JANUARY 31 2003
synchronizable interrupters (not pulse generators, unless used as a conventional
A data logger which records potential measurements very quickly (from four
to several thousand readings per second) may be used in combination with
conventional current interrupters, manually synchronizable, or advanced synchronizable
interrupters (not pulse generators, unless used as a conventional interrupter), then
analyzing the data (sometimes using computer software) to determine the instant-OFF
A waveform analyzer may be used together with a pulse generator to
calculate the OFF potential.
A high speed data logger and oscilloscope (or similar very high speed
recording device) may be used to analyze the unfiltered signal on the structure to obtain
the potential of the structure when the DC output waveform is at zero current output.
This technology may only be applicable with potential measurements that are affected
by only one single-phase rectifier, with all filters and chokes disconnected from the
rectifier output and may not remove all anode gradient errors.
Types of Interrupters. The type of interrupter used depends on the number
of rectifiers or DC current sources and the type of equipment used to perform the
instant-OFF measurements. Manual interruption of a cathodic protection rectifier using
an AC power switch, rectifier circuit breaker, or other means is generally not
recommended. The time to manually open and close contacts, coupled with interruption
of the AC side of the circuit does not provide accurate or repeatable measurements
under many conditions within the first second of interruption, resulting in significant
inaccuracies in the measurement obtained. Manually operated relay contacts are
sometimes used, and may produce repeatable results if they have a quick open and
closed mechanism and are installed in the DC output of a rectifier. Using radios to
signal operators to open and close contacts, breakers or switches could also produce
erroneous readings due to RF signals from the radio which may induce voltages in the
meter being used to measure or record the measurement, especially with analog meters
or unfiltered digital meters.
Conventional Interrupter. The conventional current interrupter provides for
timed interruption of a rectifier DC output current. This is normally accomplished by
opening the circuit either between the anodes and the rectifier, or the structure and the
rectifier. Units normally provide independent control of open and closed time intervals.
Variations in the accuracy of the interruption timing, current ratings of the relay contacts,
and the selection increments of the on and off cycles vary between units. These units
are normally installed temporarily in the rectifier output circuit during testing, but
sometimes are optionally installed in the rectifier cabinet. Quartz crystal controlled units
are usually accurate to within one second a day. These units are typically powered by