|
partial dismantling of an item of electrical equipment,
including measurement and non-destructive testing as
required, in order to arrive at a reliable conclusion as to
its condition and safety.
Isolation
– involves cutting off the electrical supply from
all or a discrete section of the installation by separating
the installation or section from every source of electrical
energy. This is the normal practice so as to ensure the
safety of persons working on or in the vicinity of elec-
trical components which are normally live and where
there is a risk of direct contact with live electricity.
Competent electrical person
– a person possessing
suffi cient electrical knowledge and experience to avoid
the risks to health and safety associated with electrical
equipment and electricity in general.
12.3
Electrical hazards and injuries
Electricity is a safe, clean and quiet method of trans-
mitting energy. However, this apparently benign source
of energy when accidentally brought into contact with
conducting material, such as people, animals or metals,
permits releases of energy which may result in serious
damage or loss of life. Constant awareness is necessary
to avoid and prevent danger from accidental releases of
electrical energy.
The principal hazards associated with electricity are:
➤
electric
shock
➤
electric
burns
➤
electrical
fi res and explosions
➤
arcing
➤
portable electrical equipment
➤
secondary
hazards.
12.3.1 Electric shock and burns
Electric shock is the convulsive reaction by the human
body to the fl ow of electric current through it. This sense
of shock is accompanied by pain and, in more severe
cases, by burning. The shock can be produced by low
voltages, high voltages or lightning. Most incidents of
electric shock occur when the person becomes the
route to earth for a live conductor. The effect of electric
shock and the resultant severity of injury depend upon
the size of the electric current passing through the body
which, in turn, depends on the voltage and the electrical
resistance of the skin. If the skin is wet, a shock from
mains voltage (220/240 V) could well be fatal. The effect
of shock is very dependent on conditions at the time but
it is always dangerous and must be avoided. Electric
burns are usually more severe than those caused by
heat, since they can penetrate deep into the tissues of
the body.
The effect of electric current on the human body
depends on its pathway through the body (e.g. hand to
hand or hand to foot), the frequency of the current, the
length of time of the shock and the size of the current.
Current size is dependent on the duration of contact
and the electrical resistance of body tissue. The elec-
trical resistance of the body is greatest in the skin and
is approximately 100
000
ohm, however, this may be
reduced by a factor of 100 when the skin is wet. The
body beneath the skin offers very little resistance to
electricity due to its very high water content and, while
the overall body resistance varies considerably between
people and during the lifetime of each person, it averages
at 1000 ohm. Skin that is wounded, bruised or damaged
will considerably reduce human electrical resistance and
work should not be undertaken on electrical equipment if
damaged skin is unprotected.
An electric current of 1 mA is detectable by touch
and one of 10 mA will cause muscle contraction which
may prevent the person from being able to release the
conductor, and if the chest is in the current path, respira-
tory movement may be prevented causing asphyxia.
Current passing through the chest may also cause
fi brillation of the heart (vibration of the heart muscle)
and disrupt the normal rhythm of the heart, though
this is likely only within a particular range of currents.
The shock can also cause the heart to stop completely
(cardiac arrest) and this will lead to the cessation of
breathing. Current passing through the respiratory centre
of the brain may cause respiratory arrest that does not
quickly respond to the breaking of the electrical contact.
These effects on the heart and respiratory system can
be caused by currents as low as 25 mA. It is not possible
to be precise on the threshold current because it is
dependent on the environmental conditions at the time,
as well as the age, sex, body weight and health of the
person.
Burns of the skin occur at the point of electrical
contact due to the high resistance of skin. These burns
may be deep, slow to heal and often leave permanent
scars. Burns may also occur inside the body along the
path of the electric current causing damage to muscle
tissue and blood cells. Burns associated with radiation
and microwaves are dealt with in Chapter 15.
Do'stlaringiz bilan baham: |
