30 CFR § 27.6
Certification of components.
In accordance with §27.4
, manufacturers of components
may apply to MSHA to issue a letter of certification. To qualify for
certification, electrical components shall conform to the prescribed
inspection and test requirements and the construction thereof shall be
adequately covered by specifications officially recorded and filed with MSHA.
Letters of certification may be cited to fabricators of equipment intended
for use in a certified methane-monitoring system as evidence that further
inspection and test of the components will not be required.