Shock current path
As we've already learned, electricity
requires a complete path (circuit) to continuously flow.
This is why the shock received from static electricity is
only a momentary jolt: the flow of electrons is necessarily
brief when static charges are equalized between two objects.
Shocks of self-limited duration like this are rarely
hazardous.
Without two contact points on the body for
current to enter and exit, respectively, there is no hazard
of shock. This is why birds can safely rest on high-voltage
power lines without getting shocked: they make contact with
the circuit at only one point.
In order for electrons to flow through a
conductor, there must be a voltage present to motivate them.
Voltage, as you should recall, is always relative between
two points. There is no such thing as voltage "on" or
"at" a single point in the circuit, and so the bird
contacting a single point in the above circuit has no
voltage applied across its body to establish a current
through it. Yes, even though they rest on two feet,
both feet are touching the same wire, making them
electrically common. Electrically speaking, both of the
bird's feet touch the same point, hence there is no voltage
between them to motivate current through the bird's body.
This might lend one to believe that it's
impossible to be shocked by electricity by only touching a
single wire. Like the birds, if we're sure to touch only one
wire at a time, we'll be safe, right? Unfortunately, this is
not correct. Unlike birds, people are usually standing on
the ground when they contact a "live" wire. Many times, one
side of a power system will be intentionally connected to
earth ground, and so the person touching a single wire is
actually making contact between two points in the circuit
(the wire and earth ground):
The ground symbol is that set of three
horizontal bars of decreasing width located at the
lower-left of the circuit shown, and also at the foot of the
person being shocked. In real life the power system ground
consists of some kind of metallic conductor buried deep in
the ground for making maximum contact with the earth. That
conductor is electrically connected to an appropriate
connection point on the circuit with thick wire. The
victim's ground connection is through their feet, which are
touching the earth.
A few questions usually arise at this point
in the mind of the student:
-
If the presence of a ground point in the
circuit provides an easy point of contact for someone to
get shocked, why have it in the circuit at all? Wouldn't a
ground-less circuit be safer?
-
The person getting shocked probably isn't
bare-footed. If rubber and fabric are insulating
materials, then why aren't their shoes protecting them by
preventing a circuit from forming?
-
How good of a conductor can dirt
be? If you can get shocked by current through the earth,
why not use the earth as a conductor in our power
circuits?
In answer to the first question, the
presence of an intentional "grounding" point in an electric
circuit is intended to ensure that one side of it is
safe to come in contact with. Note that if our victim in the
above diagram were to touch the bottom side of the resistor,
nothing would happen even though their feet would still be
contacting ground:
Because the bottom side of the circuit is
firmly connected to ground through the grounding point on
the lower-left of the circuit, the lower conductor of the
circuit is made electrically common with earth
ground. Since there can be no voltage between electrically
common points, there will be no voltage applied across the
person contacting the lower wire, and they will not receive
a shock. For the same reason, the wire connecting the
circuit to the grounding rod/plates is usually left bare (no
insulation), so that any metal object it brushes up against
will similarly be electrically common with the earth.
