The latest edition of the IET Regulations introduces a number of revisions relating to the selection of specific types of RCD based on the characteristics of the loads /devices incorporated within the installation. Maintenance staff must have a clear understanding of these requirements to reduce the risks of electrocution for non-skilled works /users of electrical equipment on site.
The IET Regulations in theory still allow the use of Type AC RCDs in some applications however this is only on the bases that it is appropriate for the application- See chapter 33 specifically clause 331.1. Most European countries demand Type A RCDs as a minimum requirement for RCD related applications.
Before we look at the various Types, let us recap briefly on the basic principles relating to the use of RCDs (RCD generic term for RCCBs – EN61008 and RCBOs – EN61009)
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. Circuits in offices/labs containing computers or other IT equipment 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, affects the tripping characteristics of standard RCDs, leading to increased risk of electrocution.
Regulation 415: Protection in the event of direct contact – Basic Protection
RCDs with a trip rating not exceeding 30 mA with the inclusion of any supplementary bonding requirements are used to meet this requirement. Regulation 531.2.4 highlights a common problem associated with installations incorporating fixed machines and portable equipment. Correct segregation of circuits and the selection of appropriate RCD protection devices, taking into account the nature of the equipment to be connected and safety requirements for staff protection, whilst maintaining the availability of the supply is a fundamental design requirement. See Fig 1
Types of RCCB recommended in the IET Wiring Regulations
The Types detailed below are generally available in RCCB format, but may not be available in RCBO format.
Type A: Designed to operate with residual sinusoidal alternating currents and pulsating direct currents with <6 mA smooth DC. For an application example see 710.411.3.2. – for instance applies to medical rooms in the work place. Many basic installations now require the use of Type A devices, due to the increasing use of electronic goods with switch mode power supplies and lighting installations with variable lighting level controls.
Type B: (EN 62423) Applications where in addition to the above; high frequency AC leakage due to capacitive loads and smooth DC residual currents (inverter bridge faults) are present under certain fault conditions. Most industrial sites would now require the use of Type B RCDs where the installation includes 3 phase inverters for speed control and UPS for critical power supplies.
Type S: Selective or time delayed RCD’s are used in applications where RCDs have to be connected in series to achieve discrimination (Fig 2) between upstream and downstream devices – reference clause 188.8.131.52 and example applications see 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 test feature.
Type KV: Have an immunity to surge currents <3kA 8/20µs, 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 30 mA 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 through increased risk of fire. Refer to the IET Wiring Regulations for guidance on the selection and use of RCCB’s to meet the requirements for new and existing installations.
This article is based on the product standards covering the design of RCCBs: EN61008 and EN62423 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 WEB sites. For B Type RCCB Doepke 60 page Technical App. Doc.
Chaz Andrews – Technical Manager, Doepke UK Ltd www.doepke.co.uk