ELECTRIC SHOCK PROTECTION

The nervous system of the human body controls all its movements, both conscious and unconscious. The
system carries electrical signals between the brain and the muscles, which are thus stimulated into action
The signals are electro-chemical in nature, with levels of a few millivolts, so when the human body
becomes part of a much more powerful external circuit, its normal operations are swamped by the outside
signals. The current forced through the nervous system of the body by external voltage is electric shock.
All the muscles affected receive much stronger signals than those they normally get and operate very
much more violently as a result. This causes uncontrolled movements and pain. Even a patient who is still
conscious is usually quite unable to counter the effects of the shock, because the signals from his brain,
which try to offset the effects of the shock currents, are lost in the strength of the imposed signals.
A good example is the 'no-let-go' effect. Here, a person touches a conductor which sends shock currents
through his hand. The muscles respond by closing the fingers on the conductor, so it is tightly grasped.
The person wants to release the conductor, which is causing pain, but the electrical signals from his brain
are swamped by the shock current and he is unable to let go of the offending conductor.
The effects of an electric shock vary considerably depending on the current imposed on the nervous
system, and the path taken through the body. The subject is very complex but it has become clear that
the damage done to the human body depends on two factors:
1. - the value of shock current flowing, and
2. - the time for which it flows.
These two factors have governed the international movement towards making electrical installations
safer.
 Resistance of the shock path
In simple terms the human body can be considered as a circuit through which an applied potential
difference will drive a current. As we know from Ohm's Law, the current flowing will depend on the voltage
applied and the resistance of the current path. Of course, we should try to prevent or to limit shock by
aiming to stop a dangerous potential difference from being applied across the body. However, we have to
accept that there are times when this is impossible, so the important factor becomes the resistance of the
current path.
The human body is composed largely of water, and has very low resistance. The skin, however, has very
high resistance, the value depending on its nature, on the possible presence of water, and on whether it
has become burned. Thus, most of the resistance to the passage of current through the human body is at
the points of entry and exit through the skin. A person with naturally hard and dry skin will offer much
higher resistance to shock current than one with soft and moist skin; the skin resistance becomes very
low if it has been burned, because of the presence of conducting particles of carbon.
In fact, the current is limited by the impedance of the human body, which includes self capacitance as
well as resistance. The impedance values are very difficult to predict, since they depend on a variety of
factors including applied voltage, current level and duration, the area of contact with the live system, the
pressure of the contact, the condition of the skin, the ambient and the body temperatures, and so on.

Figure above is a simplified representation of the shock path through the body, with an equivalent circuit
which indicates the components of the resistance concerned. It must be appreciated that the diagram is
very approximate; the flow of current through the body will, for example, cause the victim to sweat,
reducing the resistance of the skin very quickly after the shock commences. Fortunately, people using
electrical installations rarely have bare feet, and so the resistance of the footwear, as well as of the floor
coverings, will often increase overall shock path resistance and reduce shock current to a safer level.
Guidance Note 7 (Special Locations) provides data on the impedance of the human body. However, the
figures are complicated by the fact that values differ significantly from person to person; it would be
sensible to assume a worst case possibility which suggests that the impedance of the human body from
hand to foot is as low as 500 Ohms. Since this calculates to a body current of 460 mA when the body has
230 V applied, we are considering a fatal shock situation.
There are few reliable figures for shock current effects, because they differ from person to person, and for
a particular person, with time. However, we know that something over one milliampere of current in the
body produces the sensation of shock, and that one hundred milliamperes is likely quickly to prove fatal,
particularly if it passes through the heart.
If a shock persists, its effects are likely to prove to be more dangerous. For example, a shock current of
500 mA may have no lasting ill effects if its duration is less than 20 ms, but 50 mA for 10 s could well
prove to be fatal. The effects of the shock will vary, but the most dangerous results are ventricular
fibrillation (where the heart beat sequence is disrupted) and compression of the chest, resulting in a
failure to breathe.
The resistance of the shock path is of crucial importance. The Regulations insist on special measures
where shock hazard is increased by a reduction in body resistance and good contact of the body with
earth potential. Such situations include locations containing bath tubs or showers, swimming pools,
saunas and so on. .
Another important factor to limit the severity of electric shock is the limitation of earth fault loop
impedance. Whilst this impedance adds to that of the body to reduce shock current, the real purpose of
the requirement is to allow enough current to flow to operate the protective device and thus to cut off the
shock current altogether quickly enough to prevent death from shock.
How quickly this must take place depends on the level of body resistance expected. Where sockets are
concerned, the portable appliances fed by them are likely to be grasped firmly by the user so that the
contact resistance is lower. Thus, disconnection within 0.4 s is required. In the case of circuits feeding
fixed equipment, where contact resistance is likely to he higher, the supply must be removed within 5 s.
For situations where earth contact is likely to be good, such as farms and construction sites,
disconnection is required within 0.2 s.

