Guide to Residual Current Devices-September2003

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The RCD Handbook BEAMAInstallation Guide to the Selection and Application of Residual Current Devices © BEAMAInstallation 2003 INDEX 1 Overview 1.1 1.2 1.3 4 For the Non-Specialist Principle of RCD Operation Types of Residual Current Device 8 2 Effects of Electricity 2.1 Risk of Electrocution 2.2 Types of Electrocution Risk 2.3 Effects of Electric Shock on the Human Body Electric Shock Protection 3.1 Principles of Shock Protection 3.2 Earthing Systems 3.3 Protection against Direct and Indir
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  The RCD Handbook BEAMA Installation Guide to theSelection and Application of Residual Current Devices © BEAMA Installation 2003  INDEX 1Overview4 1.1For the Non-Specialist1.2 Principle of RCD Operation1.3Types of Residual Current Device 2Effects of Electricity8 2.1 Risk of Electrocution2.2 Types of Electrocution Risk 2.3Effects of Electric Shock on the Human Body 3Electric Shock Protection12 3.1Principles of Shock Protection3.2Earthing Systems3.3Protection against Direct and Indirect Contact3.4 RCDs and Indirect Contact Shock Protection3.5 RCDs and Direct Contact Shock Protection3.6 RCDs in Reduced and Extra-low Voltage Applications 4Fire Protection19 4.1General4.2Protective Measures as a Function of External Influences 5Case Studies20 5.1General5.2Typical Risks5.3Case Histories 6RCD Selection23 6.1RCD Selection Criteria6.2RCD Selection Guides 7Operation and Maintenance29 7.1Testing by the End User7.2Testing by the Installer7.3 Troubleshooting7.4 Detailed Fault-Finding in RCD Installations (See Chapter 9) 8RCD Construction33 8.1Voltage Independent RCD8.2Voltage Dependent RCD 9Detailed fault-finding on RCD-protected installations34 9.1Mains-borne Transients and Surges9.2Capacitance to Earth9.3Double-pole Switching9.4Cables and Overhead Lines9.5Neutral-to-Earth Faults9.6Double Grounding9.7Conclusions 10Annex 44 10.1Fire Protection - DTI Report (Reference Section 5)10.2Capacitance and inductance in Overhead Lines and Cables10.3References10.4Terms and Definitions BEAMA Installation RCD Handbook Acknowledgements BEAMA Installation acknowledges the support andcontributions of the Institution of Electrical Engineers,theHealth and Safety Executive and the National InspectionCouncil for Electrical Installation Contractors in thepreparation of the RCD Handbook.BEAMA Installation also would like to thank IEC,BSI,IEE andDTI for references to their standards and publications.  1 To appreciate the importance of an RCD it is helpful to understand how much electricalenergy it takes to kill a human being.The smallest fuse used in a normal electric plug is 3Amps;it takes less than one twentieth of that current to kill an adult in less than one tenthof a second. RCD Operation The operation of an RCD can be understood by taking an analogy from the water flowingin a central heating system.A leak may occur when the pipework is damaged or punctured.In the same way a ‘leak’,of electricity can occur when the cable insulation in a circuit is faulty or damaged.In a central heating system the ‘flow’ pipe takes the water from the boiler to the radiators;if the installation is sound the same amount of water will return to the boiler.However,if there is a leak,there will be less water in the return pipe than in the flow pipe.If thesystem had flow detectors in the flow and return pipes,these could be coupled to a valveso that the valve closed when the rate of flow in the return pipe was less than that in theflow pipe.The rate of flow of water can be compared with the current in an electrical circuit and thewater pressure can be compared with the voltage.When the line and neutral currents areequal,the RCD will not trip but when it senses that the neutral current is less than the linecurrent it will trip.In both cases the leakage is detected without actually measuring the leak itself.It is theflow and return rates that are measured and compared.An RCD compares the line andneutral currents and switches off the electricity supply when they are no longer equal.With an RCD the line (brown) and neutral (blue) conductors pass through the core of asensitive current transformer,see Figure 3,the output of which is electrically connected toa tripping system.In a healthy installation the current flows through the line conductor andreturns through the neutral conductor and since these are equal and opposite the coreremains balanced.However,when a leakage of electric current occurs,see Figure 4,the lineand neutral currents are no longer equal;this results in an output from the transformerwhich is used to trip the RCD and disconnect the supply. 5 OVERVIEWResidual Current Devices The use of electricity is so much a part of every day life that it is often taken for grantedand the risks associated with its use at home and at work are underestimated ormisunderstood.In a typical year,19 people die from electric shock in the home and a similar number die inother buildings.Fire brigades are called to 10,000 incidents attributed to electrical faults,of which half are in the home.These domestic fires result in about 600 serious injuries and23 deaths.Residual Current Devices (RCDs) are electrical devices which afford the highest degree of protection against the risks of electrocution and fire caused by earth faults.However,theyare not a panacea for all installation problems;it is therefore important to understand whatthey can and cannot do.Furthermore the different types of RCD available on the marketcan be confusing.This publication has been produced by BEAMA Installation Members for use by specifiers,installers and end users,to give clear guidance on the selection and application of the widerange of RCDs now available.Guidance is also given on the installation and maintenanceof RCDs,including many of the installation conditions that cause ‘unwanted tripping’.Anumber of case studies have been included to demonstrate the benefits of fitting RCDs andthe possible consequences of failing to do so.Most chapters begin with a section that is designed for the non-specialist or end-user.These,and other sections for the end-user,are picked out in blue type.When read in conjunction with BS 7671 Requirements for Electrical Installations (The IEEWiring Regulations Sixteenth Edition),the guidance in this publication will contribute tosafe and reliable installations.There can be no doubt that RCDs give protection against electrocution and can reduce therisk of fire arising from insulation failure in the electrical installation.This level of protectioncan never be equalled by circuit-breakers or fuses alone.The effect on safety,measured byfewer electrocutions and fewer fires,means that RCDs are not only here to stay but theiruse is likely to increase greatly. 