MSHA - Code of Federal Regulations - 30 CFR 56.6403
30 CFR § 56.6403
(a) If electric blasting includes the use of branch circuits, each branch
shall be equipped with a safety switch or equivalent method to isolate the
circuits to be used.
(b) At least one safety switch or equivalent method of protection shall be
located outside the blast area and shall be in the open position until persons
[58 FR 69596, Dec. 30, 1993]