TM 5-811-7
M. C. Miller multicombination corrosion engineer's meter that has ammeter scales as large as 10 amperes or
as small as 20 milliamperes or less and a number of small voltmeter scales should be used. The total current
required in a resistance bond to clear the interference current leaving the foreign structure can be calculated
by using the following simple algebraic formula:
)Et
)Er
'
(eq. F-1)
Jd
It
and:
Er Id
It '
,
)Et
where )Er = Efn - Efr = change in structure-to-earth potential required to clear interference; Et = Efn - Efrd
= change in structure-to-earth potential caused by the temporary bond; Efn = natural structure-to-earth
potential of the foreign structure; Efr = structure-to-earth potential of foreign structure with all affecting
rectifiers on; Efrd structure-to-earth potential of foreign structure with temporary bond in; Id current through
temporary bond; and Tt = total current required through final resistance bond to correct cathodic interference
caused by all of the rectifiers. The minimum size of wire required should also be calculated, but wire smaller
than No.12 AWG should not be used because of poor mechanical strength. The permanent test station should
include two wires thermit welded or brazed to the foreign structure and two wires thermit welded or brazed
to the protected structure. This will allow one wire to be used as a current drain while the other is used to
measure potential.
(2) After the permanent test station has been installed, the resistance bond can be most easily installed
as follows: bare nichrome resistance wire in the range of ohm per foot, or 1 ohm per foot, or 10 ohms per
foot, depending on the total resistance needed, should be used.
(a) Since as many as three rectifiers may be involved in an interference bond in the hydrant refueling
area, a current interrupter in the rectifier circuit for interrupting all the rectifiers simultaneously would not
be practical. All the rectifiers should be turned on and off manually if more than one rectifier is involved. If
only one rectifier causes the interference at a particular site, a current interruptor should be placed in the a.c.
circuit to interrupt the current output.
(b) Place a copper-copper sulfate half-cell electrode on the surface of the earth at the point of
greatest exposure. Measure and record the natural structure to-earth potential of the foreign structure with
all interfering electrical circuits off.
(c) After all interfering sources are turned on, insert a portion of nichrome resistance wire between
the foreign structure wire and the protected structure wire and observe the structure-to-earth potential of the
foreign structure. Increase or decrease the amount of nichrome in the circuit until the structure-to-earth
potential of the foreign (the interfered-with) structure becomes equal to the natural potential measured in b
above. The current flowing through the resistance bond is now the correct amount and the interference has
been cleared. No ammeters or additional test leads can be in series with the resistance bond when this test
is made.
(d) Mark or note the exact contact of the nichrome resistance wire with the test station lead wires.
Add or inch to each end of these points and cut the nichrome. Crimp an eyelet or fork solderless
terminal at the exact points where the nichrome contacted the test station lead wires. A permanent resistor
has now been fabricated with the exact resistance required for the resistance bond. This resistance may be
installed, removed, and reinstalled for testing purposes without changing the value of the resistance. Install
the fabricated resistor in the test station and check to see that the natural structure-to-earth potential is
achieved. If the potential measured is even a slight amount off, reduce the length of the nichrome or fabricate
a longer piece until the natural structure-to-earth potential is achieved with all rectifiers on and resistance
bond in (Efn will now equal Efrd ). When the proper resistor has been fabricated, the wire should be coiled
on a pencil to make a neat, compact coil that can be installed inside of the test stations without shorting to
the sides of the test station box or the terminals. After the pencil has been removed, the bare nichrome should
not be wrapped with tape to allow proper cooling. Primer and tape may be placed inside the test station box
to prevent a short. The amount of current flowing through the bond will be equal to the value calculated in
F-3
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