Page 231 - Electrical Safety of Low Voltage Systems

214 Chapter Thirteen

The risk of fire or explosion may be present in occupancies when
all the following circumstances occur:
1. A process of generation and accumulation of electric charge
is present.
2. The gap between parts with opposite charge is small enough
to allow a spark discharge.
3. Flammable materials, such as gasses, vapors, and dusts, are
present.
4. Flammable atmospheres are present. Such atmospheres are
created by flammable materials in the optimum concentration
in air, ranging between their lower explosive limit (LEL) and
upper explosive limit (UEL). At concentrations in air above
the UEL, the mixture air–flammable material is too poor in
oxygen to start combustion; at concentrations in air below the
LEL, there is not enough flammable material to sustain com-
bustion. Flammable atmospheres can be continuously present
in the area during normal operations, or momentarily due to
accidents (e.g., rupture of a tank).
5. The electrostatic energy stored and available to be discharged
exceeds the minimum ignition energy (E MIE ) of the flammable
atmosphere, which is typically in the order of millijoule. Note
that dusts require much more energy to ignite than gasses and
vapors.
All the above elements can be represented in the fire tetrahedron of
Fig. 13.1.
Previous point 3 is represented by the fuel leg of the tetrahedron,
whereas points 1, 2, and 5 are the heat leg. Point 4, that is, the presence
of the optimum flammable atmosphere, is the chemical reaction leg. The
risk of fire or explosion is eliminated if at least one of the above legs
is removed.
Additional risk offered by charged objects is the impulsive current
that can possibly flow through persons upon touch and discharging to
ground. This current is generally well below the threshold of danger,
but in some cases might be above the tingling sensation and, therefore,
cause sudden shock and induce accidental falls.

FIGURE 13.1 The
fire tetrahedron.