SAFETY TIP 6
COMMON MANHOLE HAZARDS
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What are the hazards most commonly found in manholes?
The most common hazards which can sometimes be found in manholes are atmospheric in nature, including vapors or gases such as:
• Gasoline (i.e. from leaking underground gasoline tanks) • Natural gas (i.e. from ruptured gas mains) • Liquid petroleum gas or propane (i.e. from storage cylinders) • Nitrogen (i.e. from tanks used to temporarily pressurize telco cables)
Gases from the decaying of naturally occurring matter such as:
• Methane (very common at or near land-fills) • Carbon dioxide • Hydrogen sulfide
Gases created as a by-product of combustion from vehicles or equipment such as:
• Carbon monoxide (i.e. from vehicle or generator exhaust)
Gases used on the job during manhole work operations such as:
• Acetylene • Propane
Oxygen Levels
• Oxygen depletion • Oxygen enrichment
What are some of the affects of these hazards?
Carbon Monoxide (CO)
PPM |
|
Resulting Condition |
35 |
|
OSHA Permissible Exposure Level (PEL) |
200 |
|
Mild headaches (3-5 hrs) |
400 |
|
More severe headache and nausea (1-2 hrs) |
800 |
|
Severe headache, dizzy, nausea (3/4 hr). Collapse & possibly death (2 hrs) |
1600 |
|
Severe headache, dizzy, nausea (20 min). Collapse % possible death (2 hrs) |
3200 |
|
Severe headache, dizzy, nausea (5-10 min). Danger of death within 30 mins. |
Hydrogen Sulfide (H2S)
PPM |
|
Resulting Condition |
0.13 |
|
Minimal perceptible odor like rotten eggs |
4.60 |
|
Strong odor of rotten eggs |
10.0 |
|
Mild eye irritation |
27.0 |
|
Strong unpleasant odor, but not intolerable |
100 |
|
Coughing, severe eye irritation, loss of sense of smell after 2-5 min |
200 – 300 |
|
Marked eye inflammation & respiratory tract irritation after 1 hr |
500 – 700 |
|
Unconsciousness & possible death within 1 hr |
Gasoline, natural gas, LPG, acetylene, propane All are extremely combustible. Propane for example, can also displace oxygen
Nitrogen An asphyxiant which displaces oxygen
Oxygen depletion or “Dead Air” The normal fresh air we breathe consists primarily of approximately 78.1% Nitrogen and 21.9% Oxygen. The remainder consists of low levels of other gases such as Argon, Carbon Dioxide and Carbon Monoxide
Oxygen depletion may occur due to:
• Poor natural ventilation in a closed manhole • Natural gas (i.e. from ruptured gas mains) • Natural occurrences (i.e. excessive rust)
Oxygen depletion can be harmful or fatal as indicated in the following chart:
% Oxygen |
|
Resulting Condition |
21.9 %
|
|
Normal fresh air |
19.5 %
|
|
Minimum safe level |
16 %
|
|
Disorientation, impaired judgment & breathing |
14 %
|
|
Faulty judgment, rapid fatigue |
8 %
|
|
Mental failure, fainting |
6 %
|
|
Difficulty breathing, death within minutes |
Oxygen enrichment 23 % or more of oxygen can significantly increase the danger of fire or explosion.
What can I do to protect myself from these hazards? Personnel working in manholes must be thoroughly trained and equipped to work safely when the atmospheric hazards described above may be present. The OSHA Standards for the telecommunications industry (1910.268) and power companies (1910.269) do vary somewhat in regard to manhole work. Therefore, the specific industry safety practices and equipment used for manhole work may also be somewhat different. This is especially true for manhole atmospheric testing and monitoring procedures. For that reason, we are providing a synopsis of these practices by industry in our companion article “Teck Tips”. Please refer to the Teck
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