Contact with live conductors
In order for someone to get an electric shock he or she must come into contact with a live conductor. Two types of contact are classified.
1 - Direct contact
An electric shock results from contact with a conductor which forms part of a circuit and would be
expected to be live. A typical example would be if someone removed the plate from a switch and touched
the live conductors inside (see {Fig 3.7}). Overcurrent protective systems will offer no protection in this
case, but it is possible that an RCD with an operating current of 30 mA or less may do so.

2 - Indirect contact.
An electric shock is received from contact with something connected with the electrical installation which
would not normally be expected to be live, but has become so as the result of a fault. This would be
termed an exposed conductive part. Alternatively, a shock may be received from a conducting part which
is totally unconnected with the electrical installation, but which has become live as the result of a fault.
Such a part would be called an extraneous conductive part.
 Danger in this situation results from the presence of a phase to earth fault on the kettle. This makes the kettle case live, so that contact with it, and with a good earth (in this case the tap) makes the human body part of the shock circuit.
To sum up this subsection: Direct contact is contact with a live system which should he known to
he dangerous and Indirect contact concerns contact with metalwork which would he expected to
he at earth potential, and thus safe. The presence of socket outlets close to sinks and taps is not
prohibited by the IEE Wiring Regulations, hut could cause danger in some circumstances. It is
suggested that special care be taken, including consultation with the Health and Safety Executive
in industrial and commercial situations.

Protection from contact
Four methods of protection are listed in the Regulations.
1. Protection by separated extra-low voltage (SELV)
This voltage is electrically separated from earth and from other systems, is provided by a safety source,
and is low enough to ensure that contact with it cannot produce a dangerous shock in people with normal
body resistance or in livestock. The system is uncommon.
2. Protection by protective extra-low voltage (PELV)
The method has the same requirements as SELV but is earthed at one point. Protection against direct
contact may not be required if the equipment is in a building, if the output voltage level does not exceed
25 V rms or 60 V ripple-free dc in normally dry locations, or 6 V rms ac or 15 V ripple-free dc in all other
locations.
3. Protection by functional extra-low voltage (FELV)
This system uses the same safe voltage levels as SELV, but not all the protective measures required for
SELV are needed and the system is widely used for supplies to power tools on construction
. The voltage must not exceed 50 V ac or 120 V dc. The reason for the difference is partly that
direct voltage is not so likely to produce harmful shock effects in the human body as alternating current,
and partly because the stated value of alternating voltage is r.m.s. and not maximum.
 shows, such a voltage rises to a peak of nearly 71 V, and in some circumstances twice this
voltage level may be present. The allowable 120 V dc must be ripple free
4. Protection by limitation of discharge energy
Most electrical systems are capable of providing more than enough energy to cause death by electric
shock. In some cases, there is too little energy to cause severe damage. For example, most electricians
will be conversant with the battery-operated insulation resistance tester. Although the device operates at a lethal voltage (seldom less than 500 V dc) the battery is not usually capable of providing enough energy to give a fatal shock. In addition, the internal resistance of the instrument is high enough to cause a volt drop which reduces supply voltage to a safe value before the current reaches a dangerous level. This
does not mean that the device is safe: it can still give shocks which may result in dangerous falls or other physical or mental problems.
The electric cattle fence is a very good example of a system with limited energy. The system is capable of providing a painful shock to livestock, but not of killing the animals, which are much more susceptible to the effects of shock than humans.


Direct contact protection
The methods of preventing direct contact are mainly concerned with making sure that people cannot
touch live conductors. These methods include:
1. - the insulation of live parts - this is the standard method. The insulated conductors should be further protected by sheathing, conduit, etc.
2. - the provision of barriers, obstacles or enclosures to prevent touching (IP2X). Where surfaces are
horizontal and accessible, IP4X protection (solid objects wider than 1 mm are excluded applies
3. - placing out of reach or the provision of obstacles to prevent people from reaching live parts
4. - the provision of residual current devices (RCDs) provides supplementary protection  but only when contact is from a live part to an earthed part.
Indirect contact protection
There are three methods of providing protection from shock after contact with a conductor which would not normally be live:
1. - making sure that when a fault occurs and makes the parts live, it results in the supply being cut off within a safe time.
2. - cutting off the supply before a fatal shock can be received using a residual current device{5.9}.
3. - applying local supplementary equipotential bonding which will ensure that the resistance between
parts which can be touched simultaneously is so low that it is impossible for a dangerous potential
difference to exist between them. It is important to stress that whilst this course of action will eliminate the danger of indirect contact, it will still be necessary to provide disconnection of the supply to guard against other faults, such as overheating.