30 CFR Part 57Public Domain
§ 57.6406 — Duration of current flow
Subpart E — Explosives
If any part of a blast is connected in parallel and is to be initiated from powerlines or lighting circuits, the time of current flow shall be limited to a maximum of 25 milliseconds. This can be accomplished by incorporating an arcing control device in the blasting circuit or by interrupting the circuit with an explosive device attached to one or both lead lines and initiated by a 25-millisecond delay electric detonator.
Public Domain: 30 CFR Part 57 (MSHA Safety and Health Standards — Underground Metal and Nonmetal Mines) is a U.S. government publication and is not subject to copyright. Text extracted from the official source ↗. Figures are served from the eCFR; if one does not load, use the link in its place.