Page 235 - Electrical Safety of Low Voltage Systems

218 Chapter Thirteen

becomes insignificant, as it approaches zero when t approaches infin-
ity. In this case, the expression of the stored energy can be simpli-
fied by neglecting it and results proportional to the capacitance-to-
ground.

13.4 Mitigation Strategies
In flammable atmospheres, bonding and earthing can be effectively
employed to prevent the accumulation of static charge over conduc-
tive bodies.
Equipotential bonding conductors bridge the gap existing be-
tween two metal parts likely to be charged, keeping them at the same
potential. In doing so, the risk of spark discharge is eliminated or
greatly reduced.
Earthing, instead, by solidly connecting metal bodies likely to be
charged to ground, allows the draining of the static charge, preventing
its accumulation. This connection to ground must have a resistance
low enough so that the energy possibly being accumulated is lower
than the minimum ignition energy E MIE of the flammable atmosphere.
To this end, we can calculate the maximum permissible value for R,by
assuming t Q > 3RC, thereby neglecting the exponential and solving
Eq. (13.6) for R. Thus, we obtain the following inequality:

1 2E MIE
R < (13.7)
I C
For instance, let us calculate the minimum safe resistance-to-
ground of a tank, whose capacitance-to-ground is C = 100 nF, bleeding
a current I of 50 A, and surrounded by a flammable atmosphere of
minimum ignition energy E MIE = 10 mJ. We obtain

1 2 × 10 −3
R < = 2.8M (13.8)
50 × 10 −6 100 × 10 −9
For static charge purposes, therefore, even high values of ground
resistance are suitable to prevent dangerous accumulation. Applicable
standards conservatively recommend that the earth resistance should
not exceed 1 M .
Equipotential bonding conductors and earthing conductors used
to bleed-off the charge will only carry currents in the order of the
microamperes, therefore, there are no issues regarding their current-
carrying capability. The mechanical strength of these conductors,
though, must be guaranteed by using minimum cross-sectional ar-
2
2
eas of at least 4 mm , if aluminum, and 2.5 mm , if copper.