Exponent

Stories

ISO adds Class L fire rating for lithium-ion as New York enforces 1 kWh storage rule

Published guidance interpreting ISO Class L for US stakeholders

Lithium-ion battery fires now have their own fire class, after ISO published the third edition of ISO 3941 in January 2026 adding Class L for cells and batteries that contain no metallic lithium. New York City had already set a lower regulatory bar: its amended NFPA 855 requires FDNY approval for lithium-ion storage above 1 kilowatt-hour, against 70 kilowatt-hours for lead-acid.

Updated Oct 21

San Francisco went dark—and the same substation has burned before

Built World

As of May 2026: Conducting independent investigation of Mission substation fire for PG&E

San Francisco is the kind of city that feels unstoppable—until the lights go out. On Saturday, a substation fire near 8th and Mission helped trigger a blackout that spread across neighborhoods and knocked out traffic signals. Restoration took more than two days and sparked political backlash and raised questions about whether a modern city can tolerate single-point failures in critical infrastructure.

Updated May 16