ATS and static transfer switches.
A transfer switch exists to guarantee that killing one source does not kill the load — which means it exists to defeat the first instinct of every lockout ever written. Two sources is the design intent. It is also the hazard.
The two-source problem, stated plainly
An electrically safe work condition at a transfer switch means proving both power sources open, control power dead, and — on a static switch — internal stored energy released, before the first cover comes off. Article 120's sequence does not care that source two 'should' be in the off position; it requires disconnection of all sources and verification of absence of voltage at the point of work. Transfer gear is where technicians meet a live terminal after a 'complete' lockout more than anywhere else we teach, because the second source path runs through logic they did not draw.
Treat the bypass-isolation handles as what they are: disconnecting means operated under Exception No. 2 to reach the safe condition — with the risk assessment that exception requires — not as maintenance conveniences outside the standard.
The closed-door trigger was written for this equipment
110.2(B)(2) fires when an employee interacts with equipment whose conductors are not exposed but where the likelihood of arc flash injury is elevated. A transfer switch mid-retransfer, a switch with a failed controller being manually operated, a unit whose last exercise cycle ended in an alarm — that is interaction with equipment in an abnormal state, doors closed or not. The likelihood table's split is exact: operating a switch under normal conditions is a No; the same handle after an abnormal event, or the first operation after service, is a Yes.
The seven conditions of a normal operating condition decide which side of that line you are on, and condition of maintenance is the one a transfer switch fails silently — contacts exercised monthly but never inspected, control batteries past date, an ATS that has never once been maintained because it could never be scheduled out.
Inside a static switch, the hazard outlasts the sources
An STS is power electronics: SCR stacks, snubbers, and dc-side capacitance that holds energy after both sources are open. Article 320's stored-energy thresholds start at fractions of a joule at elevated voltage, and the release of stored energy is an explicit step in establishing the safe work condition under Article 120. The absence-of-voltage test on an STS is not one measurement — it is both source buses, the output, and the internal caps, with a meter rated for the circuit, after the manufacturer's discharge interval has actually elapsed.
A pristine dual-source lockout — both breakers, both locks, both tags — and control power still up because it feeds from a third circuit nobody documented. The absence-of-voltage test caught it; the paperwork never would have. The finding is not the third circuit. It is that the one-line stopped being true in 2019.
Working an ATS? The walk's source-isolation and switching-step questions are the whole job here. Run it with the one-line open.
This is the material of the 8- and 16-hour NFPA 70E classes — taught on the 2027 edition, against data center topology, about 55 times a year. Private onsite days run against your one-lines.