Industrial Robots and Manufacturing Automation

Topics: Residual-current device, Circuit breaker, Electric current Pages: 7 (2025 words) Published: June 2, 2005
Since early 2003 the Electrical Inspectorate has been working to raise public awareness of Residual Current Devices (RCDs), with the aim of eventually enforcing the use of these devices as recommended by the standard adopted by the St. Lucia Bureau of Standards for wiring of buildings SLNS/BS 7671: 2001- Requirements for Electrical Installations ¡V IEE Wiring Regulations ¡V Sixteenth Edition. These Regulations though only recently adopted by the Bureau of Standards is the Regulations referred to by the St. Lucia Electricity regulations of 1973 as the standard of wiring of buildings. For a very long time the cost of these devices had proven prohibitive for the enforcement of the requirement. However, it is now believed that safety concerns as it relates to electrical hazards such as fire and electrocution far outweigh the cost of these devices

What is an RCD?

An RCD is an electrical safety device specially designed to save life from Electrocution by immediately switching the electricity off when electricity ¡§Leaking ¡¨ to earth is detected at a level that maybe harmful to a person using electrical equipment.

How Electricity Kills

Electrocution occurs when a small, specific amount of electrical current flows through the heart for 1 to 3 seconds. 0.006-0.2 Amps (i.e. 6-200 milliamps) of current flowing through the heart disrupts the normal coordination of heart muscles. These muscles lose their vital rhythm and begin a process known as ventricular fibrillation. Death soon follows.

0.25 Amps/250mA is equal to the current flowing through a 60W Bulb.

What is ventricular fibrillation?

The heart beats when electrical signals move through it. Ventricular fibrillation is a condition in which the heart's electrical activity becomes disordered. When this happens, the heart's lower (pumping) chambers contract in a rapid, unsynchronized way. (The ventricles "flutter" rather than beat.) The heart pumps little or no blood therefore the probability of death is high.

What is the advantage of RCDs over Breakers and Fuses?

Fuses or over current circuit breakers protect wires from damage not human beings and lifestock. On the other hand RCD with a maximum tripping current of 30mA provide protection from electrical shock.

Circuit breakers and fuses provide equipment and installation protection and operate only in response to an electrical overload or short circuit. Hence these devices cannot detect earth fault currents below their operating current. If they are the only means of protection, it is possible for sufficient earth fault current to flow undetected to constitute an electrical hazard (fire and electrocution). This earth fault current also represents wastage of electricity which would inevitably result in higher electricity bills.

By using an RCD, the flow of an earth fault current, above the tripping current of the RCD, is prevented, and thus eliminating any electrically related risk.

How Does an RCD Work?

RCDs work on the principle "What goes in must come out". They operate by continuously comparing the current flow in both the Live (supply) and Neutral (return) conductors of an electrical circuit.

If the current flow becomes sufficiently unbalanced, some of the current in the Live 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 live parts may occur by touching:

„«bare conductors; or
„«internal parts of an appliance; or
„«external parts of an appliance that have become "live" because of an...
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