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1.8 FORCED AIR VENTILATION
Enabling Objective: State the conditions that
require forced air ventilation of a manhole.
If combustible gases or an oxygen deficient
atmosphere is present, the confined space must be ventilated for a minimum of 10 minutes. The Figure shown is used to determine the purging time for vari- ous sized manholes as a function of blower capac- ity. By lining a straight edge between our two plotted points, one on the volume (ft3) scale and the other on the blower capacity scale, the point of
intersection on the time scale denotes the minimum ventilating interval.
If combustable gases or vapors are detected the
local control center must be notified immediately.
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