Thank you Mr Dipil for your mail.
A 24 Volt light source is the safest way to start any activity in a confined space.
It is just a beginning.
As you rightly pointed out we need to take higher voltages to carry out maintenance work.
This is normally done after ensuring adequate safety. There are instances when you have to build scaffolding. Step by step these activities are to be carried out.
I am also not exactly aware about the four times volume sweep. However the logic is not difficult to understand.It is something like the concept of factor of safety. What is your satisfactory number? Two or Three..? The factor of safety is also fixed by experience.
You can take the thickness of LIFT/ ELEVATOR ropes as example. While higher numbers might give more degree of comfort, often experience emboldens one to go for proven economical designs.
In a confined space you can expect Heat, Dust, Toxins, Odour, Vapours etc. More air is required to mitigate all these effects and ensure that adequate air supply is there for persons to breathe safely. While more than four is welcome, four might have been fixed based on reliable past experiences. Perhaps OHSAS covers this aspect.
Some of the biggest operations I have witnessed include the Under ground Mining of Non Ferrous Metals like Zinc, Silver, Cadmum , Lead etc. Scores of People work several meters below MSL. Very huge machinery driven by diesel / electric power operate
below the ground level.
Adequate ventilation is provided by forcing fresh air from the base at ground level. DGMS
( Director General of Mines and Safety) is a Statutory authority that governs these operations. One can learn about high standards of safety from these operations.
I came across one interesting presentation and a good article from the internet.
Both are attached to this post.
Hope these add some value.
V.Raghunathan