The latest addition of the IET Wiring Regulations introduces a number of revisions relating to the selection of RCDs. Chaz Andrews of Doepke explains that installers who do not have a clear understanding of these requirements can be faced with costly re-work if they have to return to site to fit the correct product
The use of Type A RCDs is still allowed by the IET Regulations in some applications. However, this is only on the bases that it is appropriate for the application. Installers should take note of chapter 33 – specifically clause 331.1. Most European countries demand Type A RCDs as a minimum requirement for domestic applications.
Before we look at the various Types, let us recap briefly on the basic principles relating to the use of RCDs designed to EN 61008 (RCCBs) and EN 61009 (RCBOs).
Regulation 411: Protection in the event of indirect contact – referred to as Fault Protection. RCDs are designed to protect persons against the hazards associated with indirect contact where exposed conductive parts of the installation are connected to a suitable earth electrode. Regulation 411.3.3 is based on the selection of the correct type of RCD (Regulation 331.1) e.g. a class room containing a number of desk top computers using switch mode power supplies would require the selection of a 30mA Type A RCD. The pulsating direct current introduced onto the AC waveform by the power supplies would affect the tripping characteristics of a standard AC RCD.
Regulation 415: Protection in the event of direct contact – basic protection. RCDs with a trip rating not exceeding 30mA with the inclusion of any supplementary bonding requirements are used to meet this requirement. Regulation 531.2.4 highlights a common problem we come across associated with PV installations and domestic properties. If the cabling between the PV inverter and the distribution board is initially selected and installed on the basis of using a 30mA RCD, the owner is likely to experience nuisance tripping – most PV inverters require a minimum of 100mA to meet regulation 531.2.4. In this case cabling would have to be replaced to allow for the use of a 100mA RCD – Regulation 522.6.
Types of RCD recommended
The Types detailed here are generally available in RCCB format, but may not be available in RCBO format.
Type A: These RCDs are designed to operate with residual sinusoidal alternating currents and pulsating direct currents with >6mA smooth DC (Fig 1). For an application example see 710.411.3.2. More and more installations now require the use of Type A RCDs, due to the increasing use of electronic white goods with switch mode power supplies and lighting installations with variable lighting level controls.
Type B: (EN 62423) – Used in applications where, in addition to the above, smooth DC residual currents above 6mA are present under certain fault conditions – example 712.411.3.2.1.2. With the exception of PV installations, most domestic properties would not require the use of Type B RCD. However, there is always an exception so care must be taken. For instance, some washing machine manufactures do recommend Type B RCDs due to the design of the variable speed control board within the washing machine. Many commercial and public buildings would now require the use of Type B RCDs where the installation includes 3-phase inverters e.g. speed control in HVAC installations.
Type S: Selective or time delayed RCDs are used in applications where RCDs have to be connected in series to achieve discrimination (Fig 2) between upstream and downstream devices – see clause 5.31.2.9 and example applications 740.411, or 534.2.6. Selective RCDs are also designed to have immunity to transient wave forms <5kA 8/20µs. This can help in applications where you have high inrush currents on start-up. Selective RCDs are not manufactured with 30mA trip for obvious safety reasons – see appendix 3 table 3A. If you experience problems when carrying out trip tests, check that the test instrument you are using is suitable for testing Type S RCCBs and is set on the correct feature.
Type KV: Have an immunity to surge currents <3kA 8/20µs, and are used in applications where transient currents would cause a standard RCD to trip under normal operation – see clause 534.2.6. These units can be manufactured with a 30mA trip suitable for use in applications involving SPDs, where a Type S would not meet the disconnection times required under the IET Wiring Regulations.
RCDs must operate reliably when other means of protection have failed. Installing/signing-off an installation which is incorrectly protected could endanger people’s lives and put property at risk. Refer to the IET Wiring Regulations for guidance on the selection and use of RCCBs to meet the requirements for new and existing installations.
This article is based on the product standards covering the design of RCCBs – EN 61008 and EN 62423 and BS 7671: 2008 incorporating Amendment No 1. For further reading on RCDs from independent sources refer to the BEAMA and Electrical Safety Council’s (ESC’s) websites.
Doepke
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