Grounding Myths

Ground rod assists in clearing a 120V or 277V ground-fault. False. Because the earths’ resistance is so high, sufficient fault current will not flow to open the circuit’s protection device. Example: The maximum fault current for a 25 ohm ground rod is only 4.8A, if the voltage is 120V.

But… The earth can be used to clear a high- voltage transmission power line ground fault. I = E/Z, = 7,200/25 ohms = 288A

Ground Rod Reduces Touch Potential Myth: Because electrons take the path of least resistance, a 25 ohm ground rod would reduce touch voltage from a ground-fault to a safe value. False: Electrons do not take the path of least resistance, they take all available paths. A ground rod does not reduce touch voltage to a safe value.

Ground Rod Reduces Touch Potential Myth: A ground rod can be used to reduce touch voltage from a ground-fault to a safe value by creating an equipotential plane. False: Because the earth is a resistor, not a conductor, touch voltage will not be reduced to a safe value.

Ground Rod Protects Luminaires Myth: Ground rod at metal poles assists in protecting the fixture, lamps, ballast, etc. from lightning. False: If lightning strikes a metal pole, the luminaire will be destroyed, with our without a ground rod.

Ground Rod Protects Concrete Pole Base Myth: Ground rod at metal pole assists in protecting the concrete pole support structure from lightning. False: Lightning is short duration high- frequency event (100kHz), therefore it travels on the outside of conductive surfaces and does not heat them up.

Ground Rod Protects Interior Wiring Myth: Ground rod at metal poles outdoors assists in protecting interior wiring and equipment from high-voltage transients from lightning. False: To limit high-voltage transient voltages on interior wiring, a transient voltage surge suppressor (TVSS) must be installed to prevent the transients from entering from the outside circuits.