Circuit grounding ensures that at least one
point in the circuit will be safe to touch. But what about
leaving a circuit completely ungrounded? Wouldn't that make
any person touching just a single wire as safe as the bird
sitting on just one? Ideally, yes. Practically, no. Observe
what happens with no ground at all:
Despite the fact that the person's feet are
still contacting ground, any single point in the circuit
should be safe to touch. Since there is no complete path
(circuit) formed through the person's body from the bottom
side of the voltage source to the top, there is no way for a
current to be established through the person. However, this
could all change with an accidental ground, such as a tree
branch touching a power line and providing connection to
earth ground:
Such an accidental connection between a
power system conductor and the earth (ground) is called a
ground fault. Ground faults may be caused by many
things, including dirt buildup on power line insulators
(creating a dirty-water path for current from the conductor
to the pole, and to the ground, when it rains), ground water
infiltration in buried power line conductors, and birds
landing on power lines, bridging the line to the pole with
their wings. Given the many causes of ground faults, they
tend to be unpredicatable. In the case of trees, no one can
guarantee which wire their branches might touch. If a
tree were to brush up against the top wire in the circuit,
it would make the top wire safe to touch and the bottom one
dangerous -- just the opposite of the previous scenario
where the tree contacts the bottom wire:
With a tree branch contacting the top wire,
that wire becomes the grounded conductor in the circuit,
electrically common with earth ground. Therefore, there is
no voltage between that wire and ground, but full (high)
voltage between the bottom wire and ground. As mentioned
previously, tree branches are only one potential source of
ground faults in a power system. Consider an ungrounded
power system with no trees in contact, but this time with
two people touching single wires:
With each person standing on the ground,
contacting different points in the circuit, a path for shock
current is made through one person, through the earth, and
through the other person. Even though each person thinks
they're safe in only touching a single point in the circuit,
their combined actions create a deadly scenario. In effect,
one person acts as the ground fault which makes it unsafe
for the other person. This is exactly why ungrounded power
systems are dangerous: the voltage between any point in the
circuit and ground (earth) is unpredictable, because a
ground fault could appear at any point in the circuit at any
time. The only character guaranteed to be safe in these
scenarios is the bird, who has no connection to earth ground
at all! By firmly connecting a designated point in the
circuit to earth ground ("grounding" the circuit), at least
safety can be assured at that one point. This is more
assurance of safety than having no ground connection at all.
In answer to the second question,
rubber-soled shoes do indeed provide some electrical
insulation to help protect someone from conducting shock
current through their feet. However, most common shoe
designs are not intended to be electrically "safe," their
soles being too thin and not of the right substance. Also,
any moisture, dirt, or conductive salts from body sweat on
the surface of or permeated through the soles of shoes will
compromise what little insulating value the shoe had to
begin with. There are shoes specifically made for dangerous
electrical work, as well as thick rubber mats made to stand
on while working on live circuits, but these special pieces
of gear must be in absolutely clean, dry condition in order
to be effective. Suffice it to say, normal footwear is not
enough to guarantee protection against electric shock from a
power system.
Research conducted on contact resistance
between parts of the human body and points of contact (such
as the ground) shows a wide range of figures (see end of
chapter for information on the source of this data):
-
Hand or foot contact, insulated with
rubber: 20 MΩ typical.
-
Foot contact through leather shoe sole
(dry): 100 kΩ to 500 kΩ
-
Foot contact through leather shoe sole
(wet): 5 kΩ to 20 kΩ
As you can see, not only is rubber a far
better insulating material than leather, but the presence of
water in a porous substance such as leather greatly
reduces electrical resistance.
In answer to the third question, dirt is not
a very good conductor (at least not when it's dry!). It is
too poor of a conductor to support continuous current for
powering a load. However, as we will see in the next
section, it takes very little current to injure or kill a
human being, so even the poor conductivity of dirt is enough
to provide a path for deadly current when there is
sufficient voltage available, as there usually is in power
systems.
Some ground surfaces are better insulators
than others. Asphalt, for instance, being oil-based, has a
much greater resistance than most forms of dirt or rock.
Concrete, on the other hand, tends to have fairly low
resistance due to its intrinsic water and electrolyte
(conductive chemical) content.
-
REVIEW:
-
Electric shock can only occur when contact
is made between two points of a circuit; when voltage is
applied across a victim's body.
-
Power circuits usually have a designated
point that is "grounded:" firmly connected to metal rods
or plates buried in the dirt to ensure that one side of
the circuit is always at ground potential (zero voltage
between that point and earth ground).
-
A ground fault is an accidental
connection between a circuit conductor and the earth
(ground).
-
Special, insulated shoes and mats are made
to protect persons from shock via ground conduction, but
even these pieces of gear must be in clean, dry condition
to be effective. Normal footwear is not good enough to
provide protection from shock by insulating its wearer
from the earth.
-
Though dirt is a poor conductor, it can
conduct enough current to injure or kill a human being.
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