7
Theory of Personal Protective Grounding
Section 7
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Theory of Personal Protective Grounding
Isolate/Insulate
misleading as the contact points are not at
equal voltage values as the name implies.
The points are held to a preselected voltage
difference by knowing the available current
and calculating the maximum resistance of
the connecting grounding cables.
The only method of providing absolute protection to a worker is to completely eliminate
any current path through the body. There are
two ways of doing this.
The first is to isolate the worker so that contact with an energized part cannot be made.
While effective, this also eliminates the ability
to work, so this often is not a viable method.
This “equipotential zone” limits voltage across
the body to a suitably low value to provide
the required measure of safety. Again referring to Equation 2 (I = V / R), by estimating
the body resistance and keeping the voltage
below the safe level selected by the employer,
the desired measure of safety can be achieved.
The reduction in body voltage is achieved by
limiting the maximum voltage that can be
developed across the parallel circuit composed
of both the body and the jumper. Information
on the personal protective jumper is a known
quantity. The jumper also will carry the largest amount of current compared to the body
and can be used to develop the needed parallel
voltage level. Again, it is the responsibility of
each employer to specify a level of acceptable
body voltage. At present, there are no standards that specify a value to be used.
The second method uses suitably rated insulation to eliminate the body as a current
path. This is the principle used when doing
energized distribution voltage maintenance
using rubber gloves. The gloves provide the
insulation to eliminate the body as a current
path. An alternate means is to completely
cover all energized components with an insulating device to prevent any worker contact.
While insulating products are available, they
cannot be used in many of the maintenance
tasks encountered by a lineman working aloft
or by the ground man in support. Present
insulating products are limited to distribution
voltage applications.
The key to a successful equipotential
zone protection method is to place the
worker in a parallel path with a conductor of sufficiently low resistance such
that the rise in voltage is held at or below
the selected level. The maximum jumper
voltage is shown by Equation 2 (V = I X
R). Shunting the fault current around
the body, through the low resistance
path, is the first key. Remember that
some current will flow in every possible
path, but it divides in inverse proportion to the path’s resistance. The use
of a low resistance jumper is the major
factor. The second key factor is to have
the line protection equipment provide
fast fault removal.
Equipotential Protection
A practical and more universal method is
to provide a means of keeping the body extremities at the same or nearly the same
voltage. If the difference in voltage across
the body can be eliminated, the flow of current is eliminated, remembering Equation
2 (I = V / R). Without a difference in voltage
there is no current flow. This is a theoretical solution that cannot be fully achieved in
practice. If current flows through anything
with resistance, a voltage drop will be developed. However, the principle of maintaining
a sufficiently low level of voltage across the
body is the basis for the development of an
“equipotential zone.” The term is slightly
7-2
The use of the system neutral provides a low
resistance path for the return of a fault current
if it occurs. This does two things: It maximizes
the fault current and tends to lower the voltage
at the worksite. The maximum fault current
ensures the fastest clearing possible of the
fault by the system’s protective equipment,
such as circuit breaker, reclosers, fuses, etc.
The reduction in voltage occurs because the
neutral conductor resistance is of a similar
magnitude as the source conductor. The source
and neutral conductors form a series circuit
of two resistances, and a division of voltage
results. Figure 7-1 illustrates this. The voltage
at the worksite is reduced to that represented
by the neutral resistance as a fraction of the
total series circuit resistance (see Section 5
for a discussion of series resistances).
I
R1
IJ
IM
V1
V
SOURCE
VJ
VM
VN
RN
Voltage Division Using the Neutral
Fig. 7-1
V1 is about equal to VN if they are about the
same length and conductor size. The voltage of
the conductor and neutral connections at the
worksite will be about equal voltage because
of the small voltage drop of the jumper, which
we will discount.
V1
=VN = VSOURCE x [RN / (R1 + RN)]
but R1 = RN
so
V1
=VSOURCE x [RN / (RN + RN)] or
V1 =VSOURCE x [RN / 2RN] or
V1
=VSOURCE / 2
and
=ISOURCE
=(VSOURCE - (V1 – VN)
[a very small difference]
or VJ =VM
IJ
VJ
The connection to overhead static or shield
wires are of questionable benefit for use as
a low resistance path for the return of fault
current and should be evaluated before use.
Many are not continuous to the power source,
therefore, cannot be considered a full current
return path. Most are steel conductor, which
has a much higher resistance than a conductor designed to efficiently carry current. The
higher resistance may become hot enough to
fuse, depending upon the current level, resulting in its loss as a return path for protection
if used alone.
They may be used as a secondary current
return path in addition to a primary return
path as a means of increasing the margin
of safety by providing multiple paths to and
through the Earth. If the static or shield wire
is included as part of the “work area” it should
be electrically connected to the personal protective grounds at the worksite to extend the
equipotential work zone.
The use of the Earth alone represents a usable
current return path for personal protective
grounding. It has higher resistance than a
conductor designed to carry current. This will
lower the fault current because its resistance
is greater than the conductive neutral, but
possibly not to such a level that the system
protective equipment would fail to recognize
the fault. However, the resistance of the Earth
varies widely. In areas of dry, sandy soil conditions the resistance may approach several
hundred ohms. In a moist soil it may be in
the low to mid teens. At the Hubbell Power
Systems research laboratory, Centralia, MO,
the Earth resistance approaches 18 Ohms.
If the neutral is broken or fuses during a fault
and it was the only return path to the source,
worker protection could be lost. A current return path through the Earth could be used as
a back-up path for the system neutral. The use
of multiple jumpers and return paths is encouraged. Because this presentation is about
accidents and avoiding accidents, a “belt and
suspenders” approach may be prudent.
7-3