Regulation 72 states:
A person conducting a business or undertaking must, while work is being carried out in a confined space, ensure that the concentration of any flammable gas, vapour or mist in the atmosphere of the space is less than 5% of its LEL, so far as is reasonably practicable.
If it is not reasonably practicable, and the concentration of any flammable gas, vapour or mist in the atmosphere of the confined space:
Where a flammable atmosphere may exist in a confined space and there is a risk of fire and explosion, all ignition sources in the vicinity must be eliminated.
A fire or explosion requires the presence of three elements: an ignition source, air and a fuel (gas, vapour or mist) capable of igniting. A flammable atmosphere is one in which the flammable gas, vapour or mist is likely to exceed its lower explosive limit (LEL).
Flammable atmospheres in confined spaces may result from the evaporation of a flammable residue, flammable materials used in the space, a chemical reaction (such as the formation of methane in sewers), or from the presence of combustible dust (such as that in flour silos).
If an ignition source, such as a sparking electrical tool or static on a person, is introduced into a space containing a flammable atmosphere, an explosion is likely to result.
Examples of potential ignition sources, both inside and outside the space, include:
- open flames and hot surfaces
- electrical equipment
- internal combustion engines
- metal tools striking metal surfaces
- spark-producing equipment for example grinding wheels
- static electricity.
What your gas monitor reads regarding flammable gas.
The Lower Explosive Level of methane is 5% by volume = LEL
Your gas monitor is set up to read from 0% to 5% methane (CH4).
So the scale is from 0% – 5%, but shows on screen as a percentage of that scale. In other words the low alarm goes off at 5% of the LEL, (so that is 5% of 5%) which equals .25% methane by volume and the high alarm is 10% of the LEL which is .5% methane by volume.
Any reading above the LEL is consider explosive, so the reason the alarm goes off so early, is to allow time for the worker to exit the space.
5% of LEL – low alarm, no entry permitted, but allowed to remain in space if already there.
10% of LEL – high alarm, exit space immediately.
Industry best practice states, that if there is an ALARM, exit the space immediately.