Nearly a quarter of all reportable electrical accidents involve portable equipment. The majority of these accidents result in electric shock; others result in fires, eg nearly 2000 fires in 1991 were caused by faulty leads to appliances. A major cause of such accidents is failure to maintain the equipment. The likelihood of accidents occurring and their severity will vary, depending on the type of electrical equipment, the way in which it is used, and the environment in which it is used.
Under no circumstances should a person use electrical apparatus if they have any doubts as to its safety. If in doubt they should consult their Supervisor/Tutor/College or Service Safety Officer (SSO) as appropriate or the Health & Safety Office directly.
Specialised appliances frequently require special precautions to be taken and reference should always be made to the manufacturer's instructions.
In this guidance:
Electric shock is the effect produced on the body, particularly its nervous system, by an electrical current passing through it. The extent of injury depends upon the current strength which in turn depends upon the voltage, the path the current takes through the body, the surface resistance of the skin (much reduced when wet) and several other factors. A voltage as low as 15V can produce discernible shock effects and 70 V has been known to cause death. But, generally speaking, fatalities involve domestic voltages (240Vac) and currents of 25-30 milliamps. The most common cause of death from shock is suffocation and accordingly it is highly desirable that those dealing with electricity should be trained in resuscitation. Minor shocks in themselves may not be serious but they can lead to serious consequences; for example, the associated muscle contraction may lead to falls from working platforms or ladders.
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. Electrical burns are a very unpleasant form of burn and require immediate medical attention.
The main causes of electrically induced explosions are:
- In situations where flammable gases or vapours are present so that a spark could initiate an event. In such environments all electrical equipment should be flame-proofed.
- Where electrical arcing takes place in a confined space causing intense local heating with consequent bursting of the enclosure by the expansion of trapped air.
A large percentage of fires are of electrical origin, caused by one or more of the following:
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.
An arc is a much larger and brighter discharge where the current flow may be hundreds of amps. It usually arises when a circuit is broken or when a conductor melts or fractures leaving a gap across which the current continues to flow. When an arc is struck, the air in the vicinity becomes ionised and forms a conductor which may allow current to flow to a nearby metal framework. Any combustible material in the vicinity could therefore lead to a fire.
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 a contact between the conductors. Insulation may therefore be burned and set fire to adjacent flammable material.
Assessment of risk
|High Risks||would result from the use of an electrically powered pressure water cleaner outside, powered by 240 volt electrical supply, with the cable trailing on the ground where it can be damaged by vehicles and other equipment, and where water is present. Damage to the cable or other parts is likely to result in the operator or others receiving an electric shock. Similar risks result when other electrical equipment such as drills and portable grinders are used in harsh environments, eg construction sites, where there is a high probability of mechanical damage resulting in danger.|
|Medium Risks||would result from floor cleaners, kettles, hand held office equipment, which are usually used in a more benign environment, eg offices, but can be subject to intensive use and wear. This can eventually lead to faults which can also result in a shock, burns or fire.|
|Low Risks||would result from infrequently moved but reasonably regularly used items such as desk lamps analytical instruments, vacuum pumps, heaters)|
|Very Low Risks||arise from specialised equipment, eg information technology (IT) equipment (computers and printers), photocopiers, fax machines etc. They are usually double insulated, are used in dry clean environments and are hardly ever moved or insulation stressed.|
Equipment which is held by hand or is handled when switched on will present a greater degree of risk because, if a dangerous fault occurs, then the person holding it will almost certainly receive an electric shock.
The risk of receiving an electric shock will be greater when the equipment user is standing on the ground outside or a concrete floor, scaffolding or similar which is a good conductor, than if standing on a wooden floor or dry carpet and not in contact with earthed metal work (ie using double insulated appliances or 110 volt tools which have a centre tapped transformer to give 55 volts between live and earth).
Because the consequences of an accident are so serious – potentially fatal electric shock, or fire affecting the whole premises – the inspecting and testing system is designed to be proactive , ie planned to prevent incidents arising, rather than reactive where action is taken following an incident/accident. The frequency of inspection and testing is directly related to risk.
The greatest overall reduction of risk will take place when the inspection and testing regime is first put into practice. Thereafter it will take time to establish the appropriate test frequency based on experience. A low failure rate would indicate that the test interval can be increased and a high failure rate that the interval should be shortened.