SCHOOL OF CRYSTALLOGRAPHY

ELECTRICAL SAFETY

Everybody must concern themselves with the subject of electrical safety, since hazards arise from electrical installations or apparatus of any kind. Broadly speaking, the hazards may be categorised as:-

1. Injury to persons

2. Fire

1. Injury to persons

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

(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 the normal domestic and industrial voltage of 240V A.C. and from currents of 25-30 milliamps.

The most common cause of death from sock 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 vapours 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 vapours - 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 vapours 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 fire 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 alterations or additions are required, the installed cable must always be checked by a competent electrician, and replaced 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 cables which provide the basic services within a building are normally housed in conduit or troughs: particular care is required where apparatus is wired up from 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 the rewirable type, or alternatively, may incorporate a wire embedded in 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:-

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

(ii) to conduct any current away by a safe path;

(iii) 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 in normal industrial use, and every precaution for safe handling must be taken. This category would include:

Lamps and measuring instruments

 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:

i) Fully insulated, with switches and terminals enclosed and protected.

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

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

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

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

iv) 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.

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