5) Safety devices are important in household circuits - hscnotes

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Oct 7, 2013 (3 years and 11 months ago)

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5) Safety devices are important in household circuits.




Discuss the dangers of an electric shock from both a 240 volt AC mains
supply and various DC voltages, from appliances, on the muscles of the
body.


The use of electricity in the home is potentially d
angerous. The risks that must be
understood and minimised are fire and electric shock.


The Alternating Voltage Supply


The electrical supply is delivered to a home through the cabling. These are
connected to a switchboard where there is a switch that con
trols the home
electricity supply. From here, the electricity passes to a fuse box (or circuit
breaker box) where it is divided into a number of parallel circuits through the
home.


An alternating voltage is delivered to the home by two wires called the
ne
utral
wire

and the
active wire (also known as hot wire).

The neutral wire is
maintained at earth potential by being connected to the Earth at the power
station. The active wire is one of the wires that brings electricity to a household
from a power statio
n. The voltage of the active varies between 340 and
-
340V
with respect to the neutral wire.








As

the voltage between the active lead and the neutral lead is varying, an
average value is used.
The household supple is called a 240V AC supply.


A cycle i
s a complete variation in voltage


Electric Shock


Electric Shock
, or electrocution, is caused when an electric current passes
through the body and causes a violent disturbance of the nervous system. The
disturbance of the nervous system produces effects o
n the muscles of the body.


Electric shock in household is most common when a person touches the active
wire.


Some appliances also produce DC voltages that can cause electric shock.


An electric current through the body can cause muscles to contract. If
the
muscles required for breathing are involved, this can prevent breathing and may
cause death. The effect of mains AC currents on the heart are much more
complicated.


Fibrillation
is a condition in which the heart stops beating regularly and
oscillates
rapidly. The heart is no longer able to pump blood, and death follows in
a matter of minutes. Currents above 200 mA will not cause fibrillation as the
muscles in the heart will clamp.
Direct currents are less dangerous than
alternating currents, as they do

not lead to fibrillation. The most
dangerous type of shock is from your hand to the opposite foot.



describe the functions of circuit breakers, fuses, earthing, double
insulation and other safety devices in the home


A household circuit can cause fire if i
t is overloaded or a short circuit occurs.


A circuit is overloaded if it carries a higher than the maximum safe value
for which it was designed.


If a wire is overloaded by passing excessive current through it, it can become
sufficiently hot to cause a fi
re.


A short circuit is where an active wire comes in contact with the neutral
wire or is earthed.


When a short circuit occurs, a circuit is formed with very little resistance, leading
to a very high current. Sufficient heat can be generated to cause a fi
re.


Double Insulation


All household electrical wiring must be covered with a flexible insulator.
Furthermore, electrical appliances have a casing of rigid plastic to act as
insulation. This is a double insulation, which provides more protection from
elec
trocution, because if the inner casing fails, the outer casing will still be
protective.


Fuses


Fuses are used to prevent overloading of household circuits. A fuse is made of a
resistance wire with a low melting point. This is connected in series with the

active wire.

As the fuse in series the total current will pass through the fuse.


The fuse has a higher resistance than the other wiring, thus its temperature will
increase more than wire. When the current gets excessive the fuse will melt, thus
breaking
the circuit. Fuses must be replaces after each circuit overload.


Circuit Breakers


Circuit breakers have the same function as fuses. A circuit breaker uses an
electro
-
magnet to mechanically break the circuit when the current exceed the
max value. Circuit
breakers are advantageous in that they can be reset after they
have been tripped

















Residual Current Devices (RCD)


A residual current device detects any leakage of current to the earth either
through the body or through any conductor. It is d
esigned to switch off the
current very quickly before it reaches a harmful level. It is also known as a
safety switch
as it reduces the risk of a electric shock in most cases.


Earth Wire


The function of the Earth wire is to provide protection from electr
ic shock for
people using electrical appliances. When the household wiring is installed, a good
conductor is used to connect the fuse box with the Earth.
















Power outlets have three wires connected to them from the fuse box


the active
wire, t
he neutral wire and the earth wire.


The active and neutral wires provide power to the appliance. The Earth wire is
connected to the casing of the appliance.


















The Earth wire provides protection incase the active wire comes in contact with
t
he casing of the appliance. Without the earth wire, a person touching the casing
would receive an electric shock. With the Earth wire in place, as soon as the
active wire comes into contact with the casing, a large current will flow through
the Earth wire
which will blow the fuse or circuit breaker.


Fuse/ circuit breaker are always part of the active wire so that current always
passes though them.