Ground Rod Does Not Reduce Touch Potential
Contrary to what we all have read, a ground rod will not reduce touch voltage to a safe level!
By Mike Holt, Published in EC&M Magazine
To understand how a ground rod is useless in reducing touch voltage to a safe level, we need to know (1) what touch voltage is, (2) at what level touch voltage is hazardous, and (3) how earth surface voltage gradients operate.
1. Touch Voltage - The IEEE definition of touch voltage is "the potential (voltage) difference between a grounded metallic structure and a point on the earth 3 feet from the structure."
2. Hazardous Level - NFPA 70E - Standard for Electrical Safety Requirements for Employee Workplaces, cautions that death and/or severe electric shock can occur whenever the touch voltage exceeds 30V.
3. Surface Voltage Gradients - According to IEEE Green Book Std 142 [4.1.1], the resistance of the soil outward from a ground rod is equal to the sum of the series resistances of the earth shells. The shell nearest the rod has the highest resistance and each successive shell has progressively larger areas and progressively lower resistances. This following Table lists the % of total resistance (%R) and the Touch Voltage (T-V) based on a 120V fault. The table's % of resistance is based on 25 feet representing 100% of the total earth resistance for a 10-foot ground rod having a diameter of 5/8 inches.
Distance from Rod %R T-V
1 Foot (Shells 1) 1 foot 68% 82V
3 Feet (Shells 1-2) 3 feet 75% 90V
5 Feet (Shells 1-3) 5 feet 86 % 103V
With the intention of providing a safer installation, some, if not most, in the electrical industry think a ground rod can be used to reduce touch voltage. However, the voltage gradient of the earth drops off so rapidly, a person in contact with an energized object (at any building wiring voltage) can receive a lethal electric shock.
As we can see in the above table, the approximate touch voltage three feet from the energized electrode will be about 90V. Because the resistance of the earth is so great, very little current (1 to 10 amperes) will return to the power supply via the earth and the circuit overcurrent protection device will not open [250-2(d) and 250-54]. Result, energized metal parts will remain energized and the touch voltage is at a lethal level waiting for someone to make contact with it and the earth, Fig. 1.
DANGER: Scary as it might be, this is the generally accepted grounding practice for street lighting and traffic signaling. That is, ground the metal parts to a ground rod and not provide a low impedance fault current path! This is one of the reasons so many people get killed with street lighting in the United Sates and I'm sure there are thousands of energized metal poles, just waiting for someone to make contact with them. For a case study on this subject, click here.
It is critical that metal parts of an electrical system have a low impedance fault current path (equipment grounding conductor) from the metal parts to the source in accordance with the NEC [250-2(d)]. A proper low impedance fault current path (not the earth) ensures that a line-to-case fault (energized metal parts) will be cleared in less than 1 second.
Next month, I'll discuss how "Electricity doesn't take the path of least resistance."