ETL 1110-9-10(FR)

5 Jan 91

LW RMFT

RW '

1000 ft

Where:

LW =

240 ft (After reviewing figure 2-20,

it is estimated that 240 ft of wire

will be required to

connect

the

rectifier to the riser anode string)

RMFT =

0.640 ohm (Wire resistance per 1000

lin ft of No. 8 AWG HMWPE insulated

wire)

RW =

240 ft x 0.640 ohm

= 0.15 ohm

1000 ft

Since this anode string is very long, the metal

of

the anode can represent a significant

resistance. Since the current discharges all

along the anode, one half of its length is used

in the resistance calculation.

Manufacturer's

data

show

the

longitudinal

resistance of a single 4-ft anode segment to be

0.053 ohm.

Effective resistance of the string

is:

0.053 ohm/anode x 28 anodes

= 0.74 ohm

2

This resistance is very high compared to the

anode resistance (greater than 10 percent) as a

result, too much current will discharge near the

top of the anode and not enough current will be

discharged near the bottom of the anode.

Therefore, either a double-end feed method will

have to be used or copper-cored rods must be

used. (Note: This is usually only a problem in

fresh water applications when the anode rod

length is greater than 30 ft.) In this case, we

will elect to use copper cored 1/8-in. diameter

rods. Manufacturer's data show the longitudinal

resistance of a single 4-ft long copper-cored rod

of this diameter is 0.0034 ohm.

Effective

resistance of this string is therefore:

= 0.048 ohm

0.0034 ohm/anode x 28 anodes

2

99

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