Nace CP 1, Course Manual

Page: 393 / 535

RP0177-2000
NACE International 15
6.3.3 The AC current in the structure to be protected
may flow to ground through cathodic protection
equipment. Current flowing in the cathodic
protection circuits under normal AC power system
operating conditions may cause sufficient heating to
damage or destroy the equipment. Heating may be
significantly reduced by the use of properly designed
series inductive reactances and/or shunt capacitive
reactances in the cathodic protection circuits.
6.3.3.1 Rectifiers should be equipped with
lightning and surge protection at the AC input
and DC output connections.
6.3.3.2 Resistance bonds for the purpose of DC
interference mitigation should be designed for
the maximum normal AC and DC current flow in
order to prevent damage to the bond.
Installation of polarization cells or other devices
in parallel with DC resistance bonds may prevent
damage to bonds. Installation of
semiconductors in DC interference bonds
between cathodically protected structures may
result in undesirable rectification.
6.3.3.3 When bonds to other structures or
grounds are used, polarization cells or grounding
cells should be used, as required, in order to
maintain effective levels of cathodic protection.
6.3.3.4 Semiconductor drain switches for the
mitigation of stray DC from traction systems
should be provided with surge current protection
devices.
6.3.4 In DC stray current areas, the grounding
methods should be chosen to avoid creating
interference problems.
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Section 7: Special Considerations in Operation and Maintenance of
Cathodic Protection and Safety Systems
7.1 Introduction
7.1.1 This section outlines safe maintenance and
testing procedures for cathodic protection systems
on structures subject to AC influence.
7.2 Safety Measures for Operation and Maintenance of
Cathodic Protection Systems
7.2.1 Cathodic protection rectifiers that are subject
to damage by adjacent electric utility systems should
be checked for proper operation at more frequent
intervals than rectifiers not subject to electric system
influence.
7.2.2 Cathodic protection testing or work of similar
nature must not be performed on a structure subject
to influence by an adjacent electric utility system
during a period of thunderstorm activity in the area.
7.2.3 When repeated rectifier outages can be
attributed to adjacent electric utility system
influences, positive measures must be taken to
maintain continuous rectifier operation. One or more
of the following mitigative measures may be
employed:
(a) Self-healing lightning arresters across the
AC input and DC output terminals.
(b) Heavy-duty choke coils installed in the AC
and/or DC leads.
7.2.4 If galvanic anodes are used for cathodic
protection in an area of AC influence and if test
stations are available, the following tests should be
conducted during each structure survey using
suitable instrumentation:
(a) Measure and record both the AC and DC
currents from the anodes.
(b) Measure and record both the AC and DC
structure-to-electrolyte potentials.
7.2.5 At all aboveground pipeline metallic
appurtenances, devices used to keep the general
public or livestock from coming into direct contact
with the structure shall be examined for
effectiveness. If the devices are found to be
ineffective, they shall be replaced or repaired
immediately.
7.2.6 In making test connections for electrical
measurements, all test leads, clips, and terminals
must be properly insulated. Leads shall be
connected to the test instruments before making
connections to the structure. When each test is
completed, the connections shall be removed from
the structure before removing the lead connection
from the instrument. All test connections must be
made on a step-by-step basis, one at a time.
7.2.7 When long test leads are laid out near a power
line, significant potentials may be induced in these
leads. The hazards associated with this situation
may be reduced by using the following procedures: