Chapter 7 – Earth Ground

Publisher Summary This chapter provides guidelines for constructing a suitable and safe earth ground. An adequate earth ground system is extremely important to both user safety and correct operation of a control system. An earth ground, in parallel with the equipment ground system, safely conducts stray or induced electrical currents to earth. For digital switching circuits, several control system industry sources recommend a ground system that has a resistance of one ohm or less between the control system ground and true earth with a maximum resistance of three ohms. The control system ground must be at least as good as a ground associated with any other system. If a ground used with a radio communication system has a one-ohm resistance to true earth, the control system ground must have a one-ohm or less resistance to true earth. The electrical resistance of an earth ground system must be tested to ensure that the ground meets the requirements for control systems. The total resistance of the ground system is comprised of the resistance of the metal used in the plant earth ground grid, the resistance of ground cable connection to the grid, the resistance of the contact between the grid and earth, and the resistance of the earth itself.