Mikes Blog: Electrical Protective Devices

There are various protective devices that can be used with portable electric power tools. These include the isolating transformer, the earth-leakage circuit breaker and the RCD.

Isolating Transformer

The isolating transformer plugs into the power supply and provides one of the safest ways to use a portable power tool. A portable power tool connected to an isolating transformer is fully isolated from earth. If a fault occurs, you will not receive an electric shock. You must plug the transformer into the power outlet (wall socket) so that it isolates the whole circuit. This is especially important when you’re using tools connected to extension cords. If you plug the transformer into the end of an extension cord, the cord is not protected.

Earth Leakage Circuit Breaker

An earth leakage circuit breaker also plugs into the power supply. It will cut off the power very quickly if a live part is touched or otherwise earths.

Residual Current Devices (RCD)

RCDs are devices that can sense a leakage of current in a circuit, and switch the circuit off. All RCDs legally require a test button to check whether they are operating efficiently and correctly. You should test an RCD regularly to ensure that it will work properly should a fault develop in the circuit. Types of RCD include

The plugin RCD adaptor is suitable for use with most portable electric tools and appliances. You plug the tool, appliance or extension cord directly into the RCD adaptor
The inline RCD extension cord is a safer alternative to a normal extension cord. You can plug it into any power outlet, and it protects the user of a power tool or appliance.
The fixed wired wall mounted RCD is permanently wired into bathrooms, kitchens and laundries. You use this type of RCD to run shavers, hairdryers, smoothing irons and other similar appliances.
The switchboard mounted RCD is permanently installed at the switchboard and can be wired to provide protection to a complete circuit.

Double Insulated Tools

The international symbol for double insulation is a square within a square. Double insulated tools have additional insulation between any exposed metal parts and the insulation of the current-carrying parts, hence the term “double insulation”. A double insulated tool may be used inside or outside, in wet or dry conditions or on earthed metals, but it is sensible to use an isolating transformer with double insulated tools. The lead supplying power to the tool needs to be electrically safe as well, either through an isolating transformer or RCD. Even when double insulted tools marked with a double square are used. Double insulation will not protect the user from risks caused by faulty leads or plugs.