UFC 3-560-01
6 December 2006
Including change 1, 7 December 2006
11-6.2.3.3 Toxic gases typically encountered are hydrogen sulfide, carbon monoxide,
and mangrove gas (southern coastal areas).
11-6.3
Atmospheric Test Equipment.
11-6.3.1 Test the manhole or other underground structure with an approved tester
prior to entry. Determine that the test instrument is in proper working order and
correctly calibrated. Make tests as soon as the manhole cover is removed.
11-6.3.2 Perform oxygen deficiency tests with a safety lamp or an oxygen deficiency
indicator. The safety lamp cannot be used to indicate the presence of carbon dioxide.
Operate the indicator instrument in accordance with the manufacturer's instructions.
11-6.3.3 Do not rely on your sense of smell. Use an approved portable unit to
measure the amount of combustible and toxic gases in the manhole atmosphere.
11-6.3.4 Never enter a manhole until test results indicate that the manhole is safe for
entry.
11-6.3.4.1 If tests made upon removing the manhole cover indicate that the atmosphere
is satisfactory, the manhole or vault can be entered and worked in. Additional tests
must be made when each crew begins work. The test interval must not exceed 8 hours.
When the manhole is covered with a tent or tarpaulin, the test interval must not exceed
2 hours. Place the tent or tarpaulin so that an opening is left in the covering for
ventilation.
11-6.3.4.2 If tests made upon removing the manhole cover indicate that the atmosphere
is unsatisfactory, ventilate the manhole or vault with a power blower for a minimum of
10 minutes if more than the allowable trace of gas is found on the initial test, then make
a second test with the blower running. If the test is satisfactory, the manhole or vault
may be entered. Make this test away from the direct blast of the blower. If gas is again
found on the second test, continue to ventilate the manhole with a power blower until
the test is satisfactory. Work can then be started in the manhole, provided adequate
power blower ventilation is continued. There must be enough ventilation to hold the
quantity of gas in the manhole to within an allowable value until the work has been
completed and the cover replaced. While working in a manhole being ventilated with a
power blower because of previous gas detection, test the atmosphere every hour. If the
blower stops, leave the manhole at once and do not re-enter until ventilation has been
restored and the atmosphere test is satisfactory. Operate the blower outside of a
manhole tent or tarpaulin down wind of the manhole.
11-6.4
Stray Electrical Voltage Testing. From outside the manhole, use a non-
contact audio and visual voltage detector attached to a live line tool to detect for stray
voltages (electric field)/ground leakage currents. If stray voltage is detected, there may
be a cable with a bad shield. Try and determine the source of the stray voltage. If the
faulty cable cannot be found with the voltage detector, use an infra-red camera (gun) to
11-6
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