Not the link I posted above as well
"In order for a surge arrestor to be effective, it must be grounded to the water strata. Water strata is the actual water underground. Any surge in the system is looking for the easiest path to true water ground. The faster this surge is directed to ground, the less damage it can cause to your system. Grounding the arrestor to only a driven ground rod may not be an adequate ground as the resistance through the soil is higher in some areas than others. Higher resistance means the surge will look for an easier path to ground, which may be through your motor. Connecting the ground wire from the arrestor directly to the motor is the best ground available. A ground wire from the motor has been a US National Electrical Code (NEC) requirement since 1990, so the wire should be readily available. Other potential ground sources are metal well casings and metal drop pipes that are in direct contact with the well water. However, not all wells are cased all the way to the water, such as a rock well. In some situations, a metal well casing or drop pipe may be adequate ground for a surge arrestor, but a motor that is not grounded to the metal casing and the service entrance does not meet 1990 and 1993 NEC requirements."