RCD description
An RCD ‘Residual Current Device’ is an electrical device that monitors the current flowing through either an individual circuit such as an RCBO ‘residual current operated circuit-breaker with integral overcurrent protection’ or multiple circuits within an installation using an RCCB ‘residual current operated circuit breaker without integral overcurrent protection’. Its purpose is to detect any current in a circuit that may be leaking to earth either through Line to Earth or Neutral to Earth.
This is done by monitoring the current in the Line and Neutral conductors to ensure they are balanced, meaning what current flows into a circuit via the Line conductor should be the same as the current flowing out through the Neutral conductor. If there is any difference between the Line and Neutral current, it would indicate that some current within the circuit must be leaking to Earth.
This leakage current is measured in milliamps ‘mA’ (1/1000 amp) and if the leakage current reaches a pre-determined level, usually 30 mA ‘0.03 A’ the device will operate and isolate the supply from the circuit.
As it is a much more sensitive device than a fuse or circuit breaker which require large amounts of currents before they trip under fault conditions, an RCD provides protection to the user of the installation minimising the chances of them receiving a potentially lethal electric shock which is often regarded as 50 mA ‘0.05 A’ or above.
Common types of RCDs
AC: Ensures tripping for residual AC currents and is found in most domestic installations.
A: Ensures tripping for all types of AC currents and pulsating DC currents.
B: Ensures tripping for residual AC currents, pulsating DC currents and smooth DC currents.
B+: Ensures tripping for residual AC currents, pulsating DC currents and smooth DC currents. It also ensures no unwanted tripping and detection of high frequency faults up to 20 kHz.
Why we need different types
The abundance of modern technology we use these days can cause problems when selecting what type of RCD to use.
This technology includes…
- smart home and data networks
- USB socket outlets
- solar PV panels
- electric vehicle charging equipment
When using switch mode power supplies, present in all types of electronic devices used to convert AC to DC, the problem arises is that DC tends to leak to earth which can affect the operation of some RCD devices a making them non-operable i.e., they won’t work
For example…
When installing electric vehicle charging equipment within a domestic installation care needs to be taken as possible leakage to earth on the DC side of 6mA or more my cause the Type A or AC RCDs already within the installation to be blinded by this DC leakage coming from the car and this could leave the other circuits within the installation to not have any residual current protection, which could have severe consequences.
This can be overcome using an RDC-DD (Residual Direct Current – Disconnecting Device) within the electric vehicle supply equipment (EVSE) which will automatically disconnect the EVSE from the supply and vehicle upon detecting a DC leakage current of 6mA or more. This means that the Type A or AC RCDs within the installation will not be affected and will continue to operate.
However, if the EVSE does not contain an RDC-DD then the installation will need to use Type B or B+ RCDs as they can detect this DC leakage and still work and disconnect if required.
BS7671:2018 Amendment 2 ‘2022’ testing RCDs…
To comply with the Wiring Regulations BS7671:2018+A2:2022 it is important that the RCD disconnects the supply from the circuit it protects within a predetermined time. This requires that the device is tested before being put into service.
The second amendment to the 18th Edition of the wiring regulations has re-written the testing requirements for RCDs within an electrical installation.
Gone is the previous requirement for testing at 5 x the rating of RCD with a max disconnection time of 40ms, as there is only a need to test at 1 x its rating with a max disconnection time of 300ms for general non-delay type and at 1 x its rating with a maximum disconnection time between 130ms to 500ms for a delay ‘S’ type RCD. (See Part 6, Regulations 643.7.1 ‘General’)
*Note, where a 30 mA RCD is used as additional protection (See 415.1.1), which is usually the case then a maximum disconnection time of 300ms at 1 x its rating applies. (See Part 6 Regulation 643.8 ‘Additional Protection’)