DC power plants.
The −48 V plant is where 'low voltage' does its most dangerous work as a phrase. Below the 50 V scope line, mostly beneath the shock thresholds — and behind it, short-circuit power measured in tens of kilowatts against a burn threshold of one.
Exception No. 5 has a test, not a blanket
Below 50 volts, Exception No. 5 relieves the de-energizing obligation — but only where the source capacity and overcurrent protection have been considered and it is determined there is no increased exposure to electrical burns or to explosion due to electric arcs. That determination is the whole exception. A −48 V plant bus fed by paralleled rectifiers and a battery string is capacity by design; run the arithmetic before claiming the relief.
The arithmetic is short. Short-circuit power is voltage times available short-circuit current divided by four: a 48 V plant capable of 5 kA computes to 60 kW against a 1000 W contact-thermal threshold. The burn-exposure determination fails by a factor of sixty, and the exception was never available. What Exception No. 5 legitimately covers in a data center is the genuinely limited stuff — control circuits and sources whose capacity actually was considered and actually is small.
The threshold with no voltage term
Of 310.2's three dc thresholds, contact thermal — 1000 W — is the one that ignores voltage entirely. Shock needs 100 V and 40 mA together; arc flash needs over 150 V; a −48 V plant can clear both hurdles and still hold metal-vaporising energy behind every bus bar. This is the entire safety story of dc plants: the hazard that remains when shock is off the table. It is also why the 2027 edition's thermal hand protection requirement at 130.7(C)(7)(e) — worn wherever exposure to a contact thermal hazard is possible — reads like it was written in a telecom plant. A hand across −48 V bus will not feel a shock. It will feel the wrench welding.
The working rules follow directly: insulated tools rated for the work, nothing conductive on hands or wrists, covered bus wherever the design allows, and torque sequences planned so the tool is never bridging.
Plant work is battery work
The plant's string answers to Article 360 — the same sectionalizing logic, charger lockout, and complex-plan requirements as the big rooms, scaled down. And the same relief: battery maintenance below 600 V where conductor separation exceeds 1 mm per volt is a No on the likelihood table, which a 48 V plant meets with 48 mm of separation. Most tidy telecom plants qualify; most retrofitted, cable-crowded ones are the exception that proves the measurement is worth taking.
A plant expansion where the new rectifier shelf doubled available short-circuit current and nobody re-ran the 1000 W math, because '48 volts is 48 volts.' Capacity is the variable that moves. The voltage never changing is exactly what lulls the re-check out of the procedure.
Claiming the under-50 V relief? The walk asks Exception No. 5's determination question directly. Answer it with the ÷4 number in hand.
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.