UFC 3-560-01
6 December 2006
Including change 1, 7 December 2006
Table 11-1. Precautions Before Entering Underground Structures
1. Secure the work area.
2. Use the probes of an atmospheric tester to check air. First check for moisture on
the end of the probe. If none found, test for air.
3. If air is acceptable, remove manhole cover.
4. Inspect the structure interior for the presence of water, oil, gasoline, or other
contaminants that must be removed before work can proceed. Check the
structure by lowering the probe as far as possible.
5. If air is acceptable, set up the lifting tripod on the manhole.
6. Enter (and leave) the structure by means of a ladder or climbing device for
structures more than 4 ft (1.2 m) deep. Never climb in or out by stepping on
cables or their supports.
7. Test the air at each corner of the structure and at each of the duct entrances.
8. Provide additional ventilation as necessary. Continuous monitoring of air quality
should be performed for work taking longer than 15 minutes. Whenever the cover
has been replaced, repeat air testing.
9. Lower tools using a handline. Use the tripod for heavy tools and parts, or when a
manhole rescue must be made.
11-6.2
Atmosphere Testing Before Entering Underground Structures.
11-6.2.1 Test the atmosphere in the underground structure prior to entry to ensure it is
safe for workers. Toxic or combustible gases can be present, or there might be a lack
of oxygen in underground structures. No one is permitted in unvented underground
vaults, manholes, or other structures unless the atmosphere is found to be safe by
testing for both oxygen deficiency and the presence of toxic or explosive gases or
fumes.
11-6.2.2 Manholes and other underground structures containing less than 19.5 percent
by volume oxygen must not be entered without the use of supplemental breathing air.
Entering a manhole or other underground structure with an oxygen deficiency can
cause sudden unconsciousness and death by hypoxia (oxygen starvation).
11-6.2.3 The following applies to toxic and combustible gases.
11-6.2.3.1 Consider subsurface structures hazardous until tested clear. They are
subject to the accumulation of combustible or toxic gases.
11-6.2.3.2 Combustible gases found in manholes or vaults are usually natural gas or
vapors from hydrocarbon fuels.
11-5
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