Confined Space

For the construction industry, OSHA defines a confined or enclosed space as any space having a limited or restricted means of entry or exit, is large enough for an employee to enter and perform assigned work, and is not designed for continuous occupancy by the employee. OSHA's Construction Regulations also mandate that all employees required to enter into confined or enclosed spaces shall be instructed as to the nature of the hazards involved, the necessary precautions to be taken, and in the use of protective and emergency equipment required. The employer shall comply with any specific regulations that apply to work in dangerous or potentially dangerous areas. 
Confined spaces may include, but are not limited to:
Underground vaults
Tanks and storage bins
Ventilation or exhaust ducts
Sewers and manholes
Containment cavities
Electrical transformers 
Heat sinks
Open top spaces more than 4 feet in depth such as pits, diked areas, tubs, vaults, and vessels
Confined space within a confined space such as tanks within pits
Throughout the construction jobsite, contractors and workers encounter both inherent and induced hazards within confined workspaces. 
Inherent hazards, such as electrical, thermal, chemical, mechanical, etc., are associated with specific types of equipment and the interactions among them. Usually inherent hazards cannot be eliminated without degrading the system or equipment, or without making them inoperative. Therefore, emphasis must be placed on hazard control methods. 
Induced hazards arise and are induced from a multitude of incorrect decisions and actions that occur during the actual construction process. 
Following are typical examples of inherent and induced hazards applicable to confined workspace in construction.
Oxygen-deficient atmosphere
Explosive or toxic gases, vapors, or fumes
Electrical shock
Purging agents such as nitrogen and argon may enter the confined space displacing oxygen to the extent that it can asphyxiate workers almost immediately 
Materials or equipment falling in and on 
Radiation generated by equipment 
High or low temperatures
High noise levels 
High-pressure vessels and lines rupturing 
Omission of protective features 
Conditions that may cause unintentional worker contact with electrical energy sources
Lack of safety factors in structural strength 
Flammable atmospheres 
Hazards in one space entering another space - such as toxic fumes entering from venting systems
Further guidance may be obtained from American National Standard ANSI Z117.1-1989, Safety Requirements for Confined Spaces.
Source: OSHA