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RCD Basics: Selecting the correct Type of RCD - …

Doepke 1 Doepke Selecting the correct type of RCD_ Amendment 3 BS7671 v1 April 16 Technical Publication 08 RCD basics : Selecting the correct Type of RCD Amendment 3 BS7671 The Electrical Safety Council have estimated, that around million people receive an electric shock each year, of which 350,000 receive a serious injury. The appropriate application and use of the correct Type of RCD, reduces the risk of electrocution in the event of receiving an electric shock. They are also used to reduce the fire hazard associated with residual currents. Selecting the correct Type of RCD and sensitivity requires an understanding of the existing Regulations, RCD characteristics, the installation design / layout /risks and the characteristics of the loads that will be connected to the RCD protected circuit.

Doepke 1 Doepke Selecting the correct type of RCD_ Amendment 3 BS7671 v1 April 16 Technical Publication 08 RCD Basics: Selecting the correct Type of RCD – Amendment 3 BS7671 The Electrical Safety Council have …

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Transcription of RCD Basics: Selecting the correct Type of RCD - …

1 Doepke 1 Doepke Selecting the correct type of RCD_ Amendment 3 BS7671 v1 April 16 Technical Publication 08 RCD basics : Selecting the correct Type of RCD Amendment 3 BS7671 The Electrical Safety Council have estimated, that around million people receive an electric shock each year, of which 350,000 receive a serious injury. The appropriate application and use of the correct Type of RCD, reduces the risk of electrocution in the event of receiving an electric shock. They are also used to reduce the fire hazard associated with residual currents. Selecting the correct Type of RCD and sensitivity requires an understanding of the existing Regulations, RCD characteristics, the installation design / layout /risks and the characteristics of the loads that will be connected to the RCD protected circuit.

2 Terms associated with RCD selection (simplified definitions to aid basic understanding for this paper) RCD Generic term for a fault breaking device operated by residual current (I n), common formats include RCCBs and RCBOs. RCCBs (EN61008) do not include short circuit and overcurrent protection, must be provided by external fuses and or MCBs. RCCBs are manufactured in ratings <125A with a wide variation in sensitivity and types . RCBOs (EN61009) include short circuit and overcurrent protection, but are normally limited by their characteristics to final circuit protection applications < 50A. CBRs (EN947-2 Apx.)

3 B) MCCB with an in-built residual current protection module for applications >100A. Note: RCMs (Residual Current Monitors) cannot be used in place of RCDs Earth Leakage Current (Protective Conductor Current) This is the current that should flow to earth via the protective (earth) conductor under normal operation. Electrical current behaves a bit like water, in that it will flow to earth via all paths of insufficient resistance, not just the lowest path of resistance. In the event of the loss of the PE conductor, a person touching uninsulated parts of the equipment would be subject to the leakage current that normally flows through the PE conductor.

4 Leakage current increases as capacitance* increases and as a function of frequency harmonic currents generated at higher frequencies will produce higher leakage currents for the same load. *Capacitance: Natural circuit capacitance + capacitance associated with electrical loads connected to the supply. Residual Current Term used to distinguish between the earth leakage current present in a healthy circuit and current that flows in the event of an insulation fault to earth. The location of the fault within the equipment/circuit will determine the basic characteristics of the residual current, which may be significantly different to the leakage current produced in normal operation.

5 Refer to the comments later, relating to equipment manufactures installation instructions and RCD selection. Note: RCDs do not distinguish between residual currents and earth leakage currents. Rated residual operating current I n RCD sensitivity I n is defined in milliamps: RCDs must not trip when subjected to a 50% I n, but must trip within defined time in multiples for I n. If the residual current >50% of I n the RCD may trip. Time/current characteristics are summarised in BS7671 Appendix 3 Table 3A. RCDs with an I n of 30mA can be applied in clean and dry environments, as an additional protection measure , for basic protection and fault protection: See 410 and 415.

6 Applications requiring 30mA must only use non delay RCDs, In the UK, Time delay or Selective RCDs S must only be applied in ratings 100mA and above See Regs Apx 3 Table 3A . RCDs with an I n 100mA provide fault protection through automatic disconnection of the supply / fire protection only. Note : Where risks of fire exist and the wiring system is protected by an RCD, I n must be 300mA. Nuisance tripping RCDs may trip with no apparent fault present: This can be the result of supply transients, harmonics and the accumulation of earth leakage currents, due to incorrect installation design and segregation of circuit: See Some applications and locations may require transient resistant RCDs to reduce the risk of nuisance tripping see Table 1.

7 Doepke 2 Doepke Selecting the correct type of RCD_ Amendment 3 BS7671 v1 April 16 Technical Publication 08 types of RCD The equipment connected in circuit determines the Type of RCD required, based on the characteristics of the leakage current and residual current. Electrical loads are characterised by the current they draw from the supply, and the effects on the shape of the supply waveform linear or non-linear load - see examples below. Linear loads connected to 50Hz supply produce sinusoidal residual currents at 50Hz. Non-linear loads produce complex leakage and residual currents with various characteristics including High frequency AC, Pulsed DC and Smooth DC components.

8 Type AC RCDs are only designed for use with linear loads that produce sinusoidal residual currents at 50Hz. It is dangerous to connect non-linear loads to circuits using Type AC RCDs. Using the wrong type of RCD can result in total loss of protection RCD Blinding . For an explanation of this term see Technical Publication 06. Manufacturers of equipment containing power electronic converter systems (PECS*), must define clearly the safety requirements for RCD compatibility when their equipment is connected to and RCD protected circuit. Safety is a basic requirement of the UK Product Safety Regulations.

9 (*Safety Reference Standard for PECS IEC62477-1 Annex H). If this information is not provided, Type B RCDs would have to be used by default. Table 1 summarises the main characteristics of the different types of RCD based on the protection performance requirements of existing regulations and standards. For specific manufacturers characteristics refer to their data. Doepke 3 Doepke Selecting the correct type of RCD_ Amendment 3 BS7671 v1 April 16 Technical Publication 08 Selecting the correct Type of RCD Non-linear loads including Ovens, Washing machines, AV/PC power supplies, HVAC, Ground source heat pumps, Jacuzzis, Lighting control, EVCP and PV Inverters, use power electronics to control the output of the equipment.

10 The residual current under fault conditions will be defined by the internal design and components included in the equipment and the location of the earth fault. Consequently only the manufacture can define accurately the characteristics of the residual current, under fault conditions and therefore the Type of RCD. If this information is not available, the Designer / Constructor to meet BS7671, would have to use an RCD that covers all of the possible scenarios Regulation : The protective devices shall operate at values of current, voltage and time which are suitably related to the characteristics of the circuits and to the possibilities of danger.


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