Chapter Seven - Electrical Safety
and Lock-out/Tag-out

Dangers and Injuries Associated with Electricity

There are several ways in which personal injury may be caused:

1.  Personal Injury

(a) Shock

Electric shock is the effect produced on the body, particularly its nervous system, by electric current passing through it, and its effect depends on current strength (which in turn depends on voltage) the path the current takes through the body, the surface resistance of the skin and several other factors.

A voltage as low as 15V can produce discernible shock effects and 70V has been known to cause death.  Generally speaking however, fatalities occur from this cause at t the normal domestic and industrial voltage of 240V A.C. and from currents of 25-30 milliamps.

The consequences of electric accidents are first of all related to the type of electric risk to which a person is exposed.  In second place, to the physical characteristics of the person involved, and, finally, to the context of the installations and the environment, which can escalate from a simple scare by contractions caused by a brief touch, up to loss of life.

Results of Physical Contact with Current/Voltage

k (60 Hz)	Sensation
Less than 1.0 mA	Perception Limit
From 6.0 to 8.0 mA	Tingling, annoyance
From 8.0 to 25 mA	Discomfort, cramps
From 25 to 50 mA	Loss of control, asphyxia
From 50 to 100 mA	Ventricular fibrillation
More than 100 mA	Shock, cardiac arrest

Consequences of Low-Voltage Contacts

Contrary to their name, low voltages in normal service are very risky due to their intensive use in all daily activities, which increases the probability of accidents.

• The most dangerous Direct Contacts, are those which occur under humid conditions, followed by dry contacts.  In both cases, a prolonged contact of more than 3 seconds causes, first of all, ventricular fibrillation and then death by asphyxia and/or cardiac arrest.  The blood and lymph liquids and muscular water continue boiling and vaporizing until carbonization occurs.

• The insertion of a resistance, in the case of Indirect Contacts, usually permits an instinctive evasive reaction by the person involved, causing only sharp contractions and distensions of the striated muscles, giving the impression of having been strongly "thrown", which could cause another type of accident due to the fall.

Consequences of Medium and High Voltage Discharges

Despite the gradients that originate the discharge process, Medium and High Voltages in normal service are statistically a lower risk probability because they are not massively used and involves crossing safety or security barriers.  Nevertheless, their consequences, although not always lethal, can be disastrous for the victim.

• In discharges from an energized conductor, the electric power arc that is formed has a current equivalent to that of a short Grounding-Phase circuit, but it closes at the end of its action time, which permits speedy rescue.

• In discharges from a charged but out-of-service conductor, the electric are involves transmission of a large amount of electric power in a brief instant, with a current that diminishes drastically and tends to remain at low intensity.  Rescue must be carried out very carefully.

Consequences of Voltages due to a Major Grounding Fault

Accidents by potentials due to the dispersion of soil grounding faults, both in substations and in electric line structures, have a low case history and low probability of occurrence, both due to adequate designs of grounding systems and because they are sometimes distant from the pedestrian traffic in such installations, which are also surrounded by clearance or easement areas.  However, accidents by poor control of these potentials are usually lethal due to shock or cardiac arrest for the people and any large animals involved.

• Touch Voltages are the ones that cause the larger voltage gradients inside substation yards or at the foot of electric line structures or supports.

• Passing Voltages inside substations originate lower voltage gradients.  However, these can be higher in the periphery of the same and of electric line structures and supports.

Death

The most common cause of death from shock is suffocation and it is highly desirable that persons dealing with electricity should be trained in resuscitation, with practice in both artificial respiration and in cardiac massage.

Minor shocks may not in themselves be serious, but can lead to serious consequences, for example, the muscle contraction which they cause may lead to falls from working platforms or ladders.

(b)  Burns

These are caused 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 intense heat generated by arcing from a short-circuit.  All cases of burns require immediate medical attention.

(c)  Explosion

Where flammable gases or vapors are present, special care is necessary in the design and selection of electrical equipment.  In such areas, all equipment should be fully flameproof.

In some cases it is simpler and more economic to isolate the electrical equipment from the flammable vapors - for example with refrigerators used to store flammable solvents the thermostat should be mounted external to the cabinet so that any sparking which occurs is harmless.  DO NOT STORE SOLVENTS IN NON-SPARK PROOF REFRIGERATORS OR FREEZERS.

