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RCD Selection

Residual Current Device (RCD) Selection

RCD Selection Criteria 

Sensitivity 

For every RCD there is normally a choice of residual current sensitivity (tripping current). This defines the level of protection afforded. Protection is divided into two broad categories:

Personal protection (additional protection of persons or livestock against direct contact) This is ensured when the minimum operating current of the RCD is no greater than 30 mA and the RCD operates to disconnect the circuit, within the specified time, in the event of an earth leakage. Installation protection This is associated with devices that are used to protect against the risk of fire caused by an electrical fault. RCDs which operate at residual current levels up to and including 300 mA provide this type of protection.






The term RCDs covers a range of products some of which are listed below:

RCCB (Residual Current Operated Circuit-Breaker without Integral Overcurrent Protection)

RCBO (Residual Current Operated Circuit-Breaker with Integral Overcurrent Protection) 

SRCD (Socket-Outlet incorporating a Residual Current Device) 

FCURCD (Fused Connection Unit incorporating a Residual Current Device) PRCD (Portable Residual Current Device) 

CBR (Circuit-Breaker incorporating Residual Current Protection) IC-CPD (In-Cable Control and Protective Device for mode 2 charging of electric road vehicles) 

MRCD (Modular Residual Current Device)

There are two other classifications of RCD that need to be considered, general and time-delayed operation each having Type AC, A, F or B characteristics. 



General RCDs 

Operate ‘instantaneously’, i.e. they do not have an intentional delay in operation and thus cannot be guaranteed to ‘discriminate’. This means that where there are two or more general RCDs installed in series in an installation; more than one device may trip in the event of an earth leakage current. This would result in healthy circuits being disconnected even though the initial fault occurred in a different part of the installation. 

Discrimination is essential in installations where it is important to ensure that a complete system is not ‘shut down’, for example in domestic installations to ensure that lighting and other circuits are not disconnected if an earth leakage occurs in a power circuit. 


Time Delayed RCDs 

Provide discrimination in circuits where RCDs are connected in series. It is essential to install devices which incorporate a time delay upstream of the general device, so that the device nearest a fault will trip. RCDs with built in time delays should not be used to provide personal protection.

For RCCBs complying with BS EN 61008 and RCBOs complying with BS EN 61009 the time delay feature is indicated by the letter ‘S’


This article was extracted from "Technical Bulletin Residual Current Device (RCD) Selection
" published by BEAMA


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