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usually appear in your nerves to operate
muscles in your hand. This 50Hz signal
will override the control signals from
your brain and cause the hand muscles to
contract tightly. You can’t let go because
the brain’s signals have been swamped.
This 50Hz signal is likely to interfere
with other muscles that lie in the current
path, some of which are essential for
keeping you alive (heart, lungs, etc).
If someone tries to pull you away, they
are likely to suffer a similar fate, because
they will become part of the conductive
path. If they are wearing some form of
insulated footwear and/or insulated
gloves, they may have a chance.
However, the capacitances involved may
still permit enough AC current to give them
a shock. For this reason, it is essential to
turn the power off as soon as possible –
before trying to help.
Even a mild electric shock may cause
a violent, unintended movement. Then
there’s the possibility of physical injury
due to falling off a ladder or being ‘thrown’
back fromwhere you got the shock.
In reality, you are not thrown – your
affected limb muscles operate in an
uncontrolled manner due to the flow of
current through your body.
PROTECTION
A normal circuit breaker or fuse is
intended to protect fixed wiring circuits
against overloads and short circuits.
Trip-out currents typically range from
8A for lighting circuits up to 20A for most
power circuits. However, these ratings are
much too high to protect anyone coming
into contact with a live conductor.
In terms of sensing an electric shock,
a 50Hz AC current of a milliamp or so is
about the lower limit of our perception.
Much above this quickly becomes
unpleasant and can cause injury.
Most 240V fixed wiring in premises
is fitted with some form of residual
current device (RCD). This is now
required by the Wiring Rules for new
installations (but not lighting circuits and
some special purpose circuits).
The RCD senses the ‘go and return’
current in the circuit’s active and neutral
conductors. If the two differ by more than
the rated trip point of the RCD, it assumes
the difference in current is flowing through
a person due to a fault. The circuit is then
disconnected in much the same way as
with a fuse or a normal circuit breaker.
RCDs are generally set to trip about
30mA current difference within about
0.3 seconds. The current level and
trip time have been determined as
a reasonable compromise between
nuisance circuit tripping and personal
safety. Faster trip times apply for larger
current differences.
NETWORK ISSUES
Occasionally, people come into
contact with the distribution network
service provider (DNSP) side of the
electricity network.
This can be in the form of the incoming
240/415V mains (eg: where the power
comes in to a main switchboard), or with
11kV and higher voltages, as found in the
distribution and transmission parts of
networks in the street.
The danger with coming into contact
with any part of the distribution network
is that the circuit is protected only by a
fuse or other protection equipment back at
the feeding substation transformer. Such
protection devices are typically rated at
several hundred amps, offering little or no
personal protection.
Some voltage must always be present
for current to flow, as it is voltage that
drives current around the circuit. Higher
voltages will drive a higher current for a
given circuit.
If the voltage is sufficiently high. and
a live conductor comes into contact with
the human body, the person will receive
an electric shock. Also, tissue damage
can result from arcing and from Ohmic
heating in the parts of the body where
the current has passed.
Furthermore, body parts vary in their
ability to conduct electricity, so the
tissue damage will not be uniform along
the current path.
The current will keep flowing until the
person is physically removed, thereby
breaking the current flow, or the supply is
switched off by the utility.
This is why such low impedance sources
of high voltage must always be treated
with the utmost care and respect.
If such a service is accidentally short
circuited, a very large fault current flows
(1,000A or more). So, a large amount of
power is dissipated in the fault until the
circuit protection operates.
People nearby will be subjected to a
blinding arc flash and loud bang. Personal
injury can result from ultraviolet radiation
burns to exposed skin, eyesight damage
from the arc flash, wounds due to
fragments of flying molten metal,
acoustic shock caused by the arc, and
electric shock.
In addition, long-term psychological
trauma may arise from the event.
Note that when an incident involves a
phase to earth fault, despite the apparent
severity of the fault, electric shock may
not occur. This is because the active
conductor is effectively grounded by
the fault, so there is insufficient voltage
to drive a harmful current through the
body, particularly if the fault is caused by
someone operating an insulated power
tool (angle grinder, drill. etc).
In conclusion, it pays to always check
that a circuit is dead before starting work
on or near it.
It pays to always check that a circuit is dead before starting
work on or near it.
BY
GEORGE
GEORGEVITS