OSHA requires employers to follow set procedures whenever their employees are working inside a confined space. But what exactly is a confined space, and does a trench qualify?

A confined space has limited or restricted means of entry or exit and is not designed for continuous occupancy by a worker. Examples include tanks, underground vaults, manholes, tunnels, equipment housings, ductwork and pipelines.

Open trenches and excavations such as building foundations aren’t generally considered confined spaces; they are regulated under OSHA’s excavation standard, 29 CFR Part 1926, Subpart P.

“Where underground construction work can shift to the regulations of 1926 Subpart AA in an excavation or trench environment is when it’s necessary for workers to enter structures like precast pipe, manholes and vaults or other similar configurations that meet the definition of a confined space,” said Joe Wise, training manager for trench safety at United Rentals.

“An acronym for remembering the definition of a confined space is SAD — Size, Access, and Duration. It’s a space large enough to bodily enter, has limited or restricted means of entry/exit and is not designed for continuous occupancy. Under those circumstances it’s required that the competent person, who is knowledgeable about testing and confined space hazards, identify and evaluate the space. That evaluation begins before entry with testing and includes both atmospheric and physical hazards that must be eliminated or controlled.

“If an actual or potential physical or atmospheric hazard exists that can’t be eliminated or controlled, the space is considered a permit required confined space.”

Examples of potential hazards include low oxygen, hazardous fumes or toxic gases, a risk of being engulfed by fire or water, or exposed electrical wires. The “permit” refers to the plan the contractor must develop.

The OSHA regulations about construction in a permitted confined space are very detailed. The agency has provided several resources for contractors who need guidance about confined spaces in construction. They include Frequently Asked Questions; Protecting Construction Workers in Confined Spaces: Small Entity Compliance Guide; Confined Spaces in Construction: Sewer Systems; and Confined Spaces in Construction: Pits

Download the United Rentals Guide to Trench Safety

Freelance writer Mary Lou Jay writes about business and technical developments in a variety of industries. She has been covering residential and commercial construction for more than 25 years.