Commissioning hiring
Interview questions for electrical and mechanical commissioning leads
We use these questions when a resume says the candidate led Level 4 functional testing or Level 5 integrated systems testing (IST). Each question comes with what a strong answer includes and a red flag. Electrical and mechanical commissioning are separate specialties, so use the set for the discipline you are hiring.
For what happens at each level, see Commissioning levels L1 to L5.
By Czarina Tabayoyong, Founder, Critical Path Hiring. Former Amazon, AWS and Google recruiter.
Last reviewed · First published
Reviewed as terms, pay and markets change.
Interview format
Ask the candidate to pick one recent project and keep every answer on that project. Ask for the site size in MW, the equipment makes, who they reported to and which levels they led. Follow-up questions go deeper on the same equipment. A candidate who led the testing can tell you what the script said, what the readings were and what failed. A candidate who watched it will describe the event in general terms.
Electrical commissioning questions
1. Walk me through a UPS functional test you scripted. Which operating modes did you test?
Strong answer: names the UPS make and the topology (for example 2N, N+1 or distributed redundant). Lists the modes: normal, battery, static bypass, maintenance bypass and return to normal. Says what load was on the UPS during the test, what was measured on a power quality meter during each transfer, and whether battery run time matched the design.
Red flag: "We tested the UPS and it passed," with no modes, no load level and no readings.
2. How did you load test the generators, and what did you record?
Strong answer: describes the load steps and how long each was held. Knows that NFPA 110 calls for a two-hour full-load test on site at initial acceptance and explains how they met it. Says whether the load banks were resistive or reactive and why. Recorded voltage and frequency recovery after each load step, plus fuel, coolant and exhaust readings. On paralleled generators, describes how synchronizing and load sharing were tested.
Red flag: doesn't know whether the load banks were resistive or reactive, or what percentage of nameplate the generators reached.
3. Describe the medium-voltage switchgear sequences you tested at Level 4.
Strong answer: explains the sequence in plain terms, such as a main-tie-main transfer when one utility feed is lost, and the interlocks that stop two sources from closing together. Checked that breaker trip settings matched the coordination study before testing. Names who operated the switchgear and who wrote the switching procedure.
Red flag: talks only about energization day and has no example of a sequence tested as a system.
4. Tell me about an ATS or STS transfer test. How did you measure the transfer?
Strong answer: says whether the ATS was open or closed transition and lists the time delay settings tested (engine start, transfer, retransfer, cooldown). Measured transfer on the downstream load with a power quality meter or recorder and compared it with the design. For an STS, explains how the test confirmed the downstream IT load stayed up during transfer.
Red flag: can't say who set the time delays or what they were.
5. How did you test power distribution from the UPS down to the rack?
Strong answer: covers PDUs, RPPs or busway. Checked breaker labels against the one-line diagram. Dropped the A feed and then the B feed to confirm each rack stayed powered from the other side. Did an infrared scan under load. Confirmed branch circuit readings appeared correctly in the EPMS.
Red flag: testing stopped at the UPS output.
6. How were the load banks set up for the data hall test, and how much load did you test with?
Strong answer: gives the number and size of load banks in kW, where they were placed, the percentage of design IT load reached and the steps used to get there. Knows who supplied and cabled them. Explains that the heat from the load banks also tested the cooling. On liquid-cooled halls, describes how load was applied to the liquid loop.
Red flag: "Full load" with no figure, and no idea who supplied the load banks.
7. Who did the NETA acceptance testing, and what did you do with the reports?
Strong answer: names the testing firm and the scope (switchgear, transformers, cables, breakers). Reviewed the reports against ANSI/NETA ATS before signing off Level 3, and logged any failed or missing test as an issue. Holds NETA certification or can explain the difference between the NETA technician's work and the commissioning agent's review.
Red flag: has never read a NETA report, or treats NETA testing and Level 4 functional testing as the same thing.
8. What did your test plan require before anyone worked on or near energized equipment?
Strong answer: the arc flash study done and labels on the equipment before energization. An electrically safe work condition with lockout and tagout for work inside the arc flash boundary. An energized electrical work permit with management approval where energized work could not be avoided, per NFPA 70E. Written switching procedures, qualified persons doing the switching and PPE matched to the label. Explains who was allowed in the room during IST.
Red flag: talks about PPE only, or says safety was the contractor's job.
9. Walk me through the loss-of-utility test at your last IST.
Strong answer: describes the sequence step by step. Utility breaker opened, UPS carried the load on battery, generators started and took load, transfer completed, cooling kept running or restarted, monitoring reported each event, then return to utility. Names the readings taken at each step, who stood where, what the stop criteria were and what failed.
Red flag: describes the test from the control room only and can't say what happened in the electrical rooms.
Mechanical commissioning questions
1. How did you test chiller staging in the chilled water plant?
Strong answer: names the chiller type (air-cooled, water-cooled, magnetic bearing) and the plant arrangement. Describes staging chillers up and down as load changed, lead-lag rotation and what happened when a running chiller tripped. Trended chilled water supply temperature through the test. If the plant had economizer or free cooling, tested the changeover.
Red flag: describes the manufacturer's startup (Level 3) as the test.
2. How did you test the CRAHs or CRACs in the data hall?
Strong answer: explains how fan speed was controlled (differential pressure or supply air temperature). Failed one unit with load banks running and confirmed the others ramped up and rack inlet temperatures stayed within the design range. Checked containment and sensor locations.