2.  Fires

Fires may be caused by any of the following:

(a)  Sparks

A spark arises from a sudden discharge through the air between two conductors, or from one conductor to earth. The current produced is usually small, so that serious fires are unlikely unless explosive gases or vapors are present, or highly flammable material is in contact with the conductor.

(b)  Short Circuits

A short circuit is formed when the current finds a path from the outward conductor wire to the return wire other than through the equipment to which it is connected.  The current flow may be large because of the low resistance of the leads, and arcing often occurs at the contact between the conductors.  Insulation may, therefore, be burned and set fir to adjacent flammable material.

(c)  Overloading and Old Wiring

Wiring must not be overloaded, otherwise it will overheat and the insulation will be damaged.  This can lead to a short circuit at some point in the length of the conductor, or more likely at connection points.

The insulation of wiring which has been in use for a number of years tends to become brittle, and where alteration or additions are required, the installed cable must always be checked by a competent electrician, and replace completely if there are indications of failure of the insulation.

3.  Safety Measures

(a)  Protection

Cables must be of sufficient size to carry the current which can flow through them in both normal and abnormal conditions and must be adequately insulated for reasons of safety and of preventing mechanical damage.  Those cable which provide the basic services within a building are normally housed in conduit or troughs:  particular care is required where apparatus is wired up form socket outlets, and where no such permanent protection is feasible.  Such cable must be sufficiently robust to withstand the wear and tear of laboratory use, and fully waterproof where water supplies may be available within the vicinity of the apparatus.  Protection against insulation failure must be provided by either fuse or circuit breaker.

• Fuse:  This device will open a circuit when a predetermined excess of current flows.  It may be able to be rewired, or alternatively, may incorporate a wire embedded in the insulating powder within a cartridge case.  The cartridge fuse is generally more satisfactory.

• Circuit Breaker:  This is a form of switch which opens automatically if the circuit it controls is overloaded: it may operate on either a thermal or magnetic principle.  It is essential to select the correct rating of fuse or circuit breaker for any particular current.

(b)  Earthing

The external metal casing of electrical apparatus, cables and conduit must be earthed as a legal requirement.  The reasons for this are:

1. to prevent the casing rising to dangerous voltage if some fault arises, for example, a short-circuit between conductors and casing;

2. to conduct any current away by a safe path;

3. to ensure that a faulty circuit is automatically disconnected from the supply by drawing sufficient current to blow the fuse or operate the circuit breaker.

New equipment should always be checked to ensure that it is properly earthed before putting it into use.

(d)  Obstruction

The circulation space in laboratories and workshops must be kept clear to prevent hazards from tripping.

(e)  Small Equipment and Tools

Electrical equipment and tools in laboratories and workshops should be regarded as being normal industrial use, and every precaution for safe handling must be taken.  This category would include:

• Lamps and measuring instrument

• Electrical machines to provide mechanical loads or drives

• Power tools and soldering irons to work on apparatus

In all instances the connection of these items of equipment to the mains must be correctly made by a competent person.

If you are connecting a plug, make sure the wires are connected to the correct terminals.

Remove only the required amount of insulation so that no bare conductors are exposed when the connections are made, and remove any "whiskers" which may be present.

In general, permanent apparatus having an incidental use in experimental and research work should be:

1. Fully insulated, with switches and terminals enclosed and protected.

2. Correctly fused, so that the maximum current required can be supplied by any fault is limited to the minimum possible.

3. Correctly connected to the supply, the line being fused and switched with the earth pin connected.  The switch must be inserted into the line of live lead.

4. Inspected and tested at regular intervals of about a year for earth continuity and general condition.

5. Provided with isolating switches, fuses or plugs, so that they may be removed before the equipment is dismantled.

6. Not overloaded, a proper consideration of the load magnitude should be made before the apparatus is connected to the supply.

Do not take chances, if in doubt seek assistance and advice.

Reference:  http://www.cryst.bbk.ac.uk/~ubcg17a/safety/jim006.html

 

Last Update: 2006-09-21