Tech Tips
Step and Touch Potentials
by Jeff Jowett
Megger
F
or theoretical considerations, “true earth” can be viewed as a conductor
deep in the ground, one with virtually no resistance. With this understanding, ground resistance can be thought of as the resistance between, say, a
ground mat or substation grid and true earth. By this definition, the surface then
is not true earth. Current can and will flow along the surface, but has a tendency
to go deep. Horizontal travel tends to be over comparatively short distance, but
adverse soil conditions such as rocky ground can develop voltages that can be
hazardous to workers during fault conditions. Accordingly, fault current on a
transmission tower will travel down the grounding conductor and legs and then
spread out over the surface before going deeper, thereby posing a potential hazard
to utility workers in the vicinity. Not surprisingly, the closer to the tower legs,
the greater the concentration of current and the higher the voltage. The wider
apart the person’s feet or the greater the distance from the hands (if touching
the structure) to the feet (if standing at ground potential), the greater the voltage
gradient across the body. It is for this reason that “step” and “touch” potentials are
evaluated and measures taken to have them brought within safe limits.
Grounded
Perimeter
Fence
Substation
E
P
Substation
Ground
System
Figure 1
www.netaworld.org Current flow causes a voltage
drop at the earth’s surface. A person
standing with feet apart will develop
a portion of this potential difference from foot to foot. Resistance
increases as current flows away from
the point of entry into the soil at
a ground rod or tower leg, but the
increments become progressively
smaller as the overall volume of soil
increases with distance. Therefore,
the risk to personnel is greatest near
the point of entry, with the voltage
drop over the same span becoming
less and less with distance. Hence,
the voltage gradient across the span
of a typical human step is referenced
in the literature as “step potential”.
A related problem is referred to as
“touch potential.” This is the potential that can be established between
the point at which a person is standing on the ground and the point at
which some contact is made with
remote hardware, such as by placing
the hand on a substation fence.
There are complex software programs available based on computer
modeling of the entire substation
that can be employed to calculate
step and touch potentials, but these
will be discussed separately at another time. For basic fieldwork, step
and touch potentials can be calculated readily from measurements
easily obtained with a standard
Summer 2007 NETA WORLD
four-terminal ground tester. One current lead from the
tester is connected to the system ground at some convenient
point of contact, and the other current lead is clipped to a
probe driven into the ground at a convenient remote position outside the substation fence. One potential terminal is
then connected by a lead to an appropriate position on the
fence, and the second potential terminal is connected by a
lead to a probe driven into the ground about three feet (the
length of a typical adult human stride) outside the fence
(Fig. 1). The tester is energized and a resistance measurement obtained. By employing Ohm’s law,
V = IR
and using the maximum fault current from the system
design along with the measured resistance, the touch potential across a human body is calculated. For step potential,
the connection to the fence is taken down and moved to a
second probe driven into the soil about three feet from the
other probe. The other three terminals remain connected as
they were, and the tester is reenergized (Fig. 2). Again, the
resistance reading from the tester is employed in Ohm’s law
as shown above, and the step potential is calculated. These
two values indicate the maximum voltage to which a person
(utility worker or hapless passerby) could be subjected if
touching a substation fence or walking close to it while a
fault condition is occurring. These calculations typically yield
about a 20 percent accuracy, with more rigorous techniques
available, as mentioned above, where warranted.
For worker safety, protective mats can be utilized. These
can be either insulating or conducting mats. They produce
the desired effect through opposite means. The former, of
course, isolates the person by interrupting the circuit path,
while the latter maintains constant potential over the worksite area. A conducting mat moves the problem area, with the
voltage gradient now starting at the mat’s edge. Maximum
step potential exists at the edge, so a nonconductive ladder
may be laid on the ground to act as a safe approach. Substation yards are covered by coarse stone to increase protection
by diverting fault current into the more conductive soil
beneath. A grade of stone one inch or larger is used in order
to provide air gaps that act as further insulation between
stones. Otherwise the stone tends to pack tight and defeat
the purpose. It also prevents the growth of vegetation, which
presents a safety hazard in its own right.
Substation grounding must dissipate normal and fault
currents without exceeding operating or equipment limits
or adversely affecting continuity of service in addition to assuring safety of workers and civilian personnel who happen
to be walking in the vicinity. It was once assumed that any
ground connection, however crude, made an object safe to
touch. But there is no simple relationship between ground
resistance of a system as a whole and maximum shock current to which a person might be subjected. Examples of
this seeming conundrum can be illustrated accordingly. A
substation supplied from an overhead line will experience
NETA WORLD Summer 2007
Substation
Grounded
Perimeter
Fence
E
A
B
Substation
Ground
System
Figure 2
total ground fault current entry into the earth, causing a
steep rise of local ground potential. Since current is entering
via an aerial conductor and air is a good insulator, substantial
fault current will travel on the grounding conductor and
enter the earth at a defined point, thereby leading to high
voltage gradients in the immediate vicinity. A low system
ground resistance is therefore critical.
Where an underground cable feeder is present, a major part of fault current returns through the enclosure or
cable sheaths directly to the source, thereby establishing
a low-resistance metallic link parallel to the earth return.
This lessens the magnitude of ground potential rise. So in
the one case, a danger to personnel might still exist despite
the presence of what might otherwise be considered a low
ground resistance, while in the other instance, a much higher
ground resistance might still provide a safe environment. It
can be seen, therefore, that the design of the overall system,
not just that of the grounding grid itself, can contribute to
a safe facility, even in what might otherwise be considered
poor grounding soils. By extension, the ultimate design is
one where the product of maximum short-circuit current
and ground resistance represents a voltage low enough to
be contacted safely. The number of factors that contribute
to ground fault clearance and the complexity of their interrelationships indicate that a grounding system might not be
adequate in terms of safety even while capable of sustaining
fault current in magnitude and duration as permitted by
protective relays. Safe grounding can be viewed as a balance
between two parallel systems: the permanent one of buried
electrode and the accidental one established by personnel
contact in the vicinity.
Critical factors, as indicated above, include:
• relatively high fault current in relation to the size of
ground system and its resistance
• soil resistivity and the manner in which it affects distribution of ground current flow
• the coincidence of the presence of an individual at such
time and position as to bridge two points of high voltage
gradient
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• absence of sufficient contact or series resistance in such
an instance
• duration of the contact, and hence current flow through
the body, for a sufficient time to cause harm
• coincidence of various combinations of the factors
above
In the next column, we will examine some practices that
will mitigate these potential risks.
Information provided courtesy AVO Training Institute,
Dallas, TX; avotraining.com
Jeffrey R. Jowett is Senior Applications Engineer for Megger in
Valley Forge, PA, serving the manufacturing lines of Biddle®, Megger®,
and Multi-Amp® for electrical test and measurement instrumentation.
He holds a BS in Biology and Chemistry from Ursinus College. He was
employed for 22 years with James G. Biddle Co. which became Biddle
Instruments and is now Megger.
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