Introduction to Health and Safety at Work: The Handbook for the NEBOSH National General Certificate, Second Edition

These Regulations concern any work which is carried on in a place which is substantially (but not always entirely) enclosed, where there is a reasonably foreseeable risk of serious injury from conditions and/or hazardous substances in the space or nearby. Every year about 15 people are killed and a number seriously injured across a wide range of industries ranging from simple open top pits to complex chemical plants. Rescuers without proper training and equipment, often become the victims.
Confined space - means any place, including any chamber, tank, vat, silo, pit, trench, pipe, sewer, flue, well or similar space in which, by virtue of its enclosed nature, there arises a reasonably foreseeable specified risk.
Specified risk - means a risk to any person at work of:
serious injury arising from a fire or explosion
loss of consciousness arising from an increase in body temperature
loss of consciousness or asphyxiation arising from gas, fume, vapour or the lack of oxygen
drowning arising from the an increase in the level of liquid
asphyxiation arising from a free flowing solid or because of entrapment by it.
Duties are placed on employers to:
comply regarding any work carried out by employees and
ensure, so far as is reasonably practicable, that other persons (e.g. use competent contractors) comply regarding work in the employer's control.
The self-employed also have duties to comply.