The primary purpose of grounding the metal drop pipe
and/or metal well casing in an installation is safety. It is
done to limit the voltage between nonelectrical (exposed
metal) parts of the system and ground, thus minimizing
dangerous shock hazards. Using wire at least the size of
the motor cable wires provides adequate current-carrying
capability for any ground fault that might occur. It also
provides a low resistance path to ground, ensuring that
the current to ground will be large enough to trip any
overcurrent device designed to detect faults (such as a
ground fault circuit interrupter, or GFCI).
Normally, the ground wire to the motor would provide the primary path back to the power supply ground for any
ground fault. There are conditions, however, where the
ground wire connection could become compromised.
One such example would be the case where the water
in the well is abnormally corrosive or aggressive. In this
example, a grounded metal drop pipe or casing would
then become the primary path to ground. However,
the many installations that now use plastic drop pipes
and/or casings require further steps to be taken for
safety purposes, so that the water column itself does not
become the conductive path to ground.
When an installation has abnormally corrosive water
AND the drop pipe or casing is plastic, Franklin Electric
recommends the use of a GFCI with a 10 mA set-point.
In this case, the motor ground wire should be routed
though the current-sensing device along with the motor
power leads. Wired this way, the GFCI will trip only when
a ground fault has occurred AND the motor ground wire
is no longer functional.