Battery rooms.
The densest concentration of hazard categories in the building — chemical, thermal, shock, arc flash, and for lithium, fire and deflagration. Four of those five live in one NFPA 70E article. The fifth is a different standard entirely.
A 48 V string is below the DC shock threshold and can still cause a severe burn. Contact thermal hazard is triggered above 1,000 W of short-circuit power, and short-circuit power is (voltage × short-circuit current) ÷ 2. Almost every stationary string in a data center exceeds it. Check yours →
§ Four hazard categories — Article 360
Chemical. Sulfuric acid in lead-acid, potassium hydroxide in nickel-cadmium. Review the SDS. Sealed batteries may limit exposure unless damaged.
Contact thermal. Burns from current heating through a dropped tool, a ring or a watch band. Threshold 1,000 W. Thermal gloves, not rubber gloves.
Electric shock. 100 V and 40 mA DC; 50 V and 5 mA AC.
Arc flash. 150 V DC and 1.2 cal/cm², with the 1 mm-per-volt separation guidance for likelihood.
§ Access and PPE
- Authorized personnel only. Article 360 requires a notice prohibiting access by unauthorized personnel on the signage.
- Signage must now carry a thermal hazard warning in addition to shock and arc flash — new in the 2027 edition. Plus chemical warnings and a PPE notice.
- PPE depends on which categories are triggered — potentially rubber insulating gloves and thermal gloves and chemical splash protection and arc-rated clothing, simultaneously.
- Non-conductive tools, and no jewellery, watches or metal-frame glasses.
- Never work alone where a permit specifying PPE is required — 130.2(A)(2).
§ The room itself
| Requirement | Source | Note |
|---|---|---|
| Emergency eyewash and shower | OSHA 1910.151(c) | Within 10 seconds of travel where corrosives are present. Flushed weekly, tested annually |
| Ventilation | NEC Art. 480, building code | Prevent accumulation of an explosive hydrogen mixture |
| Working space | NEC 110.26 | Battery racks encroach constantly |
| Spill containment and neutralization | Building and fire code | Flooded cells |
| Disconnecting means | NEC Art. 480 | Within sight of the battery system |
| Thermal runaway protection | NFPA 855 | Lithium — spacing, venting, gas detection, suppression |
| Warning signage | 70E Art. 360 | Electrical (incl. thermal), chemical, PPE and access notice |
§ What goes wrong
Signage printed once at commissioning and never revisited — so it carries shock and arc flash and no thermal warning, and where the room has been re-populated with a second chemistry, the chemical warning reflects the wrong worst case.
- Nitrile gloves on a 48 V string because "it's only 48 volts." The voltage is irrelevant to the thermal hazard.
- Eyewash present but never flushed, or blocked by a battery cabinet.
- The battery omitted from UPS isolation procedures — see UPS rooms.
- Mixed chemistries in one room without the assessment being redone.
NFPA 855 governs fire. NFPA 70E governs shock, arc flash and thermal. Neither lets you turn the battery off. A DC string is a stored-energy hazard that persists with every breaker in the building open — which is why the room is treated as permanently energized and access-controlled rather than as something you lock out.
Article 360 in the 16-hour class
Four categories, thresholds, signage and the NFPA 855 boundary against real UPS and BESS configurations.
See the 16-hour class Bring it onsite