JANUARY 31 2003
UNSCHEDULED MAINTENANCE REQUIREMENTS
INTRODUCTION. Impressed current cathodic protection systems require a
higher level of maintenance than sacrificial (galvanic) CP systems. More things can, and
do, go wrong. There are five major components to the operational impressed current
system: the rectifier, the anode bed, the structure lead, the anode lead (header cable),
and the structure. There are two major components to the operational sacrificial
system: the anode and the structure lead. If adequate cathodic protection does not
exist on the protected structure, then troubleshooting must be accomplished to
determine the cause of this lack of protective current.
Troubleshooting. The starting point for all troubleshooting for impressed
current systems is at the rectifier. Indications of all problems are present at this
location. The greatest aids to troubleshooting are historical data and drawings of the
system. Usually, the fault may be isolated, then verified by testing.
Procedures. For impressed current systems, there are sufficient test points
on the face plate of the rectifier to isolate the faulty component. Follow the
troubleshooting procedures in paragraph 5-2; the typical rectifier wiring diagram, Figure
5-2; the troubleshooting block diagram, Figure 5-1; the shunt multiplication factor,
Figure 5-3; and the anode gradient samples, Figure 5-6. The starting point for all
troubleshooting for galvanic systems is at the anode (or anode connection). For
galvanic systems, there must be an anode test lead for conclusive testing of the
anodes. For isolation of a fault to the major component, follow the troubleshooting
procedures in paragraph 5-4.
TROUBLESHOOTING IMPRESSED CURRENT SYSTEMS
WARNING: All connections should be made with alligator clip leads with the rectifier
circuit breaker or power switch OFF. If needlepoint leads are used with power ON,
employ electrical safety practices for working with live circuits.
DC Voltage. Measure the DC voltage output of the rectifier with a handheld
multimeter. With power ON, scale on DC volts, measure voltage from N4 to P4 (Figure
5-2). One of three conditions may exist: voltage may be near zero (proceed to
paragraph 5-2.1.1), near half of normal (proceed to paragraph 5-2.1.2), or near normal
(proceed to paragraph 5-2.1.3).
No DC voltage indicates that one of the components in the rectifier is faulty
or there has been a loss of AC power (proceed to paragraph 5-2.5).
Half the normal voltage output indicates defective diodes/selenium plates