Nace CP 1, Course Manual

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RP0177-2000
NACE International 3
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Section 3: Exposures and Effects of Alternating Current and Lightning
3.1 Introduction
3.1.1 This section outlines the physical phenomena
by which AC, AC power systems, and lightning may
affect metallic structures.
3.2 Resistive Coupling (Electrolytic)
3.2.1 Grounded structures of an AC power system
share an electrolytic environment with other
underground or submerged structures. Coupling
effects may transfer AC energy to a metallic structure
in the earth in the form of alternating current or
potential. Whenever a power system with a
grounded neutral has unbalanced conditions, current
may flow in the earth. Substantial currents in the
earth may result from phase-to-phase or phase-to-
ground faults. A metallic structure in the earth may
carry part of this current. Also, a structure in the
earth coated with an insulating material may develop
a significant AC potential across the coating.
3.3 Capacitive Coupling
3.3.1 The electric field associated with potentials on
power conductors can develop a potential on an
inadequately grounded structure in the vicinity of the
power system. The potential that the structure
attains because of capacitive coupling varies with the
power conductor potential and depends on many
factors, including the geometric configurations of the
structures involved. During construction, when the
structure is aboveground or in an open trench, it may
reach a dangerously high potential. When the
structure is buried or submerged, the capacitive
coupling effect usually is not significant unless (1)
the soil resistivity is high, (2) the structure is
electrically isolated, or (3) the structure is very long.
3.4 Inductive Coupling
3.4.1 AC current flow in power conductors produces
an alternating magnetic field around these
conductors. Thus, an AC potential can be induced in
an adjacent structure within this magnetic field, and
current may flow in that structure. The magnitude of
the induced potential depends on many factors
including the overall geometric configuration of the
structures involved, the magnitude of the current in
the power circuit, and any current imbalance. If the
currents in a three-phase power system are equal
(balanced) and the affected structure is equidistant
from each of the conductors, the total induced
voltage is zero. This, however, is seldom the case,
and induced AC voltage is usually present on the
affected structure. Greater electromagnetically
induced potentials may occur during a phase-to-
ground or phase-to-phase fault in multiphase circuits
because of the higher magnitude of fault current in
these systems. The leakage conductance to ground,
caused by the resistive coupling of the affected
structure, allows AC current to flow between that
structure and earth. This phenomenon, combined
with other factors, results in different values of AC
structure-to-electrolyte potential along the affected
structure. The higher the dielectric strength and
resistance of the coating and the higher the soil
resistivity, the greater the induced AC potential.
3.5 Power Arc
3.5.1 During a fault to ground on an AC power
system, the AC power structures and surrounding
earth may develop a high potential with reference to
remote earth. A long metallic structure, whether
coated or bare, tends to remain at remote earth
potential. If the resulting potential to which the
structure is subjected exceeds breakdown potential of
any circuit element, a power arc can occur and
damage the circuit elements. Elements of specific
concern include coatings, isolating fittings, bonds,
lightning arresters, and cathodic protection facilities.
3.6 Lightning
3.6.1 Lightning strikes on the power system can
initiate fault current conditions. Lightning strikes to a
structure or to earth in the vicinity of a structure can
produce electrical effects similar to those caused by
AC fault currents. Lightning may strike a metallic
structure at some point remote from AC power
systems, also with deleterious effects.
3.7 Switching Surges or Other Transients
3.7.1 A switching surge or other transient may
generate abnormally high currents or potentials on a
power system, causing a momentary increase in
inductive and capacitive coupling on the affected
structures.