type.] [of the floor mounted on stand type.] [as shown.]
3.2.2
Rectifier Grounding
The grounding system for grounding rectifier cabinets shall have a
resistance to earth of not more than 25 ohms as determined by an approved
method.
3.3
PERMANENT REFERENCE ELECTRODES
3.3.1
Permanent reference electrodes shall be calibrated against a standard
electrode before installation. Calibration shall be done in a test tank
containing water with the same composition as the tank to be protected.
The permanent electrodes shall measure reference voltage agreeing with that
measured by the standard electrode within plus or minus 0.005 volt when the
sensing windows of the two electrodes being compared are not more than 4 mm
1/6 inch apart but not touching.
3.3.2
Installation
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NOTE: The requirements for installation of
permanent reference electrods are similar to those
for anode installation. See the NOTE in paragraph
Anode Installation.
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Reference electrodes shall be operable under the same icing conditions as
the anode system. Installation of permanent reference electrodes shall be
made at points in the tank which will monitor minimum and maximum
tank-to-water potentials [and as otherwise needed for automatic control
system]. Sensing windows of reference electrodes shall be located between
6 mm 1/4 inch and 13 mm 1/2 inch away from the steel surface sensed and
shall be fixed in position preventing contact with tank steel.
3.4
CRITERIA OF PROTECTION
Acceptance criteria for determining the adequacy of protection on the
internal submerged surfaces of steel water tanks shall be in accordance
with NACE RP0388 and as specified below.
3.4.1
Minimum
The criterion of protection shall be a negative voltage of at least minus
850 millivolts as measured between the tank and a saturated
copper-copper-sulphate reference electrode. Determination of this voltage
shall be made with the cathodic protection system in operation. Voltage
drops shall be considered for valid interpretation of this voltage
measurement. A minimum of minus 850 millivolts "instant off" potential
between the tank surface being tested and the reference cell shall be
achieved over 95 percent of the area of the structure. Adequate number of
measurements shall be obtained over the entire tank surface to verify and
record achievement of minus 850 millivolts "instant off." This potential
shall be obtained over 95 percent of the total metallic area without the
"instant off" potential exceeding the maximum limit indicated below.
SECTION 13111A
Page 18
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