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RCD Basics: Selecting the correct Type of RCD – Amendment 3 BS7671

The Electrical Safety Council have estimated, that around 2.5 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.

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. 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. Leakage current increases as capacitance* increases and as a function of frequency i.e. 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. 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. 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 532.1: 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 314.1. Some applications and locations may require transient resistant RCDs to reduce the risk of nuisance tripping – see Table 1.

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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.

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. (*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.

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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. 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 i.e. Regulation 132.8: “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. “ The Inspector cannot sign-off the installation if it does not meet BS7671.

The table below gives some examples of generic loads and typical RCD requirements. For specific applications refer to the equipment Manufacturer’s instructions and the requirements of the Regulations, including Section 7 for special installations and locations.

Check the equipment manufactures installation and operating instructions

Equipment sold in the UK has to comply with the Product Liability and Safety Directives. Consequently there is a requirement in Law to provide information for safe installation and use, including any risks associated with the product. To quote from the Courts in cases of prosecution, Instructions and or Labelling must be “adequate, accurate effective” and “clear”.

Importers of equipment from outside of the EU, are responsible for safety instructions provided with the equipment they are selling in the UK. Check the installation instructions, relating to the Type of RCD and operational leakage currents associated with the equipment being connected to a UK electrical supply. If this is not given in the installation instructions, ask the OEM to provide the information in writing.

Some inverter based equipment may not be suitable for use with RCDs of any Type, due to the operational leakage currents. Verify with the OEM before making decisions on final circuit designs and product purchases.

It is an offence under various UK Laws, to install equipment without the appropriate Manufacturer’s instructions.

Regulation 114.1 explains the relationship between BS7671, Codes of Practice and statutory Regulations, relating to installation design / performance and equipment used in or connected to the installation – see Reg 120.3

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Design for Safe and Reliable operation using the correct Type of RCD BS7671 gives clear and detailed advice relating to the application of RCDs to achieve fault and additional protection, based on sensitivity and maximum disconnection times. Some of the more common applications requiring different Type of RCD are detailed in the BS7671 Section 7.

The requirements to select the correct Type of RCD for ALL applications, may not be so obvious for those who are not familiar of the detailed clauses in BS7671 and the safe application of RCDs.

Reg 132.8 highlights the need for RCDs (protection devices), to operate at the required values of current and time “suitably related to the characteristics of the circuits and to the possibilities of danger” i.e. using the incorrect type of RCD, will affect the tripping characteristics of the RCD and a result in a reduction or loss of protection.

Reg 133.1.3 states that were equipment is used outside the scope of its standard, the designer or person specifying the installation is responsible for confirming that the equipment provides the same degree of safety as required by the Regulations.

Reg 314.1 (iv) deals with the basic requirement for the division of RCD protected circuits, to ensure that specific circuits and loads are segregated to achieve a safe and reliable performance during operation, maintenance and under fault conditions. See example Fig2

Reg 331.1 requires an assessment of equipment characteristics that may have harmful effects on the performance of other electrical equipment in circuit. The majority of points listed in this clause can affect RCD performance and can be addressed by using the correct Type of RCD, as already mentioned in this paper.

Reg 531.2.4 the protective conductor current (total earth leakage current for the loads connected) should not cause nuisance tripping of the RCD under normal operation.

Example: Division of circuits and segregation of specific loads - Fig 2

To meet Reg 331.1 & 531.2.4, equipment and RCDs on downstream feeder circuits must be compatible with any RCDs installed upstream. During a routine inspection on a TT installation (Condition Report required for Site Insurance), new equipment containing a VSD controlled motor has been connected on the same feeder circuit as hand held plug-in equipment. The existing feeder circuit is protected by a 30mA Type A RCD fed from the main incoming 300mA Type A “S” RCD upstream. The manufacturer of the VSD controlled equipment recommends Type B RCD protection. It is not suitable for use with Type A RCDs and therefore the installation is unsafe.

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In the example, there are three possible options to make the installation safe and enable the issuing of the Site Condition Report for insurance renewal purposes.

1. Stop using the equipment until the installation can be modified (immediate requirement to reduce the risk from electrocution and fire).

2. If the leakage current characteristics of the VSD + any other leakage currents associated with this feeder are <12mA i.e. suitable for use with a *30mA RCD, change the Type A RCD on the affected feeder circuit and the main incoming RCD to Type B. These units are always 4 modules wide and this may not be the most cost effective solution see 3 below. *Note Type B RCD may detect leakage currents that the Type A RCDs did not detect (RCD blinding).

3. The simplest option may consist of installing a separate feeder (or using an existing feeder if it can be adapted) taken off the main incoming supply, upstream of the main Type A RCD. The Type B RCD can then be rated specifically for the VSD feeder circuit and fused accordingly.

The use of equipment with non-linear load characteristics increases the site harmonic distortion levels. As a rule of thumb, for installations with low levels of harmonic distortion, the sum of the earth leakage currents should

be < 40% of I∆n. To reduce the risk of nuisance tripping the % I∆n loading, may need to be reduced proportionally as the level of harmonic distortion increases. Selective “S”, AKV, F, B and EV RCCBs incorporate a transient resistance features to reduce nuisance tripping for installations /applications where this is likely to be a problem.

Using the correct Type of RCD is critical to achieving the required disconnection times

The initial shock current will be limited by the resistance of the shock path through body + any additional resistance in the fault path. Dry and unbroken skin is a poor conductor of electricity, however if the skin blisters or is broken, the resistance path will drop significantly and the shock current will rise accordingly. Reducing the duration of shock current reduces the level of damage to nerves, muscles, body tissue and organs.

RCD operating time is critical, with regard to reducing the damage caused by the initial shock current. Using the incorrect Type of 30mA RCD could delay the operation of the RCD or prevent it tripping, resulting in an increased risk of electrocution.

RCDs also help to reduce the risk associated with fires resulting from residual currents tracking across contaminated surfaces: See on-line video “How RCCBs Prevent Fires” WWW.doepke.co.uk

RCDs must operate reliably when other means of protection have failed. Designing, Installing/ Signing-off an installation which is incorrectly protected, could increase the risk of serious injury from electrocution and or fire.

This article refers to electrical installations covered by BS 7671: 2008 incorporating Amendment No 3. Refer to the IET for the latest copy of 7671 for guidance on the selection and use of RCCB’s. For guidance on UK Safety Regulations refer to the HSE Government web site. For further reading on RCDs from independent sources refer to the BEAMA and Electrical Safety Council’s WEB sites. For information on different Types of RCD please refer to the Doepke Web site.

Chaz Andrews – Technical Manager, Doepke UK Ltd

If you would like more detailed information relating to this subject, Doepke UK have a free 60 page Technical Application Guide

Available on request, or log on to WWW.doepke.co.uk to download the guide or obtain further information on RCCBs. For

technical support please e mail chazandrews@doepke.co.uk