One of the major changes in the 17th Edition of the IEE Wiring Regulations is the requirement of RCD’s to protect circuits where the cables are buried in walls at a depth of less than 50mm and not mechanically protected. In a domestic property this is likely to include most if not all the circuits. So what exactly is an RCD?
An RCD is an electrical safety device specially designed to immediately switch the electricity off when electricity “leaking” to earth is detected at a level harmful to a person using electrical equipment. An RCD offers a high level of personal protection from electric shock. Fuses or overcurrent circuit breakers do not offer the same level of personal protection against faults involving current flow to earth. Circuit breakers and fuses provide equipment and installation protection and operate only in response to an electrical overload or short circuit. Short circuit current flow to earth via an installation’s earthing system causes the circuit breaker to trip, or fuse to blow, disconnecting the electricity from the faulty circuit. However, if the electrical resistance in the earth fault current path is too high to allow a circuit breaker to trip (or fuse to blow), electricity can continue to flow to earth for an extended time. RCDs (with or without an overcurrent device) detect a very much lower level of electricity flowing to earth and immediately switch the electricity off.
RCDs have another important advantage – they reduce the risk of fire by detecting electrical leakage to earth in electrical wiring and accessories. This is particularly significant in older installations.
How they work?
RCDs work on the principle “What goes in must come out”. They operate by continuously comparing the current flow in both the Active (supply) and Neutral (return) conductors of an electrical circuit. If the current flow becomes sufficiently unbalanced, some of the current in the Active conductor is not returning through the Neutral conductor and is leaking to earth.
RCDs are designed to operate within 10 to 50 milliseconds and to disconnect the electricity supply when they sense harmful leakage, typically 30 milliamps.
The sensitivity and speed of disconnection are such that any earth leakage will be detected and automatically switched off before it can cause injury or damage. Analyses of electrical accidents show the greatest risk of electric shock results from contact between live parts and earth.
Contact with earth occurs through normal body contact with the ground or earthed metal parts. An RCD will significantly reduce the risk of electric shock, however, an RCD will not protect against all instances of electric shock. If a person comes into contact with both the Active and Neutral conductors while handling faulty plugs or appliances causing electric current to flow through the person’s body, this contact will not be detected by the RCD unless there is also a current flow to earth.
On a circuit protected by an RCD, if a fault causes electricity to flow from the Active conductor to earth through a person’s body, the RCD will automatically disconnect the electricity supply, avoiding the risk of a potentially fatal shock.
Examples of equipment recomended to be protected by a RCD:
• Hand held electric power tools, such as drills, saws and similar equipment.
• Tools such as jack-hammers, electric lawn mowers.
• Equipment on construction sites.
• Equipment such as appliances which move while in operation, such as vacuum cleaners and floor polishers.
• Appliances in wet areas such as kitchens, including kettles, jugs, frying pans, portable urns, food mixers/blenders.
• Hand held appliances such as hair dryers, curling wands, electric knives etc.
• Cord extension leads.