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What is the risk of injury?
Injury can occur when live electrical parts are exposed and can be touched, or when metalwork which is meant to be earthed becomes live at a dangerous voltage. The likelihood of touching live parts is increased during electrical testing and fault-finding, when conductors at dangerous voltages are often exposed. This risk can be minimised if testing is done while the equipment is isolated from any dangerous source of supply, although this cannot always be done, and care must also be taken to prevent contact with any hazardous internally produced voltages.
The most dangerous injuries are those caused by electric shock. This is because the effects of a shock are largely unpredictable and can easily lead to a fatal injury. However, there is also a risk of burn injuries resulting from arcing when conductors are accidentally short-circuited. A secondary risk can be the harm caused by a person reacting to an electrical injury, for example by falling or being traumatised by the experience.
Electric shocks occur when contact with a live conductor causes sufficient current to pass through the body to cause an injury. As a rough guide, voltages exceeding 50 V ac or 120 V ripple free dc should be considered hazardous in a dry, unconfined, non-conductive location. These voltage values must be reduced if the location is wet, confined or conductive, so where there is an adverse environment, those in charge of the work and those doing the work should be aware of the probable increase in injury risk.
In some equipment, for example microwave ovens, high voltages of several thousand volts are used and there is a very high risk of fatal injury if the exposed conductors are touched at these voltages. Injury may also be caused by currents as low as 5 mA or by stored charges.
Suitable precautions must be taken to prevent contact with stored charges in excess of 350 mJ. If the skin is pricked or cut at the point of contact, the shock current (and hence the seriousness of the injury) will be higher. Healthy skin may also become damaged at the time of contact either by the burning effect of the current or by penetration from sharp-ended conductors.
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