Nace CP 1, Course Manual

Page: 163 / 535

Field Measurements 5:16
CP 1 – Cathodic Protection Tester Course Manual
©
NACE International, 2000
02/01/05
Rectifier Current Output
Most rectifiers have a shunt on the panel. Usually the shunt rating data are
stamped on the shunt. You can determine the rectifier current output by
measuring the voltage drop across the shunt. The reading can then be
compared with that shown on the rectifier ammeter. This serves as a check
of the accuracy of the rectifier meter.
An ammeter can also be inserted into the rectifier circuit. Many rectifiers
have large outputs, however, so it is necessary to be certain you have an
ammeter capable of handling the current. There is also a safety hazard in
working with higher currents. If you do test the output this way, be certain
the rectifier is turned off when the circuit is opened and when it is closed
again.
Current Requirement Tests
You may be asked to assist in making current requirement tests. In these
tests, a current is impressed on a structure to be protected and the potential
changes brought about by that current are measured. From the data, the
amount of current required for protection can be determined.
The current in the test circuit can be determined by inserting an ammeter
into the circuit or by the use of a shunt. Here again, if an ammeter is used, it
must have sufficient capacity to handle the test current.
Current Flow on a Pipeline or Cable
A length of pipeline, cable, or similar long, thin metal structure can serve as
a shunt. Line current measurement is useful in determining the distribution
of current along a cathodically protected structure, solving stray current
problems, and in locating areas of poor coating or perhaps a short.
In this section we will explore two techniques for determining current flow
along such a structure:
•
2-wire test points
•
4-wire test points
Connections to the structure should be made by permanent test wires; probe
rods are sometimes used on bare or poorly coated piping.