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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