Testing Before Entering Underground Structures
The structure should be tested to ensure that the atmosphere is safe for
workers. Entering a manhole with an oxygen deficiency can cause
sudden unconsciousness and death by hypoxia (blood starvation).
Manholes containing less than 19.5 percent oxygen are not to be entered
without a supplemental oxygen supply.
188.8.131.52 Hazardous Conditions in Underground Structures
Toxic or combustible gases may be present or there may be a lack of
oxygen in unventilated subsurface structures.
a. Toxic or combustible gases. Since subsurface structures are subject to the
accumulation of combustible or toxic gases, they should be considered hazardous until proven
clear by test. Combustible gases found in manholes or vaults are usually natural gas or
hydrocarbon fuels. Toxic gases usually encountered are hydrogen sulfide, carbon dioxide, or
mangrove gas (southern coastal areas).
b. Lack of oxygen. No one is permitted in unvented vaults or manholes unless forced
ventilation is provided or the atmosphere is found to be safe by testing for both oxygen deficiency
and the presence of explosive gases or fumes. Provisions should be made for a continuous supply
of air when necessary.
184.108.40.206 Testers for Underground Structures
The manhole or unvented vault should be tested in accordance with Navy
or local confined space entry program requirements. Testing should be
made only by trained technicians with approved testers prior to entering,
after the technician first determines that the instrument is in proper
working order and correctly calibrated. These tests will be made as soon
as the manhole cover is removed.