Working in manholes pose potential dangers which may result in serious accidents. The common ones include falls/slips, fire or explosion, oxygen depletion, gas poisoning, heat stress, drowning, asphyxiation arising from gas, fume, vapour and entrapment by free flowing solids. Amongst which, dangers involving gases are easily overlooked or neglected, leading to serious casualties.
This blog aims to remind persons entering or working in a manhole to take appropriate measures, including measures for the prevention of oxygen depletion and gas poisoning.
Certified workers must be familiar with the ‘Code of Practice – Safety and Health at Work in Confined Spaces’, and before entering a manhole, they must take all the necessary safety precautions to ensure safety at work.
Things to note before entering a manhole
Enter a manhole only after the responsible person has issued a valid certificate and only within the period specified in the certificate. Before issuing such a certificate, a proprietor or a contractor should employ a qualified and competent person.
To conduct a risk assessment, including the use of appropriate equipment to test the air inside the manhole. Workersshould never use out-dated methods (like throwing a flame down the manhole, observing the distribution of small animals and the colour of the manhole) as a safety indicator for entering a manhole.
Safety measures for the prevention of gas poisoning include:
Blank off all pipelines connected to the manhole to prevent any dangerous gas or fume from entering the manhole; e.g. closing the valve and removing any generator that may produce toxic gases.
Remove any sewage or sludge in the manhole beforehand to prevent any toxic gases that may have accumulated in it, from being released and endangering the workers in the manhole in the course of their work
Use a ventilating blower to supply fresh air into the manhole to ensure that the air inside is maintained safe for the workers.
Ensure a sufficient supply of power outlets for the operation of the ventilation blower(s).