ERICO Practical Guide to Electrical Grounding - Manual (Page 106)

Practical Guide to Electrical Grounding
level in the mixer and/or providing a liquid conductive
medium to dissipate the charge will help. If this is not
possible, the vessel should be inerted. It should be noted
that the static accumulation in liquids should be controlled
by raising the ambient humidity.
Pebble mills present an additional hazard because the
porcelain lining is an insulator that will prevent the flow of
static charges from the liquid to ground, even if the mill is
grounded. This hazard is best controlled by inerting the
mill.
Nonconductive Plastic Containers and Stretch Film.
The use of nonconductive plastic containers in potentially
flammable locations may be an ignition hazard. Static
charge accumulations on such containers, caused by the
transfer of poorly conductive materials or by contact
charging, cannot be dissipated by bonding and grounding.
Contact (“triboelectric”) charging of a nonconducting
container in a low humidity environment creates a spark
ignition hazard by inducing charges in materials in a
container. These induced charges may cause sparking, for
example, when the material is poured into a grounded
safety can. Surprisingly, this hazard of charge induction is
greatest when the material is conductive.
For example, experience in the coating industry suggests
the following precautions:
Fiberboard Drums - No hazard of static accumulation
except for metal rims which should be grounded during
product transfer.
Kraft Paper Bags and Plastic-Lined Paper Bags - No
hazard with paper bags. Plastic-lined paper bags are usually
not hazardous, but the static electrification for each
bag/contents combination should be measured. All plastic
bags and bags with removable plastic liners should be
avoided unless measurements of electric field intensity at
the bag surface during product transfer is less than 5 kV/cm
(12.5 kV/inch).
Plastic Bottles and Nonconductive Drum Liners - Both
of these items are subject to the hazard of charge induction
as a result of electrification. Precautions must be taken to
minimize contact charging or to neutralize contact charges
before use. Removal of plastic bottles from plastic bags
may cause contact charging. Electric field intensities
greater than 5 kV/cm (12.5 kV/inch) at the surface of the
bottle or liner should be neutralized before a conductive
flammable liquid is put into the bottle. It is also important
to avoid charging a plastic bottle that even contains a small
quantity of a conductive, flammable liquid.
Stretch Wrap - Stretch wrap must be removed from pallets
in a nonflammable location. This material is usually highly
charged and represents a serious hazard in flammable
locations.
Semi-Bulk “Supersacks” - Electrostatic field intensity at
the bag surface should be less than 5 kV/cm (12.5 kV/inch).
Bags that contain metallic filaments must be grounded
during product transfer.
Conductive Plastic Liners and Containers - Although
most plastic materials are nonconductive, some conductive
plastic liners and containers are commercially available.
Conductive plastic materials must be grounded during
product transfer in flammable locations.
Bonding and Grounding Principles
Bonding and grounding are very effective techniques for
minimizing the likelihood of ignition from static electricity.
A bonding system connects various pieces of conductive
equipment and structures together to keep them at the same
potential. Static sparking cannot take place between objects
which are at the same potential. Grounding is a special form
of bonding in which the conductive equipment is connected
to the facility grounding system in order to prevent sparking
between conductive equipment and ground.
In potentially flammable locations, all conductive objects
that are electrically isolated from ground by nonconductors
such as nonconductive piping or hoses, flexible hoses,
flexible connections, equipment supports or gaskets should
be bonded. An isolated conductive object can become
charged sufficiently to cause a static spark. Objects that can
become isolated include screens, rims of nonconductive
drums, probes, thermometers, spray nozzles and high
pressure cleaning equipment.
In order to successfully achieve the objective of the same
ground potential for all materials and their containers when
there are additional and/or redundant grounding systems,
and particularly when there are supplementary grounding
electrodes, all such grounding electrodes and systems must
be interconnected as required by the NEC and NFPA
Lightning Protection Code.
Bonding and grounding conductors must be durable and of
a low resistance. Connections of bonding conductors to
equipment must be direct and positive for portable
equipment. Clamps must make contact with metal surfaces
98
Grounding Book 4/14/99 10/5/99 6:02 PM Page 98 (Black plate)