back to basics
Ground Rods
and Touch Voltage
Why won’t a ground rod reduce touch voltage to a safe level?
By Mike Holt, Mike Holt Enterprises, Inc.
T
o understand why a ground rod won’t reduce
touch voltage to a safe level, we need to
know what touch voltage is, at what level
touch voltage is hazardous, and how earth surface
voltage gradients operate. Let’s take a look.
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 ft from the structure.”
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.
Surface voltage gradients. The IEEE Green Book
Std 142 [4.1.1] says the resistance of the soil outward from a ground rod equals the sum of the series
resistances of the earth shells. The shell nearest the
rod has the highest resistance, and each successive
shell has a progressively larger area and a progressively lower resistance. The Table lists the percentage of total resistance (%R) and the touch voltage
(T-V) based on a 120V fault. That “% R” assumes
100% resistance at 25 ft from the rod. It also assumes
you are using a 10-ft rod with a 5/8 in. diameter.
Are you providing a safer installation if you use
a ground rod to reduce touch voltage? No. As the
table shows, the touch voltage 3 ft from the energized electrode will be about 90V. Because the
resistance of the earth is so great, very little current
(1A to 10A) will return to the power supply via the
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May 2001 • EC&M • www.ecmweb.com
If a person completes the circuit in an enegized metal pole by
touching the metal part and the earth, a lethal outcome is likely.
earth, and the circuit overcurrent protection device will not open [Secs. 250-2(d) and 250-54].
Thus, energized metal parts will remain energized,
and the touch voltage will remain at a lethal level.
If a person completes the circuit by touching
the metal part and the earth, a lethal outcome is
likely.
What is the generally accepted grounding practice for street lighting and traffic signaling? The
metal parts of this type of system are commonly
grounded to ground rods without a low-impedance
fault current path. This is one reason maintenance
personnel die while working on street lighting. We
have thousands of energized metal poles just waiting for someone to make contact with.
It is critical that every metal part of an electrical
system have a low-impedance fault current path
(equipment grounding conductor) from the metal
part to the source, in accordance with Sec. 2502(d). A proper low-impedance fault current path
(not the earth) ensures that a line-to-case fault (energized
metal parts) will clear in less than 1 sec.