UFC 3-570-06
JANUARY 31 2003
analyzer to accurately calculate the OFF potential. Pulse generators do not require any
synchronization. The waveform analyzer captures a digital picture of the potential
waveform by recording thousands of voltage readings on the waveform. Digital signal
processing techniques are then used to filter out any induced AC or 60-cycle noise in
the waveform and the ON potential reading is calculated. The error from the six pulse
generators having the greatest influence on the reading is determined by analyzing the
zero current pulses from all pulse generators affecting the waveform, and the OFF
potential reading is calculated by subtracting the error from the ON reading. If the
location under test is suspected to have more than six rectifiers or DC current sources
significantly affecting the potential, separate testing should be conducted to verify or
eliminate that possibility. If there are more than six sources of current affecting the
potential at a particular location, determine the six sources with the greatest influence
and turn the rest off so that their effect is not considered as part of the off potential. The
value recorded using this technique will have very little or no depolarization in the
measurement.
7-3.7.4
Using a High Speed Data Logger and a Filtered Oscilloscope. The
oscilloscope is used to analyze the rectifier output waveform, and the high-speed data
logger is used to obtain a digital picture of the potential signal on the structure. This
technology simultaneously measures the rectifier output waveform and the potential
waveform, and by comparison extrapolates the potential measurement when the
rectifier waveform is at zero current. 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. Since the data logger is
connected to the structure with a closed circuit to the anodes through the rectifier, the
potential of the anodes could still affect the potential measurement if readings are taken
in the vicinity of the anodes. On a well-coated structure, the distance required to
remove the possibility of a mixed potential (anode and structure) would be greater. This
technology will not work with a three-phase rectifier system or where more than one
rectifier is protecting the structure.
7-4
STRUCTURE-TO-SOIL POTENTIAL LIMITS
7-4.1
Excessive Cathodic Protection Current. Excessive cathodic protection
current produces hydrogen gas evolution at the surface of the cathode. If the gas is
produced faster than it can permeate the coating, bubbling of the coating will occur.
The amount of coating damage is dependent on the amount of gas generated and the
type of coating. This condition is normally called "blowing off" the coating. When this
occurs, more of the structure is exposed to the electrolyte and the circuit resistance
between the anodes and the cathode becomes lower. This causes more current to be
impressed to this location, and usually, more gas evolution. This phenomenon results
in more coating damage and is very detrimental to current distribution, since more
current goes to that location, and less current goes to other, more remote, locations.
7-17
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