Thursday, August 21, 2008

RCD - residual current device

What is RCD?
The simple definition as given here is: What current flow goes in must also come out. Other names by which these circuits are known are earth leakage circuit breakers (ELCB) or safety switches.
An RCD is an electrical safety device designed specifically to switch the electricity immediately off when an electricity "leaking" (to earth) is detected at a level harmful to a person who is using an electrical equipment. An RCD circuit 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 much lower level of electricity flowing to earth and immediately switch the electricity off.
RCDs are thus designed to prevent electrocution by detecting the leakage current, which can be far smaller (typically 5–30 milliamperes) than the currents needed to operate conventional circuit breakers or fuses (several amperes). RCDs are intended to operate within 25–40 milliseconds, before electric shock can drive the heart into ventricular fibrillation, the most common cause of death through electric shock.
How do the RCDs work?
According to the principle"what goes in must come out"(or, more technically, the sum of all the currents meeting in any point in a circuit must be zero, otherwise the circuit will be broken - the Node Law), the residual current device operates by continually comparing the current flow both in the Active(the "IN" part, the Supply) and Neutral(the "OUT" part, the Return) conductors of the electrical circuit. If the current flow becomes unbalanced(there are some leakages detected), it means that some of the current in the Active conductor is not returning back through the Neutral conductor and is leaking to the earth - this triggers the device, which automatically closes the circuit. The time frame in which the RCDs are designed to operate is within 10 to 50 milliseconds, and the harmful leakage at which, when detected, they disconnect the circuit is typically 30 mA.
The Residual Current Devices are designed in such a way that it senses, in a very quick time-wise way any earth leakage of power and automatically switches off the circuit, before it can cause injury or damage. The analyses of the electrical accidents in the past show that the most common and greatest risk of an electric shock comes from the contact between live parts(electrical devices) and earth, via either the human body or conductors(metal parts, etc).
Even though an RCD will not protect against all instances of electric shock, its function is to at least significantly reduce its risk. If someone touches both the Active and Neutral conductors(and the plugs or electrical appliances are faulty), he will cause the electric current to flow through his body - this contact will not be detected by the RCD unless there is also a current flow to earth. If a circuit is protected by RCD and a fault causes electricity to flow from the Active conductor to earth through a person's body or through a metal conductor, the RCD will automatically disconnect the electricity supply - avoiding the risk of a fatal shock or short circuit.
Many electrical fires are caused by such electrical power leakages to the earth, due to faulty wiring. Even a leakage current of 1A or a little less can start a fire; normal protection devices will not detect such low current levels. This is why there was the need of the RCDs, who detect low current level leakages to earth, and trip out the circuit, thus greatly reducing the risk of an electrical fire.
RCD sensitivity
10mA RCDs offer a high degree of protection against electrocution in an accidental shock hazard situation. They are of particular value in a high risk area where resistances external to the body are likely to restrict the earth fault current flowing through the body to less than 30mA and where 110V supply is being used
30mA RCDs offer a high degree of protection in an accidental shock hazard situation and are by far the most popular sensitivity used in the United Kingdom. In a shock situation, the current flowing through the human body at 250V 50Hz could be between 80 and 240mA, depending on the resistance of the body in question. To ensure that there are no harmful physiological effects in such a situation, it is necessary for the RCD to operate within 300mS at 30mA and 40mS at 150mA.
100mA RCDs may, in some circumstances, provide protection against electrocution in an accidental shock hazard situation. However, it is important to note that there is likelihood that the earth fault current may be below the sensitivity of the RCD. This becomes increasingly likely if additional resistances to that ofthe human body are in the current path.
300mA RCDs provide protection against the risk of fire only. They do not provide protection against electrocution in an accidental shock hazard situation. A typical application would be lighting circuits where it is deemed that the risk or electric shock is small.
It is important to note that a current of less than 500mA flowing in a high resistance path is sufficient to bring metallic parts to incandescence and, potentially, initiate a fire.
Electrical equipment recommended to be RCD protected:
Small and medium electric power tools, such as electric drills, electric saws, etc.
Large power tools such as the jack-hammers, electric lawn mowers, etc.
The equipments on the construction sites.
Equipment such as the appliances which move while in operation, such as the vacuum cleaners, the floor polishers, etc
The appliances located in wet areas such as in kitchens, bathrooms, etc, including kettles, jugs, frying pans, portable urns, food mixers/blenders, sockets , etc.
Hand held appliances such as hair dryers, curling wands, electric knives etc.
Cord extension leads, consumer units and switches , modules.
More details/other articles that talk about RCD: wikipedia, pat, idc, blue room, RCDs and examples, residual current device, and RCD.

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