Battery rooms.
The battery room breaks the habit every electrician brings to it: the source cannot be de-energized. Article 360 was rewritten around that fact, and its arithmetic — including one divisor the industry gets wrong — decides everything else.
Four thresholds, and one formula with a four in it
360.3(A) sets the exposure levels: ac at 50 V and 5 mA, dc at 100 V and 40 mA, contact thermal at 1000 W of short-circuit power, and an arc flash risk assessment required for battery systems at 150 V or more before any interaction likely to create an arc.
The contact-thermal number comes with its own arithmetic, from Informational Note No. 1: available short-circuit power is nominal voltage times available short-circuit current, divided by four. A 480 V string capable of 10 kA is 1.2 MW by that formula — three orders of magnitude over the threshold. Even a 48 V telecom string at 2 kA computes to 24 kW. Half the industry divides by two, doubling every reported figure; the divisor is four, and our battery threshold tool quotes the note so the calculation is defensible in an audit.
The millimetre-per-volt line
Table 130.5(C)(3) gives battery work its most useful likelihood call: maintenance on battery equipment below 600 V where the exposed conductor separation exceeds 1 mm per volt is a No for arc flash likelihood. The same table splits intercell connector covers by material — removing nonconductive covers is a No; inserting or removing covers for the connectors themselves sits with the Yes tasks. A 480 V string needs about half a metre of separation to earn the No; a 48 V block needs 48 mm. Measure it once and the PPE conversation for routine inspection settles itself.
Sectionalizing: the lockout that isn't one
360.4(E) is explicit that the safe-condition procedure shall not de-energize the batteries, shall not release stored energy, and shall not require charging or discharging to get there. What it builds instead is sectionalizing — breaking the string into voltage blocks below the thresholds. The ESWC sequence for batteries runs through determining the series/parallel configuration, isolating the bank from all loads and sources including chargers, verifying disconnects visually where possible, locking out the charger disconnecting means, and sectionalizing per a documented procedure — testing absence of voltage with an adequately rated portable meter where 360.4(E)(2) requires it.
A complex sectionalizing plan must identify the exposed persons and their PPE, the person in charge, the order of connections, and the resulting changes in dc voltage, short-circuit power, boundaries and incident energy at every stage. That last clause is the one to respect: every intermediate state of the plan has its own hazard numbers.
The dc boundary table is gentler — until it isn't
Table 130.4(E)(b): from 50 to 300 V dc the restricted approach boundary is simply avoid contact; from 301 V to 1 kV it is 1 ft 0 in., with the limited approach boundary at 3 ft 6 in. for fixed parts throughout. The trap is reading 'avoid contact' as 'no hazard' — the shock boundary being small says nothing about the 24 kW of contact-thermal capability sitting behind it. A wrench across a 48 V bus never crossed anyone's restricted boundary. It still removes fingers.
Rings and watches in the battery room, years after the last shock-based rule would have caught anyone. The killer at 48 V is not shock — it is contact thermal, a threshold with no voltage term. The room's PPE sign lists gloves rated in volts; it should also list what comes off your hands before you enter.
Battery task in front of you? The thresholds and sectionalizing questions are built into the battery tool and the walk — run both.
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.