5 Jan 91
Deep anode groundbed resistance graphs are available for
deep vertical ground beds. (See figure 2-9a through 2-9c
located in section 2-4.)
For a single horizontal anode:
4L 2 % 4L (2h)2 % L 2
(2 h)2 % L 2
where RA is the anode-to-electrolyte resistance in ohms, p
is the electrolyte resistivity in ohm-cm, L is the length
of the backfill cylinder in feet, d is the diameter of the
backfill cylinder in feet, and h is the depth of the
backfill cylinder in feet.
If the horizontal anode dimensions are assumed to be 6 in.
in diameter, 8 ft long, and buried 6 ft below the surface,
where RA is the anode-to-electrolyte resistance in ohms and
p is the electrolyte resistivity in ohm-cm.
For multiple vertical anodes:
[(ln (8L/d) -1)
ln (.656 N))
where RN is the anode-to-electrolyte resistance, p is the
anodes, L is the length of the backfill column in feet, d
is the diameter of the backfill column in feet, and Cc is
the center-to-center spacing of the anodes in feet. This
equation assumes a linear configuration of the groundbed
If the number of anodes used does not produce a low enough
anode-to-electrolyte resistance, the number of anodes will
have to be increased accordingly.