RCD's
RCDs are often known by other names, eg., earth leakage circuit breakers
(ELCB) or safety switches.
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.
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