Green technology: RCD specification

omestic, commercial and public buildings, designed to meet the latest energy standards, can contain a range of energy saving devices, from simple LED lighting and appliances with built-in inverters, to more complex ground source heat pumps. These devices cannot be used with standard AC RCDs and this must be taken into account when specifying, designing and completing the installation. A number of applications are not covered in detail in existing guidance, so care must be taken to reduce the risk of electrocution and fire when selecting RCDs.


WHY DOES THE LOAD AFFECT THE RCD SELECTION? Energy saving technology relies on power electronics to alter or chop the electrical supply, i.e. producing non- linear load characteristics. Type AC RCDs can only be used with loads that generate sinusoidal leakage and residual currents (linear loads) (Fig 1). Inverters used in lighting control, speed control and power supplies, produce complex leakage and residual currents affecting the tripping characteristics of standard AC RCDs. This phenomenon is commonly referred to as ‘RCD blinding’.

RISKS ASSOCIATED WITH INCORRECT RCD SELECTION RCD’s are installed to provide additional protection when a primary protection method fails. Ensuring they work could mean


the difference between life and death. Installing equipment on a circuit protected by the incorrect type of RCD, will compromise the protection, and the safety for staff using other equipment connected to the same circuit (Fig 2). The Electrical Safety Council have

Figure 1:

estimated that around 2.5 million people receive an electric shock each year, of which 350,000 receive a serious injury. The Electricity at Work Regulations make

EXISTING GUIDANCE RELATING TO RCDS – IS IT CLEAR ENOUGH? Health & Safety at Work Regulations and the Electricity at Work Regulations 1989 (EWRs) reflect legal obligations as compared to non-statutory codes of practice such as the Wiring Regulations (BS7671), which only give advice. This relationship is important when making decisions relating to circuit design and selection of RCDs, particularly for applications where the equipment connected in circuit could affect the safety performance of the RCD. BS 7671 details minimum safety

requirements, traditional aimed at improving safety through non-statutory regulation in domestic and similar premises. This may not be sufficient in commercial or industrial environments. In the event of an employee or customer receiving an electric shock from a 230V supply, just meeting the Wiring Regulations may not prevent prosecution in the event of a breach of a statutory regulation. HSE Guidance HSR25, 3rd Edition

October 2015 (EWRs) Regulation 16, places a duty of care on employers: “No person shall be engaged in any work activity where technical knowledge or experience is necessary to prevent danger or, where appropriate, injury unless he possesses such knowledge or experience.” If you are not sure check and seek

advice from your manager, equipment manufacture and/or trade association. For example BS7671 Regulation 331

it clear (EWR 2.35): “You should always consider the conventional public electricity supply voltage of 230V AC as potentially fatal”. Those involved in the specification,

Figure 2:

design and installation, have a duty of care under the Health & Safety at Work Act 1974. Companies should take note of the new sentencing guidelines issued by the Sentencing Council in February 2016, relating to breaches of H&S Regulations.

Equipment containing inverters can cause problems for RCD protected installations. Check installation instructions for the correct type of RCD

Compatibility of Characteristics covers the “assessment of characteristics of equipment likely to have harmful effects upon other electrical equipment in the installation”. There are some specific examples given in BS7671 (see Part 7) where Type AC RCDs are not suitable, but this does not cover all the possible applications. An obvious example would be the connection of three phase VSDs inverters into a circuit which has Type AC or Type A RCD protection upstream. Although it is not specifically stated in BS7671, we all know it is not safe to use three phase VSDs with anything other than a Type B RCDs. However, there are many applications

Equipment requiring Type B may be connected on a seperate feeder, upstream of any existing Type A’s

where it is not so obvious and 331 is there as ‘catch all clause’. When installing any new equipment either fixed, portable or transportable, where RDC protection forms part of the “Risk Reduction Measures”, the risk assessment must include a review of the existing installation and proposed RCD protection to ensure it is appropriate.

Deopke UK T: 01628 829133


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