Manholes & Underground Vaults

Manholes and underground vaults hold sewer, storm, water, gas, telecom and electric equipment. They are usually confined spaces, with hidden atmospheric dangers, tight access and little room to escape. Many deaths happen when a second person goes in to rescue the first.

• Construction
• Utilities
• Sewer & Water

Know the hazards

• Bad air: low oxygen, hydrogen sulfide (rotten egg smell that can disappear), methane, carbon monoxide, fuel vapors and toxic gases from sewers and soil. See Trench Atmospheres, Hydrogen Sulfide and Gas Detectors.
• Engulfment and drowning: rising water, a sudden flush of storm or sewer flow, or loose materials.
• Electrical: energized cables, transformers and switchgear in power vaults, with shock and arc flash hazards. See Underground Power Lines.
• Falls: ladders, steps and deep shafts, plus slippery rungs.
• Traffic: manholes are often in the road. See Work Zones.
• Heavy covers: many weigh 100 pounds or more, and fingers, hands and backs get crushed. See Lifting & Ergonomics.
• Other hazards: insects, rodents, sharp debris and fire from sparks near flammable gas. See Biological Hazards.

Confined space rules

A manhole or vault with a hazardous atmosphere, an engulfment risk or another serious hazard is a permit-required confined space. Construction work follows 29 CFR 1926 Subpart AA, and general industry follows 1910.146. Water and sewer utilities may also be covered by state-plan rules. See Confined Spaces. Key rules:

• Identify and label spaces, and keep unauthorized people out.
• Use a written entry permit that lists hazards, tests, controls, entrants, the attendant and rescue plan.
• Test the air before entry and keep monitoring while people are inside, testing oxygen, flammable gas and toxic gases, in that order, at different depths.
• Ventilate with a blower set up to bring in clean air, away from exhaust and traffic.
• Station an attendant outside who keeps count, talks with entrants and never leaves or enters.
• Use a harness and retrieval line with a tripod or winch when entry is vertical, unless it would add danger.
• Isolate and lock out pipes, valves, power and pumps that could send in flow or energy. See Lockout/Tagout.
• Have a trained rescue plan ready before anyone enters. Don’t depend on 911 alone.

Opening a manhole safely

1. Set up traffic control and cones. Park the truck to shield the work area.
2. Wear high-visibility clothing, gloves and boots. See High-Visibility Clothing.
3. Use a cover lifter or hook, not your fingers or a screwdriver. Lift with your legs and don’t twist.
4. Stand back after opening. Gas may rush out. Don’t smoke or make sparks.
5. Test the air from outside before looking in or entering, and ventilate first.
6. Put a barrier, guardrail or cover around the open hole whenever it’s unattended or while work is underway. See Barriers & Fall Protection at Excavation Edges.

Power and telecom vaults

• Vaults can contain energized cables and transformers. Only qualified persons may work on them, using the right PPE and procedures. See Arc Flash and Qualified vs. Unqualified.
• Water in an energized vault is an electrocution risk. Don’t enter or pump it out until qualified workers confirm it’s safe.
• Don’t use metal ladders, long metal tools or uninsulated equipment near energized parts.
• Know that communication and gas lines in the same vault may carry hazards of their own.

Before opening or entering

• Traffic control set; open hole guarded
• Space identified as permit-required or not, and permit completed
• Air tested from outside and ventilation running
• Attendant, harness, retrieval line and rescue plan ready
• Energy sources and flows isolated; electrical vaults left to qualified workers
• Cover lifter and lifting aids available

Last updated: October 4, 2026