An Excavation is any man-made cut, cavity, trench, or depression in an earth surface that is
formed by earth removal. A Trench is a narrow excavation (in relation to its length) made below
the surface of the ground. In general, the depth of a trench is greater than its width, and the width
(measured at the bottom) is not greater than 15 ft (4.6 m). If a form or other structure installed or
constructed in an excavation reduces the distance between the form and the side of the excavation
to 15 ft (4.6 m) or less (measured at the bottom of the excavation), the excavation is also
considered to be a trench.
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g) Lower Explosive Limit (LEL): LEL is the minimum concentration of a vapour in air (or other
oxidant) below which propagation of flame does not occur on contact with an ignition source. This
is usually expressed as volume percentage of the vapor in air.
h) Upper Explosive Limit (UEL): UEL is the maximum concentration of a vapor in air (or other
oxidant) above which propagation of flame does not occur on contact with and ignition source.
This is usually expressed as a volume percentage of vapors in air.
i) Pyrophoric Substance : Substances that ignites spontaneously in air at ambient temperature. In
hydrocarbon industries iron sulphide is the most commonly observed pyrophoric substance and is
called as pyrophoric iron.