ERICO Practical Guide to Electrical Grounding - Manual (Page 107)

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through most paint, rust and surface contaminates. Single
point clamps are superior to battery type and “alligator”
type clamps for making direct contact.
Caution must be exercised in the installation of static
grounding systems so that no part of the electrical current-
carrying system is used as a ground. Fires have occurred in
plants where static-control grounds were tied into the
electrical system neutrals. These neutrals must never be part
of the ground system except at the service entrance or other
approved common bonding point.
Testing and Inspection of Bonding
and Grounding Systems
The proper installation of bonding and grounding devices is
important in the protection of personnel and equipment. At
the time of installation, a resistance test is needed to
confirm electrical continuity to ground. In addition, an
effective inspection and periodic maintenance program is
needed to ensure that continuity exists throughout the
system.
In evaluating maintenance requirements, the bonding and
grounding requirements can be divided into three
categories:
1. The point type clamps equipped with flexible leads
used for temporary bonding of portable containers
to the facility grounding system.
2. The fixed grounding conductors and busbars used to
connect the flexible leads and fixed equipment to
ground.
3. The facility grounding system.
The flexible leads are subject to mechanical damage and
wear, as well as corrosion and general deterioration. For
this reason, they usually should be uninsulated and should
be inspected frequently. This inspection should evaluate
cleanliness and sharpness of clamp points, stiffness of the
clamp springs, evidence of broken strands in the conductor
and quality of the conductor connections.
A more thorough inspection should be made regularly
using an approved ohmmeter to test electrical resistance
and continuity. One lead of the ohmmeter is attached to a
clean spot on the container, the other lead is connected to
the facility grounding system. The measured resistance
should be less than 25 ohms and will usually be about 1
ohm. Shake the leads to make sure that the contact point
and the leads are sound. Do not rely on contact through dirt
or rust.
The fixed leads and the busbar are not usually subject to
damage or wear but should be annually checked with an
ohmmeter. They are checked between the leads or bus and
the facility ground. The measured resistance should be less
than 1 ohm.
Conductive hoses should be checked regularly and after any
repairs are made. The conductive segments may break or
may not be properly repaired. Nonconductive hoses with an
internal spiral conductor should be installed so that the
spiral conductor makes contact with the adjacent metallic
fittings. Shake the hose whenever possible when making
the measurements.
Facility Ground System.
The final component of the static bonding and grounding
system is the facility ground system. The facility ground
must conform to the rules of the NEC as described
elsewhere in this book.
Underground piping equipped with cathodic protection
should not be used as the grounding system.
Inerting Methods and Procedures
The introduction of an inert gas such as nitrogen into a ball
or pebble mill or mixer may prevent a flash fire if an
electrostatic spark occurs within the vessel. Care must be
exercised that sufficient inert gas is introduced to
adequately displace the oxygen (air) throughout the entire
vessel. The most common inert gases are nitrogen and
carbon dioxide (CO
2
).
Two important considerations when inerting are gas
pressure and gas velocity. High gas pressure could damage
a closed vessel. To avoid overpressurization, a relief valve
is recommended on the gas line to the mill. Inerting with
carbon dioxide is potentially hazardous, and such systems
must be carefully designed and installed. A CO
2
fire
extinguisher should never be used to inert a vessel.
Continuous automatic inerting systems are available which
can monitor the oxygen content in a vessel and can adjust
the flow of inert gas to maintain a nonflammable
environment within the vessel.
NFPA 69 “Explosion Prevention Systems” published by the
National Fire Protection Association further discusses inert
gas systems.
Chapter 6: Special Grounding Situations
Grounding Book 4/14/99 10/5/99 6:02 PM Page 99 (Black plate)