70 / 116
70 E L EC TR I C AL CONNEC T I ON
AU T UMN 20 1 7
IT’S SHOCKING
M
ost of us have experienced
some form of electric shock.
One common type on a dry
day is the zap from a car door after you
slide out of the seat.
Other types of shock have serious
consequences and, under some conditions,
can even be fatal.
Electricity flows in a circuit or loop,
and current in the circuit is governed by
the circuit elements, namely the source
voltage and the circuit resistances.
These are related by Ohm’s law:
V = I x R, where V is the source voltage,
I is the current flowing and R is the total
circuit resistance.
For a given applied voltage the
corresponding flow of current is
determined by the total circuit
resistance. This relationship applies
even when parts of the human body are
included in the circuit and form part of
the circuit resistance.
EFFECTONTHEBODY
When electricity flows through a body
that has become part of the loop, it must
have (at least) two points of contact – an
entry and an exit point.
In general, it is only the parts of the body
that lie between these two points that are
affected by the flow of current.
When taking off a jumper made of
synthetic materials in a dark room, I was
amazed to see sparks about 100mm long.
Dry air has a breakdown strength of about
1,000V/mm, so there was about 100,000V
between different parts of this jumper.
Yet I felt nothing, because electric
shock is caused by the amount of current
flowing through the body, not the applied
voltage. The voltage was high, but the
current was negligible due to the very
high circuit resistance.
The human nervous system uses
impulses of current for communication and
to operate parts of the body.
When you grip something with your
hand, a train of tiny current pulses
is sent down the relevant nerves to
operate the muscles – typically five
pulses per second. When you grip
harder, the pulses become more rapid.
Imagine you have come into contact
with, say, a electrical appliance with a
metal case that has become live due to a
wiring fault. If you happen to be standing
barefoot on a conductive surface (eg: a
concrete floor) and you grasp this drill, a
substantial 50Hz current will flow through
your body between your hand and your
feet (the entry and exit points).
This current is much larger than, but
in many ways similar to, the signals that
TINY ELECTRICAL SIGNALS ARE
CRUCIAL TO THEWORKINGS
OF THE HUMAN BODY.
GEORGE
GEORGEVITS
OUTLINESWHAT
HAPPENSWHEN A LARGE VOLTAGE
COMES CHARGING THROUGH.
TECHNICALLY SPEAKING