ETL 1110-9-10(FR)

5 Jan 91

RA

%

RN '

N

Cc

Where:

RN =

Groundbed-to-earth resistance

RA

=

21.06 ohms (Resistance of a single

anode-to-earth

from

the

previous

calculation)

=

4500 ohm-cm (Soil resistivity from

item 1 of paragraph 2-3a)

CC

=

10 ft (Estimated approximate spacing

of anodes)

PF

=

0.00212

(Paralleling

factor

from

table 3-5; assume 9 anodes; reasoning

is the same as in step 5, paragraph

2-2b)

N

=

9 anodes (Estimated number of anodes

required; from step 3, paragraph

2-2b)

RN

=

21.06

+

4500 x 0.00212

9

10

RN

=

3.29 ohms.

Resistance is too high. Additional calculations using

an increasing number of anodes (i.e., 11, 12, 13, 14,

etc.) have to be made; these calculations show that

fourteen

anodes will yield

a groundbed-to-earth

resistance of 2.26 ohms.

6)

Select the number of anodes to be used.

The numbers

determined are:

For life = nine anodes maximum required

For resistance = fourteen anodes maximum required

Therefore, the larger number of anodes, fourteen, is

selected.

7)

Determine the

total

circuit

resistance

(RT)

from

equation 1-3:

36