RCD – Residual Current Device.
An RCD is a safety device that automatically switches off electricity if there is an earth fault. According to AS/NZS 3000:2018, RCDs are devices designed to isolate the power supply to various circuits, socket outlets, or electrical equipment when there is an excess current flow to the earth. It is important to note that RCDs are not intended to be used as the sole means of protection but rather as a supplement to existing protective measures.
RCDs used for limiting earth leakage current must be capable of interrupting the protected circuit when the earth leakage current reaches a predetermined value. RCDs shall be installed for additional protection of circuits, socket outlets, wiring systems, electrical heating cables, electrical equipment, and specific electric installations as specified in AS/NZS 3001, AS/NZS 3002, AS/NZS 3003, AS/NZS 3004, AS/NZS 3012 and AS/NZS 4249.
RCD is always connected at the origin of the circuit, typically in the Main Switchboard. It consists of a summation current transformer, electronic amplifier, trip relay and circuit breaker mechanism. It’s important not to confuse the circuit breaker mechanism with over-current protection. Refer to the Difference between RCD and RCBO blog.
blog for further information. The below image shows the construction of an RCD
Importance of RCD
RCDs are an important part of any electrical installation as they break the circuit instantly (i.e in less than 40ms). RCDs are rated for leakage currents ranging generally up to 30mA.
Currents between 10mA and 30mA are not fatal, but their prolonged presence may cause breathing difficulties.
Currents above 30mA can be fatal unless the person is quickly separated from the source.
Electric currents of up to 500mA can be fatal if they flow for more than 0.5 seconds.
Currents that exceed 500mA are typically lethal, even if they occur for only a brief time.
Types of RCD
RCDs are classified according to sensitivity to various types of residual currents. They are typically classified as Type AC, Type A, Type F, Type B and Type S.
Type AC – RCD
Type AC RCDs are designed to operate only for alternating currents but most homes these days have electronic Inverters. Due to its inherent characteristics of operating only with AC residual current, type AC RCD cannot protect all parts of installations. Hence type AC RCDs are banned from April 2023 in Australia.
Type A – RCD
‘Type A’ RCD can disconnect the circuit for alternating and pulsating direct currents. It can also break the circuit for smooth direct residual current up to 6mA.
Type A RCDs are commonly used to disconnect Solar power inverters, UPS systems, Drives (VSD, VFD), EV charging stations, and LED lighting systems
Type F – RCD
These are special variants of type A with altered frequency characteristics considering high frequencies. Type F RCDs are designed to withstand surge currents without tripping, specifically designed for circuits with inrush currents.
Common applications of F-type RCDs are washing machines, dishwashers, air conditioners, tumble driers, and refrigerators.
Type B – RCD
B-type RCDs isolate all types of residual current viz. alternating, pulsating direct, smooth direct current and high-frequency alternating current. Direct residual currents may occur in industrial and commercial installations (and/or even in residential installations), where frequency inverters, photovoltaic power plants and other equipment with power semiconductor elements are applied.
Type S – RCD
These RCDs are also called time delay RCDs. They take longer to detect a fault to trip a circuit. This is because they are designed to prevent false alarms, also called nuisance tripping. The main benefit of using this type of RCD is that it offers greater immunity to transients.
However, since they take more time to isolate the circuit, they are unsuitable for shock protection. These RCDs are commonly used to protect circuits from fire.
We can tabulate the above information –
|Banned in Australia from April 2023
|Alternating and pulsating direct current
|UPS systems, EV charging stations, LED lighting systems etc.
|Alternating current, high frequency alternating current and pulsating direct current
|Washing machine, Refrigerators, dishwashers, air conditioners etc.
|Alternating current, high frequency alternating current, pulsating direct current and flat direct current
|Photovoltaic power plants, frequency inverters etc.
|Alternating current with time delay
|Protection from fire.
RCDs can be classified in another way as well, according to their combination with other apparatuses.
Combined with circuit breakers
These are also called RCBOs.
RCBO has features of both RCD and RCBO.
It can isolate the circuit when there is an earth fault and also if there is an overcurrent in the circuit.
For more information visit: Difference between RCD and RCBO
While using power tools or electrical appliances, accidents can be avoided by attaching portable RCDs to a power board or extension leads
RCDs fitted to a PowerPoint can be identified by the test button on the faceplate. They are designed to protect electrical appliances in your bathroom and workshop or medical equipment in hospitals and clinics
Summary – When it comes to choosing a residual current device (RCD) for protecting a circuit, it is crucial to select the right type of RCD based on the application or circuit you want to protect. Different types of RCDs are made for different purposes.
For instance, if you want to safeguard a circuit that includes electronic equipment, you should opt for a type A or type B RCD.
If you need to protect a motor drive circuit, you should use a Type F RCD. When choosing an RCD.