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Subchapter 20. Tunnel Safety Orders
Article 8. Tunnel Classifications
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§8425. Operation of Gassy and Extrahazardous Tunnels.

(a) (1) Before any electrical equipment or services are installed or used in places classified as Gassy or Extrahazardous, they shall be permissible, approved, or in accordance with Title 8, Electrical Safety Orders and acceptable to the Division.

EXCEPTION: In tunnels where the classification is based on toxic gas(es) which does not present a fire or explosive hazard, the provisions which address a source of ignition shall not be applied.

(2) Before any internal combustion engine is permitted to enter any place classified as Gassy or Extrahazadous, the internal combustion engine shall be of an approved, permissible safe design acceptable to the Division.

(b) Smoking shall be prohibited and the employer shall be responsible for collecting all personal sources of ignition such as matches, lighters, cameras and radios from all persons entering the tunnel.

(c) Welding, cutting, or other hot work and/or spark producing operations shall not be permitted while a probe hole is being drilled or when the tunnel face is being excavated, and shall only be done in atmospheres containing less than 10 percent LEL and under the direct supervision of qualified persons. Tests for gas and vapors shall be made before the start and continuously during such operations.

(d) Tests for flammable gas shall be conducted in the return air and measured not less than a distance of 12 inches from any surface in any open workings.

(e) Tests for flammable petroleum vapors shall be conducted in the return air and measured at a distance not less than 3 inches from any surface in any open workings.

(f) Whenever gas levels in excess of 10 percent of the LEL are encountered, the Division shall be notified immediately. Any work therein shall be conducted with extra care and steps shall be taken to increase ventilation.

(g) A fixed system of continual automatic monitoring equipment shall be provided for the heading, muck handling, transfer points and return air of tunnels using mechanical excavators. The monitors shall have sensors so situated that they will detect any anticipated gas encountered and shall signal the heading, give visual and audible warning and shut down electric power in the tunnel, except for acceptable ventilation and pumping equipment necessary to evacuate personnel, when 20 percent or more of LEL is encountered. In addition, a manual shut down control shall be provided near the heading.

(h) In tunnels driven by conventional drill and blast methods, the air shall be tested for gas prior to re-entry after blasting and continuously when employees are working underground.

(i) The main ventilation systems shall exhaust flammable gas or vapors from the tunnel, shall be provided with explosion relief mechanisms, and shall be constructed of fire-resistant materials.

(1) In any tunnel classified Extrahazardous, the main ventilation system shall contain a cutoff switch capable of stopping all electrical machinery underground automatically should the fan fail or its performance fall below minimum power needed to maintain a safe atmosphere.

(j) A refuge chamber or alternate escape route shall be maintained within 5,000 feet of the face of a tunnel classified as gassy or extra-hazardous. Workers shall be provided with emergency rescue equipment and trained in its use. Refuge chambers shall be equipped with a compressed air supply, a telephone, and means of isolating the chamber from the tunnel atmosphere. The emergency equipment, air supply, and rescue chamber installation shall be acceptable to the Division.

(k) At a tunnel classified as Gassy or Extrahazardous, the Division shall permit the tunnel to operate up to but not exceeding 20 percent of the LEL without further notification if the required precautionary measures are in effect and permission is given in writing.

NOTE: Authority cited: Sections 142.3 and 7997, Labor Code. Reference: Sections 142.3 and 7997, Labor Code.


1. Amendment of section and new Note filed 3-5-96; operative 4-4-96 (Register 96, No. 10).

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