99. Suspension of work during thunderstorms. - Whenever a thunderstorm appears to be imminent in the vicinity of the process building, every person engaged in or about the building shall be withdrawn to a safe distance and the building shall be kept closed and locked until the thunderstorm has ceased or the threat of the same has passed:
Provided that where an operation is in process, stopping of which would in itself contribute a danger in which case the operation shall be carried on to point at which it can be suspended with safety, and no such operation shall be commenced during the storm.
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