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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).
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"Industrial Robots and Manufacturing Automation." 123HelpMe.com. 22 Aug 2019
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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 internal fault.
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 Live 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 Live conductor to earth through a person's body, the RCD will automatically disconnect the electricity supply, avoiding the risk of a potentially fatal shock.
How Long have RCD existed?
Devices operating on the RCD principal as described previously have existed since the 1950¡¦s. However these devices were referred to as iELCB (differential current operated device Earth Leakage Circuit Breaker). To remove the confusion, between (the voltage operated device vELCB and the differential current operated device iELCB the IEC (International Electrotechnical Commission) decided to apply the term Residual Current Device (RCD) to differential current operated ELCBs.
Regulations for RCD
[Sections 412-06, 413-02-16 and 413-02-17, 471-08-06 and 471-08-07,471-16,514-12-02 and 531-02 to 531-04, 601-09, 602-07, 608-08]
In brief RCD protection is required for socket outlet where:
1. They are part of a TT system (no earth terminal provided by the Electricity Supply Company) the system used Locally.
2. They are installed in a bedroom which contains a shower cubicle, or where
3. The socket outlet (s) are likely to feed portable equipment used outdoors
4. They are installed for all electrical equipment in an areas within and approximately 2 meters from a swimming pool and comply with BS EN 60309-2
5. Protection by an RCD with a rating of 30 mA is required for fixed electrical equipment installed in a bathroom or 3.5 m from a swimming pool.
The choice of RCD depends upon the application of the degree of protection required.
300 mA provide the means to achieve compliance with the wiring regulations in conditions of poor earth loop impedance and also give a good level of fire risk protection.
100 mA provide the means to achieve compliance with the wiring regulations, a high level of fire risk protection and a degree of indirect shock risk protection.
30 mA for use where a higher degree of protection is required, with portable equipment or equipment used in hazardous conditions e.g. outdoors, or wet environments as per regulations.
10 mA provide a higher degree of personal protection, for use in sensitive areas such as Laboratories, schools and workshops where potential hazards exist from electrical faults caused through misuse, accidental damage or failure of electrical appliances.
Testing and Identifying RCDs
RCDs must be tested to ensure correct operation within the required operating times. RCDs are provided with a built-in self test system which is intended to be operated regularly by the user. RCDs can be identified by the test button which is incorporated into these devices.
Problems With RCDs
As far as users are concerned, the single greatest problem with RCDs is nuisance tripping. This problem will either be due to the design of the RCD or due to the installation. Nuisance tripping can be a frustrating problem for the user and there have been cases where RCDs were strapped out, removed from installations completely or replaced with RCDs with a higher rated trip current. All of these types of actions are very dangerous and should be avoided. Where the problem lies with the RCD design, it is usually attributable to factors such as voltage spikes, surges, switching transients, noise, inrush currents, etc. Over recent years, the problem of nuisance tripping attributable to the RCD itself have been addressed by IEC. New tests have been introduced into the product standards to ensure that RCDs have a reasonably high immunity to nuisance tripping. Unfortunately, all RCDs do not comply with the new requirements. To minimise this problem, ensure as far as possible that the RCD has compliance to IEC61008 or EN61008 (RCCBs) or IEC61009 or EN61009 (RCBOs) and also to IEC61543 or EN61543 (EMC requirements for RCDs).
Sometimes the problem of nuisance tripping is attributable to the installation. IEC recommend that the standing earth leakage current on the installation should not exceed 30% of the rated trip current of the RCD intended to be used on that installation. This means that for a 30mA RCD, the standing earth leakage current should not exceed 10mA. Given that a 30mA RCD may trip anywhere from 15 - 30mA, a 10mA standing leakage current will virtually prime the RCD to trip. In general, RCDs cannot tell the difference between a standing leakage current and an earth fault current. The sum of these two currents is the residual current seen by the RCD and if this aggregate current is greater than its rated trip current, the RCD will trip, unless it's faulty. (but that's another story)
The problem of standing leakage currents is increasing. This is due to two factors:
a. equipment manufacturers filtering internally generated noise to earth
b. fitting of RFI suppression to provide immunity to mains borne noise.
Recently introduced European Directives on EMC require manufacturers of products, appliances, equipment, etc. to contain within specified limits the levels of RFI type emissions produced by their products. Manufacturers often have to resort to the use of filtering circuits to meet these requirements. The filtering circuits can divert high frequency signals to earth, but can also result in the flow of significant levels of leakage current to earth at the standard 50Hz frequency. Under IEC rules, electrical appliances may allow a standing leakage current of up to 3.5mA/50Hz to flow to earth. Such current levels coupled with other sources of standing leakage currents can give rise to nuisance tripping.
Washing machines, oil or gas burners and even immersion heaters can contribute to nuisance tripping. Temperature and humidity may also be factors in nuisance tripping. Other factors contributing to nuisance tripping may be poor earth terminations, surge suppressers, neutral voltage rising above earth potential, etc. A very sneaky problem is where an earth fault current on the supply side of the RCDs manages to appear as a load side fault to the RCD, causing it to trip. This problem is more usually associated with IT systems, or on TT systems with high earth impedance. Before changing the RCD in response to a nuisance tripping problem, check the installation by measuring the standing leakage current and carrying out an earth loop test. Also try to identify equipment that is likely to contribute to standing or transient earth currents. A residual current monitor can be fitted to an installation to detect the level of the standing earth leakage current or even transient leakage currents.
As a result of problems with of nuisance tripping it is usually recommended that outlet circuits be proctected by 30mA RCDs while circuits controlling the lighting circuits are protected by RCDs with a higher rating of say 100mA.
Types of RCDs and Methods of RCD implementations
RCCB residual current operated circuit breaker. These are usually used as a main switch, thereby affording RCD protection to all sub circuits. This implementation of RCD protection is very vulnerable to of nuisance tripping except when used within a split panel where various ratings of RCD protection can be implemented.
RCBO is an RCCB which includes overcurrent. These would be used to replace breakers and as such would provide RCD protection to individual sub-circuits, which could be a set of outlet or set of lights within an installation. The method affords greater discrimination and hereby reduces the level of nuisance tripping. However typically a installation will require a number of these devices to ensure the requirements of the regulations are met, thereby increasing the cost of implementation.
SRCD socket outlet incorporating an RCD, protects only the device plugged into the socket outlet. This method of implementation is a very costly method of achieving the requirement of the regulations.
PRCD portable RCD, usually an RCD incorporated into a plug, protects only the device connected to the plug.