Static Electricity 2000 Edition

6.5 Charge Neutralization.
6.5.1 General.
Air can be made to contain mobile ions that
will be attracted to and will eliminate unbalanced static elec-
tric charge from surfaces. In the use of air ionizers, one must
consider certain factors that can influence their effectiveness,
such as environmental conditions (e.g., dust and tempera-
ture) and positioning of the device in relation to the material
processed, machine parts, and personnel. It is important to
note that these control devices do not prevent the generation
of static electric charge. They provide ions of opposite polarity
to neutralize the generated static electric charge.
6.5.2 Inductive Neutralizers.
6.5.2.1
Inductive neutralizers include the following:
(1) Needle bars, which are metal bars equipped with a series
of needle-like emitters
(2) Metal tubes wrapped with metal tinsel
(3) Conductive string
(4) Brushes made with metal fibers or conductive fibers
Each design is based on or consists of sharply pointed ele-
ments arranged for placement in the static electric field near
the charged surfaces.
6.5.2.2
A charge drawn from ground to the needle-like tips of
an inductive neutralizer produces a concentrated electric field
at the tips. If the tips are sharply pointed, the electrical field
will be sufficient (i.e., greater than 3 kV/mm) to produce a
localized electrical breakdown of the air. This electrical break-
down, known as corona, will inject ions into the air that are
free to move to distant charges of opposite polarity. The flow
of ions produced in corona constitute a neutralizing current.
(See Figure 6.5.2.2.)
FIGURE 6.5.2.2
Example of an induction needle.
6.5.2.3
Although inexpensive and easy to install, inductive
neutralizers require a minimum potential difference between
the object and the needle tip to initiate corona and the neu-
tralizing process. In the absence of this minimum charge, neu-
tralization will not occur, and a residual potential of a few
thousand volts will be left on the material when sharp induc-
tive points are approximately within about 12 mm of the sur-
face.
6.5.2.4
It is critically important that inductive neutralizers are
connected to a secure ground. If the inductive neutralizer is
not grounded, sparks from the induction bar can occur.