Red flag: tested each unit by itself and never tested the units as a group under load.
3. Have you commissioned CDUs and a liquid cooling loop? What happened before the loop was filled with coolant?
Strong answer: describes pressure testing, cleaning, flushing, staged filtration and fluid sampling for conductivity and turbidity, then filling with coolant. Then the CDU functional test: flow, pressure and supply temperature control, pump failover inside the CDU, leak detection alarms and loss of one CDU with racks under load.
Red flag: describes a CDU as a CRAH with water, or has no answer on flushing and fluid quality.
4. How did you test pump redundancy?
Strong answer: describes the pumps and VFDs, the differential pressure setpoint and lead-lag. Failed the lead pump and timed how long the standby took to reach flow. Compared measured flow with design.
Red flag: confirmed each pump starts and stops, with no failover test.
5. What did you test on the cooling towers and condenser water side?
Strong answer: fan staging, isolation valves, freeze protection and basin heaters, water treatment and loss of makeup water. If the site used air-cooled chillers or dry coolers, the candidate says so and describes those tests.
Red flag: generic answer that doesn't match the plant on their own resume.
6. Pick one sequence of operation you tested. Read it back to me and tell me how your script tested it.
Strong answer: states the sequence and its setpoints from memory or close to it. Explains which steps were tested by causing a real failure in the field and which by forcing a point in the BMS, and why. Used trend logs as the record. Checked that BMS graphics matched field devices.
Red flag: every test was done by overriding points in software.
7. During IST, how did you confirm the BMS and EPMS reported events correctly?
Strong answer: compared the alarm list and time-stamped event logs with what the team saw happen on the floor. Checked that BMS and EPMS clocks were in sync. Logged missing or wrong alarms as issues. Put the trend data in the turnover package.
Red flag: assumed the controls contractor checked it.
8. Which mechanical failure scenarios did you script for IST?
Strong answer: a list that matches the design, such as chiller trip, pump failure, loss of a CRAH or CDU, loss of a BMS controller or network switch, valves failing to their safe position and loss of power to the mechanical plant. For each, what was measured and the pass criteria.
Red flag: the only failure scenario they can name is the loss-of-utility test.
9. When utility power dropped during IST, what happened to cooling, and how long did it take to recover?
Strong answer: knows which mechanical loads are on UPS (for example pumps, CRAH fans or CDUs) and which wait for the generators. Measured chiller restart time, rack inlet temperature rise and any thermal storage or buffer tank ride-through.
Red flag: doesn't know which mechanical equipment is UPS-backed.
Shared questions
1. Who owned the issues log on your last project, and how did items get closed?
Strong answer: names the tool, the fields tracked, who could close an item and how retests were witnessed. Can say roughly how many items were open at turnover and what happened to them.
Red flag: the general contractor kept the log and the candidate added items to it.
2. Which scripts did you write from scratch, and which did you adapt?
Strong answer: wrote scripts from the sequences of operation, the owner's project requirements and the design documents, then revised them after owner and engineer review. Can name the systems.
Red flag: ran scripts other people wrote on every project.
3. Tell me about an IST that failed. What changed afterward?
Strong answer: names the failure, who decided to stop or continue, the root cause, the fix, the retest date and the change to the script or design.
Red flag: blames the contractors and can't name anything that changed.
4. Have you refused to sign off a test? What happened next?
Strong answer: a specific case, who pushed back and how it was settled.
Red flag: has never refused, or doesn't know who had the authority to stop turnover.
Verification
For each project on the resume, ask what the candidate did at each level: witnessed, executed, wrote the script, directed the test or signed off. Write it down per project. A resume that lists IST on every project usually includes some the candidate only witnessed.
Ask for a redacted script or a template they authored. Many owners don't allow scripts to leave the project, so if the candidate can't share one, ask them to walk through its structure: steps, expected results, pass criteria and sign-off.
Ask for the owner's representative or the commissioning manager on the project. Ask the reference who wrote the scripts and who directed the IST.
Check that project dates line up with the IST windows the candidate describes. Overlapping projects can mean the candidate was assigned to more than one site and present at only part of the testing.
CxA, CCP and NETA certification help. They show the candidate completed training and passed an exam.
Frequently asked questions
- Should electrical and mechanical commissioning candidates get the same interview?
- No. The equipment is different, so use the electrical or mechanical set, whichever the project needs. A commissioning manager who oversees both should be able to answer the shared questions and talk through at least one IST from start to finish.
- Who should run the technical interview?
- Someone who has led IST, such as your senior commissioning lead or the owner's engineer. If no one on your team has, ask your commissioning provider to join the interview. When we screen candidates, we check the levels they led, the scripts they wrote and the systems they tested through Level 4 and Level 5.
- Can candidates from pharma, semiconductor or Navy nuclear backgrounds pass these questions?
- Many can answer the electrical and mechanical testing questions well, because those industries also test to written scripts on critical systems. Integrated testing on a live-load data center is the hardest part to substitute, so spend more time on the IST and failure scenario questions with these candidates.
Czarina Tabayoyong
Founder and principal recruiter, Critical Path Hiring. Czarina recruits commissioning, MEP, power and critical facilities people for data center builds. She previously recruited cloud and data center architecture talent at AWS Professional Services, and PMO and project delivery talent for hyperscale data center campuses in Texas and Virginia.
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