1.1 For the Non-Specialist Readers who are familiar with the role and operation of RCDs can skip this section andmove on to section 1.2 “What is an RCD?” An RCD is a device that is designed to provide protection against electrocution orelectrical fires by cutting off the flow of electricity automatically,or actuating an alarm,whenit senses a ‘leakage’ of electric current from a circuit. 4 BoilerPumpFlowDetector 1ValveFlowDetector 2RadiatorBoilerPumpFlowDetector 1ValveFlowDetector 2RadiatorLeak  Figure 2Figure 1 Figure 1. Healthy central heating circuit.The sameamount of water flows inthe ‘flow’and ‘return’pipes. Figure 2. If there is aleak,there will be lesswater flowing in the ‘return’ pipe than in the ‘flow’pipe.This could be used to trip avalve.  The test circuit is designed to pass a current in excess of the tripping current of the RCDto simulate an out-of-balance condition.Operation of the test button checks theelectromechanical integrity of the RCD only.It is important to note,therefore,that thetest circuit does not check the circuit protective conductor or the condition of the earthelectrode.On all RCDs a label instructs the user to check the function of the RCD at regular intervalsand to observe that the RCD trips instantly. 1.2Principle of RCD Operation The basic principle of operation of the RCD is shown in Figure 5.When the load isconnected to the supply through the RCD,the line and neutral conductors are connectedthrough primary windings on a toroidal transformer.In this arrangement the secondarywinding is used as a sensing coil and is electrically connected to a sensitive relay or solidstate switching device,the operation of which triggers the tripping mechanism.When theline and neutral currents are balanced,as in a healthy circuit,they produce equal andopposite magnetic fluxes in the transformer core with the result that there is no currentgenerated in the sensing coil.(For this reason the transformer is also known as a ‘corebalance transformer’).When the line and neutral currents are not balanced they create an out-of-balance flux.Thiswill induce a current in the secondary winding which is used to operate the trippingmechanism.It is important to note that both the line and neutral conductors pass through the toroid.A common cause of unwanted tripping is failure to connect the neutral through the RCD.RCDs work equally well on single phase,three phase or three phase and neutral circuits,but when the neutral is distributed it is essential that it passes through the toroid. Test Circuit A test circuit is always incorporated in the RCD.Operation of the test button connects aresistive load between the line conductor on the load side of the RCD and the supplyneutral. 6 RCDSUPPLYEARTHLOADNLRCDSUPPLYEARTHLOADNL TRIP SUPPLY LN LOAD Test buttonSensingcoilRelay   Figure 3. In an RCD,theline and neutral conductors of a circuit pass through asensitive current transformer.If the line and neutral currentsare equal and opposite,thecore remains balanced. Figure 4. If there is anearth fault the neutral currentwill be lower than the linecurrent.This imbalanceproduces an output from thecurrent transformer which isused to trip the RCD and sobreak the circuit. Figure 5. Schematic of an RCD. Figure 3Figure 4 1.3Types of Residual Current Device RCCB - Residual Current Operated Circuit-Breaker without Integral Overcurrent Protection. A mechanical switching device designed to make,carry and break currents under normal service conditions and to causethe opening of the contacts when the residual current attains a given value under specified conditions.It is not designedto give protection against overloads and/or short circuits and must always be used in conjunction with an overcurrentprotective device such as a fuse or circuit-breaker. RCBO - Residual Current Operated Circuit-Breaker with Integral Overcurrent Protection A mechanical switching device designed to make,carry and break currents under normal service conditions and to causethe opening of the contacts when the residual current attains a given value under specified conditions.In addition it isdesigned to give protection against overloads and/or short circuits and can be used independently of any otherovercurrent protective device within its rated short circuit capacity. SRCD - Socket-Outlet Incorporating a Residual Current Device. A socket-outlet,for fixed installations,incorporating an integral sensing circuit that will automatically cause the switchingcontacts in the main circuit to open at a predetermined value of residual current. PRCD - Portable Residual Current Device. A device comprising a plug,a residual current device and one or more socket outlets (or a provision for connection).It may incorporate overcurrent protection. CBR - Circuit-Breaker Incorporating Residual Current Protection. A circuit-breaker providing overcurrent protection and incorporating residual current protection either integrally (anIntegral CBR) or by combination with a residual current unit which may be factory or field fitted. Note:The RCBO and CBR have the same application,both providing overcurrent and residual current protection.In general,theterm RCBO is applied to the smaller devices whereas CBR is used for devices throughout the current range,with ratings up toseveral thousand amperes,single- and multi-phase.The RCBO and CBR are more strictly defined by the relevant standards. RCM - Residual Current Monitor. A device designed to monitor electrical installations or circuits for the presence of unbalanced earth fault currents.Itdoes not incorporate any tripping device or overcurrent protection. MRCD -  Modular Residual Current Device. An independently mounted device incorporating residual current protection,without overcurrent protection,andcapable of giving a signal to trip an associated switching device.
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