Find every source before you hang a lock.
The most dangerous assumption in critical facilities work is that opening the upstream breaker de-energizes the equipment. On a redundant load it does not. Work the list.
§ Source checklist
§ Why the breaker lies
A data center is designed so that no single failure drops load. Every one of those design features is a path that can deliver energy to the equipment you are working on: two utility services, generators with automatic start, N+1 or 2N UPS, static and maintenance bypass, dual-corded IT loads, and busway fed from both ends.
The as-built drawings are usually one change order behind. Verify the one-line against the field before you trust it — particularly on any facility that has been through a capacity expansion.
A lock on the A side and an assumption about the B side. In incident reviews this appears as: the technician verified at the disconnect rather than at the point of work, and the B feed was never in the procedure because the procedure predates the second feed. This is exactly what the annual procedure audit under 110.3(L)(4) exists to catch.
§ Working the list
Start at the load, not the breaker. Identify the equipment, then trace every path backwards.
Defeat automatic operation. Generator auto-start, automatic transfer schemes and automatic bus ties will close on you.
Account for control power separately. It is frequently fed from a different source than the equipment it controls.
Treat stored energy as a source. DC bus capacitors and battery strings do not care that the AC is off.
Verify at the point of work. Not at any disconnect. This is the step that makes an ESWC real.
Isolation against real topology
Both classes work the ESWC process against dual feeds, UPS bypass and the battery that stays live regardless.
See the schedule Bring it onsite