Page 232 - Electrical Safety of Low Voltage Systems

Safety Against Static Electricity and Residual V oltages 215

For all of the above reasons, strategies to avoid or mitigate the for-
mation of static charge must be taken into consideration in the design
of installations, primarily to guarantee persons’ safety and, secon-
darily, to prevent economic losses. In the following sections, we will
examine the causes of generation and accumulation of static charge
and possible mitigation strategies.

13.2 Generation of Static Electricity
The common cause of the generation of static charge is the friction
between unlike materials, be solid or fluid. The relative motion of the
two materials, made of contacts and subsequent separations, allows
the transfer of electrons from one surface to the other. If either, or both,
materials are insulators, when they separate, some electrons may not
be able to return to their original location due to their elevated resis-
tance. Therefore, the electrons’ counterparts in the atoms, the protons,
are left behind not neutralized, originating ions. This process causes
a charge within the insulating material, which may even take days
to be spontaneously neutralized. Among other factors such as mate-
rial characteristics, areas of contact, etc., the speed of separation of the
parts in contact plays an important role in determining the magnitude
of their electrification.
As an example, let us consider a liquid flowing through a pipe
and filling a tank. In correspondence of the conduit’s surface, the fluid
tends to develop a positively charged layer, while negative ions will be
present in its inner part. The system is, therefore, electrically neutral,
but when the liquid abandons the tube the negative charge may be
carried out with it. This effect is particularly accentuated in fluids with
high electrical resistivity (i.e., orders of hundreds of megaohms) due
to the charges’ difficulties at moving within it. In return, the negatively
charged liquid will electrify the container that collects it (Fig. 13.2).

FIGURE 13.2
Charging of a
container by flow
of liquids.