Page 115 - Mechanical Engineers Reference Book

2/56 Electrical and electronics principles

Feedback (programmable) Any circuit brought near an induction heater will receive
energy and heat will be generated extremely rapidly. No rings
or other metal objects should therefore be worn when in the
vicinity of an induction heater, neither should any metal be
held in the hands. In general, no part of the body should come
within close proximity (about a metre) to an induction heater.

2.4.3 Rescue
To render assistance to a person undergoing an electric shock
the rescuer should first attempt to isolate the circuit by
switching off the supply. If the rescuer cannot isolate the
supply, he or she should try to break the victim’s contact with
the live apparatus by using insulating material. This material is
essential to prevent the rescuer from becoming a second
Input output
block block victim.
If the victim is unconscious the rescuer should send for
medical assistance and start artificial resuscitation. This as-
Figure 2.119 General architecture of a PLD sumes, of course, that the rescuer is trained in modern
methods of such resuscitation.
Statutory Instrument 1989, No. 635, available from HMSO.
The notes included here are based on the Health & Safety
series booklet HS(R)25, which forms part of the Electricity at 2.4.4 Protection
Work Regulations.” The regulations are made under the
Health and Safety at Work Act 1974, and the Act imposes the 2.4.4.1 Insulation
responsibility for people at work on both the employer and the Electrical cables consist of one or more metal cores which may
employee. It is partly the individual’s responsibility therefore be single wires but are more usually stranded wire and
to ensure that safe working practices are observed in the surrounded by insulation. The insulation serves to contain the
workplace. flow of current and prevent a person from touching the live
metal and thereby receiving a shock.
2.4.1 Electric shock
Most serious injuries and deaths from electric shock occur 2.4.4.2 Fuses
from contact with the mains electricity supply. In the UK, the A fuse is a device which will melt when the current exceeds a
mains supply is about 240 V a.c. and in the USA it is about predetermined value. In operation, fuses serve as current-
110 V a.c. The live wire (colour coded brown) is at a higher limiting devices, and they are used for overload protection of
potential with respect to the earth. If a person is in contact electrical equipment. Two types of fuses are generally avail-
with the ground, an electric shock can be sustained by able: rewirable and cartridge.
touching the live wire only. When a shock is received the Fuses are specified by the maximum current that they can
passage of the electric current through the body may cause transmit and the correctly rated fuse must be used at all times.
muscular contractions, respiratory failure, fibrillation of the A high-rating fuse must never be substituted for a low-rating
heart, cardiac arrest or injury from internal burns. Any one of one.
these can be fatal. The greatest danger occurs when the
current flows across the chest. This can happen either when
the current flows from one arm to the other or when it flows 2.4.4.3 Circuit breakers
from one arm to the opposite leg. A circuit breaker is a mechanical device in the form of a switch
The magnitude of an electric shock depends on the strength which opens automatically if the circuit which it controls
of the current, which in turn depends on the voltage and the becomes overloaded. Circuit breakers may be operated
ohmic resistance of the body. The resistance of the human magnetically or thermally and they can also be manually reset
body varies in different persons and is primarily dependent on and adjusted. Plug-in circuit breakers are available and are
the resistance of the skin. This variability in skin resistance recommended for use with small electric power tools.
means that a ‘safe’ voltage cannot be readily specified, and all
voltages in excess of about 50 V must therefore be regarded as 2.4.4.4 Earth-leakage protection
being ppotentially lethal. If the skin is damp, either from water
or from perspiration, then the skin resistance is considerably Normally there is no net flow of electricity to an electrical
reduced. Under such circumstances the chances of an electric device. The flow in from the live wire is exactly balanced by
shock proving to be fatal are greatly increased. the return flow in the neutral cable. If an earth fault develops,
Injury can also be caused by a minor shock, not serious in however, a leakage current will result, and this can be
itself but which has the effect of contracting the muscles detected by the earth-leakage apparatus. Modern earth-
sufficiently to result in a fall. leakage devices are so sensitive that the supply is immediately
disconnected before a lethal current can be drawn from the
2.4.2 Electric burn mains. It should be noted that earth leakage protection will
operate only when the fault occurs between line and ground.
Burns can be caused either by the passage of heavy current
through the body or by direct contact with an electrically
heated surface. They may also be caused by the arcing across a 2.4.4.5 Isolation
short circuit or as a result of an electrically originated fire or Effective means of disconnecting cables or apparatus from the
explosion. source of supply must be provided so that maintenance, repair