Nace CP 1, Course Manual

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RP0177-2000
4 NACE International
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Section 4: Design Considerations for Protective Devices
4.1 Introduction
4.1.1 This section describes various protective
devices used to help mitigate AC effects on metallic
structures, minimize damage to the structures, and
reduce the electrical hazard to people coming in
contact with these structures.
4.1.2 The methods listed can be used to mitigate the
problems of power arcing, lightning arcing, resistive
coupling, inductive coupling, and capacitive
coupling.
3, 4, 5
4.2 Electrical Shields
4.2.1 Shields are intended to protect the structures
from arcing effects that may be produced in the earth
between AC power systems and affected metallic
structures, thus reducing the possibility of puncturing
the coating and/or structure under surge conditions.
4.2.2 Among the factors that influence the design of
electrical shields are the extent to which the structure
is affected and the magnitude of the electrical
potential between the structure and earth. These
factors vary from one location to another and must
be calculated or determined for each specific
location.
4.2.3 Shields may consist of one or more electrodes
installed parallel to and/or encircling an affected
structure at specific locations or along its entire
length. Some types of shields, such as those made
of an anodic material, must be electrically connected
to the affected structure. Shields of the parallel or
encircling anode type shall be connected to the
structure at least at the end points of the shield.
Shields constructed of materials that are cathodic to
the protected structure must be connected to the
structure through a DC decoupling device.
4.2.4 Other types of electrical shields can be
designed for protection against surges on
miscellaneous underground or aboveground
structures. A long, buried, bare conductor can be
used effectively as a shield.
4.3 Grounding Mats
4.3.1 Grounding mats, bonded to the structure, are
used to reduce electrical step and touch potentials in
areas where people may come in contact with a
structure subject to hazardous potentials.
Permanent grounding mats bonded to the structure
may be used at valves, metallic vents, cathodic
protection test stations, and other aboveground
metallic and nonmetallic appurtenances where
electrical contact with the affected structure is
possible.
4.3.2 Grounding mats should be large enough to
extend through and beyond the entire area on which
people may be standing when contacting the affected
structure. They should be installed close enough to
the surface so that step and touch potentials are
adequately reduced for individuals coming in contact
with the structure.
6
4.3.3 Grounding mats, regardless of materials of
construction, must be bonded to the structure,
preferably at more than one point. If cathodic
protection of the structure becomes difficult because
of shielding, a DC decoupling device may be
installed. Connections to the structure should be
made aboveground to allow a means of testing for
effectiveness of the grounding mat in reducing AC
potentials and of its effects on the cathodic protection
system. Care should be taken to prevent the
possible establishment of detrimental galvanic cells
between the grounding mat and structures that are
not cathodically protected.
4.3.4 A bed of clean, well-drained gravel can reduce
the shock hazard associated with step and touch
potentials. The thickness of the bed should be no
less than 8 cm (3 in.). Gravel should be a minimum
of 1.3 cm (0.5 in.) in diameter. The hazards of step
potentials at the edge of a mat may be mitigated by
extending the gravel beyond the perimeter of the
grounding mat.
4.4 Independent Structure Grounds
4.4.1 Wherever a metallic structure that is not
electrically connected to an existing grounded
structure is installed, it shall have an independent
grounding system. This grounding system may
consist of one or more ground rods and
interconnecting wires. Care shall be taken to
properly interconnect all components of the structure
to be grounded. Factors considered in the design of
the grounding system of an independent structure
include the resistivity of the soil and the magnitude of
the induced potential and current which the designer
expects the structure to encounter under all possible
conditions.
4.4.2 When an independent metallic structure or its
grounding system is in close proximity to an existing
grounded structure, an electrical hazard may develop