Section 75.340 - Underground electrical installations(a) Underground transformer stations, battery charging stations, substations, rectifiers, and water pumps shall be housed in noncombustible structures or areas or be equipped with a fire suppression system meeting the requirements of § 75.1107-3 through § 75.1107-16 . (1) When a noncombustible structure or area is used, these installations shall be- (i) Ventilated with intake air that is coursed into a return air course or to the surface and that is not used to ventilate working places; or(ii) Ventilated with intake air that is monitored for carbon monoxide or smoke by an AMS installed and operated according to § 75.351 . Monitoring of intake air ventilating battery charging stations shall be done with sensors not affected by hydrogen; or(iii) Ventilated with intake air and equipped with sensors to monitor for heat and for carbon monoxide or smoke. Monitoring of intake air ventilating battery charging stations shall be done with sensors not affected by hydrogen. The sensors shall deenergize power to the installation, activate a visual and audible alarm located outside of and on the intake side of the enclosure, and activate doors that will automatically close when either of the following occurs: (A) The temperature in the noncombustible structure reaches 165 °F; or(B) The carbon monoxide concentration reaches 10 parts per million above the ambient level for the area, or the optical density of smoke reaches 0.022 per meter. At least every 31 days, sensors installed to monitor for carbon monoxide shall be calibrated with a known concentration of carbon monoxide and air sufficient to activate the closing door, or each smoke sensor shall be tested to determine that it functions correctly.(2) When a fire suppression system is used, these installations shall be-(i) Ventilated with intake air that is coursed into a return air course or to the surface and that is not used to ventilate working places; or(ii) Ventilated with intake air that is monitored for carbon monoxide or smoke by an AMS installed and operated according to § 75.351 . Monitoring of intake air ventilating battery charging stations shall be done with sensors not affected by hydrogen.(b) This section does not apply to-(1) Rectifiers and power centers with transformers that are either dry-type or contain nonflammable liquid, if they are located at or near the section and are moved as the working section advances or retreats;(3) Permissible pumps and associated permissible switchgear;(4) Pumps located on or near the section and that are moved as the working section advances or retreats;(5) Pumps installed in anthracite mines; and(6) Small portable pumps.