Energy of Electrostatic Ignition Sources

A spark is generated between two conductors when the distance between the conductors is small compared to the diameter of the conductors and when the electric field intensity between the conductors is approximately 3 MV/m. A brush discharge is generated if the distance between the conductors is large compared to the radius of curvature of the conductor. 

The energy of a spark discharge is a function of the accumulated charge (Q in coulombs) on the object, the capacitance of the object (C in farads), and the potential, or voltage (V in volts) of the object. These three variables are related by the expression C = Q/V. The actual energy (expressed in joules) associated with the discharge process is given by 

Equation 7-20 assumes a capacitance-type discharge (that is, a spark) however, capacitance and voltage are not defined in nonconductive systems. Therefore Equation 7-20 is valid only for capacitive sparks, but it is used qualitatively for the other discharges. 

A criterion that is commonly used to estimate the potential hazard of a discharge is to compare the MIE of the fuel-air mixture to the equivalent energy of the discharge. A precise experimental determination of the MIE is often required under the specific conditions of the system. MIEs are shown in Table 6-4 for a number of flammable gases and in Table 6-8 for dusts. 

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