Commissioning_
The Power-On Walkdown: A Step-by-Step Energization Procedure
This article provides the exact sequence of grounding, torque, and protection checks performed on GPU racks before first energization, including the specific order that prevents arc-flash and thermal events during commissioning.
Key facts
- All torque values are taken exclusively from the OEM documentation for the installed busbar and lug hardware.
- Grounding continuity is verified with a calibrated low-resistance ohmmeter before any protective device is armed.
- NFPA 70E requires arc-flash PPE and a documented LOTO procedure before any rack power compartment is opened.
- Busbar connections must be re-torqued only after thermal cycling if the OEM installation guide specifies it.
- Protective relay settings are confirmed against the coordination study before the main breaker is closed.
- A separate domain exists between internal copper NVLink and the fiber scale-out network; power walkdowns never reference MPO links.
- Leviathan Systems records every torque reading and continuity value with timestamped photos for traceability.
Pre-Energization Documentation and LOTO
Review the single-line diagram, coordination study, and OEM rack power drawings against the as-built installation. Confirm that every feeder, breaker, and interlock listed on the drawings matches the hardware installed in the rack.
Establish LOTO with the site safety officer and apply personal locks to all upstream disconnects. Verify zero energy state at the rack main lugs with a calibrated voltage tester rated for the available fault current.
Confirm that all personnel assigned to the walkdown have current NFPA 70E training and are wearing the required arc-rated PPE for the calculated incident energy at the rack.
Visual and Mechanical Inspection of Power Path
Open each power compartment door only after LOTO is confirmed and scan every busbar, lug, and cable termination for proper bend radius, support, and absence of foreign material. Check that phase barriers and insulation boots are fully seated and undamaged.
Verify that all rack PDUs and power shelves are seated in their guides and that retaining hardware is installed. Note any shipping brackets or temporary jumpers that must be removed before energization.
Inspect the rack frame bonding straps and confirm they remain connected to the building grounding electrode system without paint or corrosion at contact points.
Grounding Continuity and Bonding Verification
Measure resistance from each rack ground bar to the nearest building grounding electrode using a low-resistance ohmmeter. Record values and compare against the project grounding specification; any reading above the acceptable threshold requires investigation before proceeding.
Test every equipment bonding jumper and PDU chassis ground with the same instrument. Confirm that the rack frame, PDU enclosures, and cable tray sections form a continuous low-impedance path.
Perform a point-to-point continuity check between the rack ground bar and each power shelf chassis ground terminal. This step catches missing or reversed bonding straps that visual inspection can miss.
Torque Verification on All Power Connections
Use a calibrated torque wrench set to the OEM value for each fastener size and material. Start at the main lugs and work downstream through every busbar joint, PDU input, and shelf power connector. Never rely on visual marks alone.
Re-check any connection that was disturbed during the visual inspection. Document the final torque value, wrench serial number, and technician initials on the walkdown form.
If the OEM specifies a re-torque after initial heat-up, note that requirement on the commissioning checklist so it can be scheduled after first power application.
Protective Device and Interlock Checks
Confirm that every breaker and fuse rating matches the coordination study. Verify that electronic trip units are programmed with the correct long-time, short-time, and instantaneous settings before the main breaker is closed.
Test each mechanical interlock that prevents energization with doors open or shelves removed. Confirm that rack EPO circuits and ground-fault relays are wired and functional per the drawings.
Walk the rack perimeter and verify that no temporary jumpers or test leads remain across protection devices. This final sweep prevents inadvertent bypasses that would defeat the coordination study.
Common Failure Modes Encountered in the Field
The most frequent cause of delayed energization is a missed torque value on a single busbar joint; vibration during transport loosens the connection and creates a high-resistance point that fails under load. Catch it by requiring every fastener to be checked with a calibrated wrench rather than relying on installation crew marks.
Ground faults from paint under bonding lugs or reversed neutral-ground bonds appear after the first attempt to close the main breaker. These are found by completing the low-resistance ohmmeter sweep before any protective device is armed.
Interlock failures occur when a shelf is not fully seated; the mechanical plunger does not engage and the control circuit remains open. Physical manipulation of each shelf after the visual inspection reveals these issues before power is applied.
Sequential Energization and Immediate Post-Power Checks
Close the main breaker only after the signed walkdown form is reviewed by the site engineer. Monitor phase currents and voltages at the rack main lugs for thirty seconds before proceeding to individual PDUs.
Energize one power shelf at a time while watching for unexpected current draw or ground-fault indications. Record voltage at the shelf input terminals to confirm drop is within acceptable limits.
After all shelves are online, perform a thermal scan of every connection that was torqued during the walkdown. Any hot spot above ambient requires immediate de-energization and re-torque.
Standards referenced: NFPA 70E · NEC Article 250 · IEEE 142
Frequently asked_
What instrument is required for the grounding continuity checks?
A calibrated low-resistance ohmmeter capable of four-wire measurement is used. The instrument must be verified against a known low-resistance standard on the day of the walkdown. Clamp-on ground testers are not accepted for rack-to-electrode measurements because they cannot isolate parallel paths inside the rack.
When is the torque wrench calibration verified?
The wrench certificate is checked at the start of the shift and the serial number is recorded on every torque log entry. If the wrench has not been calibrated within the interval required by the project quality plan, a spare calibrated unit is substituted before any fasteners are touched.
Who signs off before the main breaker is closed?
The Leviathan Systems lead technician, the site electrical engineer, and the safety officer must all sign the completed walkdown form. No single signature is sufficient; the form is scanned and attached to the commissioning package before the rack is released for load.
How are NVLink connections treated during the power walkdown?
Internal copper NVLink links are inspected only for mechanical seating and are never part of the electrical torque or grounding checks. Power walkdown stops at the shelf power connectors; NVLink status is confirmed later through the GPU management interface after the rack is fully energized.
What happens if a torque value is missing from the OEM documentation?
Work stops and a formal RFI is issued to the OEM. No assumed torque value is applied; the connection remains un-torqued until the correct specification is received and the change is approved by the project engineer.