A rising number of AC Type residual current devices (RCDs) are being adversely affected by direct currents (DC) from electronic devices and renewable energy, says Michael Kenyon.
03 February 2020 | Michael Kenyon
Use of RCDs has grown significantly over the past few years, with AC Type RCDs often promoted as the first choice, particularly since 2001, when the 17th Edition (Amendment 1) of the IET Wiring Regulations stated that AC Type RCDs should be used for sockets that might supply outdoor equipment.
In the 18th Edition, there's updated guidance on the use of various types of RCDs and it's AC Type RCDs that are being adversely affected by DC from electronic devices and renewable energy sources.
Why we need RCDs
Currents leak to earth through appliances and equipment all the time. A typical laptop will leak around 1mA of current to earth; these currents then use the earth connection or protective conductor to return to source. An RCD is designed to detect these leakage currents and disconnect the supply when they reach a defined level, typically 30mA for protection of life. The idea is that if a faulty piece of equipment allowed a person to come into contact with a live part of the RCD, it would disconnect the supply before the current could flow at fatal levels. So how do we select the right type of RCDs?
Will it be used for overcurrent protection and protection against earth leakage? An RCBO might be required.
Alternatively, if only protection against earth leakage currents is required then an RCD might suffice.
The standard ratings of RCD are 30, 100, 300 and 500mA. If protection of life is required you'd need a 30mA device, whereas if the requirement is protection against fire a 300mA would do. Note: This is not to be confused with the rating of the overcurrent protective device (OCPD).
Different types of RCDs
The correct type of RCD must be selected. BS 7671 says that for general use AC type RCDs can be used, but this takes no account of the levels of DC leakage current in modern-day installations.
Michael Kenyon is technical manager at H&S expert Bureau Veritas