28 September 2013

Earthing or RCD - do I need both?


WHY DO I need an RCD if I have a good earth?

Earthing is a method of protection for an electrical system, whilst a RCD is a device that disconnects the electricity supply when there is a fault. The big advantage of using earth over a RCD as the primary means of earth fault protection is that earth is all around us in abundance and it connects everything. Basically we can tap into it anywhere anytime.

In contrast, a RCD is a mechanical device, and like any device it can malfunction. However, a RCD does provide some protection that earth, sometimes, cannot.

RCD protection is not a substitute for an earthing system, however well the system has been installed and maintained. One should not replace the other - they simply complement each other to improve safety.

TT earthing system
Across Spain the protective earth connection in a dwelling is provided by a local connection independent of any earth connection at the generator of the electricity utility company. This is usually achieved by installing a copper earth spike (grounding rod) near the Consumer Unit. This method is technically referred to as a TT earthing system, and whilst other systems use the neutral as a means of earthing, the advantage of a TT system is that it does not carry the risk of a broken neutral. In locations where power is distributed overhead and TT is used, installation earth conductors are not at risk should any overhead distribution conductor be fractured by, say, a fallen tree or branch.

A disadvantage of a TT earthing system is high earth fault loop impedance. Ground conditions can vary, especially during the summer compared to the winter. The resistance of soil can change and when the earth fault loop impedance is too high for a circuit breaker to disconnect the electricity supply quickly in the event of a fault, an RCD is what’s needed. Hence, TT earthing systems require proper RCD protection.

A RCD is not there simply to back up the earthing system - they both have an important job to perform. Earthing provides a direct path for electricity to dissipate into the ground rather than through you, and the RCD provides additional protection to ensure that this happens very quickly before any real danger occurs.

Always use a RCD with a sensitivity of 30 mAmps or greater, which provides protection against serious electric shock. Periodically test it say every 3 months by pushing the test button.

How to test the earth loop impedance on a TT earthing system >>>