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Double insulation
Equipment such as hand-held domestic electric drills has the added precaution of double insulation. Double insulation is another method of protection against electric shock. It consists of an extra layer of supplementary insulation over the basic insulation, and it gives basic and fault protection together. It is commonly found in power tools.
A double-insulated appliance shows a symbol of a square inside a square, for the two layers of insulation.
| Advantages | Limitations |
|---|---|
| Good level of protection | Needs regular inspection |
| Class II equipment uses double or reinforced insulation and normally needs no protective earth | Damage or unsuitable repairs can defeat protection; use and maintenance must preserve the design |
Residual current devices (RCDs)
An RCD quickly breaks an electrical circuit to prevent serious harm from a continuing electric shock. RCDs give a level of personal protection that ordinary fuses and circuit breakers cannot. An RCD constantly monitors the current flowing through the circuit or circuits it protects. If it detects electricity flowing down an unintended path, such as through a person who has touched a live part, it switches the circuit off very quickly, greatly reducing the risk of death or serious injury.
The best place for an RCD is built into the main switchboard or the socket outlet, because then the supply cables are permanently protected. If that is not possible, a plug that incorporates an RCD, or a plug-in RCD adaptor, can give additional safety. HSE refers to devices rated at no more than 30 mA for additional personal protection in relevant applications. Select the device for the system and risk; this is not a worldwide rule that every RCD has the same rating.
| Advantages | Limitations |
|---|---|
| Main purpose is to protect people from residual current | Nuisance tripping |
| Protects against earth faults as well as electric shock | Does not protect against current overloads |
| Disconnects the circuit automatically when the rated sensitivity is exceeded | Higher cost |
| Can provide additional shock protection for some residual-current faults | Does not cover every current path, such as some line-to-neutral contacts, or replace secure isolation |
Reduced and low voltage systems
One of the best ways to reduce the risk of injury when using electrical equipment is to limit the supply voltage to the lowest needed to get the job done. For example:
- temporary lighting can run at lower voltages, such as 12, 25, 50 or 110 volts
- suitable battery or air-powered equipment can reduce some mains-shock risks, but batteries can still create fire, short-circuit and ignition hazards; select equipment for the actual task
- portable tools designed to run from a 110 volt supply with a centre tap to earth are readily available
| Advantages | Limitations |
|---|---|
| Lower shock risk because of the lower voltage | Cannot be used with every appliance or in every workplace |
Use of competent people
A competent person is someone with the suitable training, skill and knowledge for the task, to prevent injury to themselves and others. More specialised work, such as maintaining high-voltage switchgear or modifying control systems, almost certainly needs additional training and experience.
Summary notes for Element 11 · Cheat sheet · Glossary. A study aid, not published or endorsed by NEBOSH.