop. $400 in damage and a delayed launch because I didn't look. The fix was 20 seconds of visual review.
Check torque on the main terminals, fully seated push-in wires, no copper whiskers outside terminals, correct polarity on every DC connector, and the CPU mode switch flipped to STOP before download.
The 12-point checklist I created after my third mistake has saved us an estimated $8,000 in potential rework. This seven-step version is the short one. It isn't elegant, and it doesn't make me look clever. It makes me look disciplined, which is better.
Most people don't realize that the 'standard' commissioning schedule already includes buffer time for the site's surprises. Don't spend that buffer on your own panel.
Five minutes of verification beats five days of correction.I've got the invoices to prove it.