(a)Energy output. Blasting units shall meet the acceptable
performance criteria of the output energy test in §7.66.
(b)Maximum blasting circuit resistance. The maximum value of
the resistance of the blasting circuit that can be connected to the firing
line terminals of the blasting unit, without exceeding its capacity, shall be
specified by the applicant. The specified maximum blasting circuit
resistance shall be at least 150 ohms.
(c)Visual indicator. The blasting unit shall provide a
visual indication to the user prior to the operation of the firing switch
when the voltage necessary to produce the required firing current is
(d)Firing switch. The switch used to initiate the
application of energy to the blasting circuit shall--
(1) Require deliberate action for its operation to prevent
accidental firing; and
(2) Operate only when the voltage necessary to produce the
required firing current is available to the blasting circuit.
(e)Firing line terminals. The terminals used to connect the
blasting circuit to the blasting unit shall--
(1) Provide a secure, low-resistance connection to the
blasting circuit as demonstrated by the firing line terminals test in
(2) Be corrosion-resistant;
(3) Be insulated to protect the user from electrical
(4) Be separated from each other by an insulated
(f)Ratings of electric components. No electric component of
the blasting unit, other than batteries, shall be operated at more than 90
percent of any of its electrical ratings in the normal operation of the
(g)Non-incendive electric contacts. In the normal operation
of a blasting unit, the electric energy discharged by making and breaking
electric contacts shall not be capable of igniting a methane-air atmosphere,
as determined by the following:
(1) The electric current through an electric contact shall
not be greater than that determined from Figure D-1.
(2) The maximum voltage that can be applied across an
electric contact that discharges a capacitor shall not be greater than that
determined from Figure D-2.
(3) The electric current through an electric contact that
interrupts a circuit containing inductive components shall not be greater
than that determined from Figure D-3. Inductive components include inductors,
chokes, relay coils, motors, transformers, and similar electric components
that have an inductance greater than 100 microhenries. No inductive component
in a circuit with making and breaking electric contacts shall have an
inductance value greater than 100 millihenries.
U.S. Department of Labor | Mine Safety and Health Administration (MSHA) | 1100 Wilson Boulevard, 21st Floor | Arlington, VA 22209-3939 www.msha.gov | Telephone: (202) 693-9400 | Fax-on-demand: (202) 693-9401