UPS systems.
The UPS is three hazard regimes in one enclosure — an ac article, a dc article, and a stored-energy article — governed in part by a chapter heading almost nobody expects: Electrical Hazards 1 Hz to 110 MHz.
The article nobody expects: 330
The 2027 restructuring of Chapter 3 put equipment containing rectifiers and inverters — UPS systems by name — under Article 330, the frequency-hazards article. That is not trivia. 330.5 imposes two requirements that bite in a UPS room: meters used for safety purposes, including zero-energy verification, must be rated for the frequency being measured, and dielectric PPE derates above 100 kHz. A technician verifying a dc link or an inverter output with a meter rated only for 50/60 Hz service is running the verification step of Article 120 on an instrument the standard says is the wrong one.
The practical translation: the meter roster for the UPS room is a procurement decision with a section number behind it.
Three regimes, one enclosure
Upstream and downstream of the conversion, the UPS is a 480 V ac machine — the boundaries and table rows of the switchgear pages apply. The dc link and battery bus answer to 310.2's three thresholds: contact thermal at 1000 W (a test with no voltage term at all), shock at 100 V dc and 40 mA together, arc flash at over 150 V dc with over 1.2 cal/cm². And the rectifier and inverter sections hold capacitor banks whose stored energy falls under Article 320 — where the shock threshold is as low as 1.0 J at 100 V, and hearing protection is triggered above 100 J regardless of voltage.
A UPS lockout that addresses only the ac sources has handled one regime of three.
Maintenance bypass is a routing decision, not a safety state
Putting the UPS in maintenance bypass moves the load. It does not discharge the dc link capacitors, it does not open the battery, and depending on topology it may leave the rectifier input live. The electrically safe work condition still requires every source opened — rectifier input, bypass input, battery disconnect, output — plus stored energy released and absence of voltage verified at the point of work, after the manufacturer's stated discharge time has actually elapsed. 'We're in bypass' is the beginning of the lockout conversation, not the end of it.
The battery side of that isolation belongs to Article 360 and its own sectionalizing rules — covered on the battery-room page of this series.
A UPS module opened for fan replacement forty seconds after transfer to bypass, dc link still holding hundreds of volts, because the procedure said 'bypass' where it needed to say 'bypass, isolate, discharge, verify.' The fan took ten minutes. The verification would have taken two.
UPS work order in hand? The walk's source and stored-energy questions do the heavy lifting here — run it before the covers come off.
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.