A 24-volt light source is the safest way to start any activity in a confined space. It is just the beginning. As you rightly pointed out, we need to use higher voltages to carry out maintenance work. This is usually done after ensuring adequate safety measures. There are instances when scaffolding needs to be built, and these activities should be carried out step by step.
I am not exactly aware of the "four times volume sweep." However, the logic is not difficult to understand. It is somewhat similar to the concept of the factor of safety. What is your preferred number, two or three? The factor of safety is typically determined based on experience.
You can consider the thickness of lift/elevator ropes as an example. While higher numbers may provide more comfort, experience often leads one to opt for proven economical designs.
In a confined space, one can expect heat, dust, toxins, odors, vapors, etc. More air is needed to mitigate these effects and ensure an adequate air supply for individuals to breathe safely. While more than four units of air exchange are preferable, the number four may have been chosen based on reliable past experiences. Perhaps OHSAS covers this aspect.
Some of the largest operations I have witnessed include the underground mining of non-ferrous metals such as zinc, silver, cadmium, lead, etc. Numerous people work several meters below mean sea level, with very large machinery powered by diesel/electricity operating below ground level.
Adequate ventilation is ensured by pumping fresh air from the ground level. The Director General of Mines and Safety (DGMS) is the statutory authority governing these operations. Valuable lessons about high safety standards can be learned from these operations.
I recently came across an interesting presentation and a good article on the internet. Both are attached to this post. I hope these additions provide some value.
Regards, V. Raghunathan