Stroma Certification’s Technical Advisor, John Peckham, provides guidance on the requirements for each type of RCD protection, as set in BS7671.
Residual Current Devices (RCDs) work by monitoring the flow of current in the live conductors of a circuit; if the flow of current becomes un-balanced, due to a flow of current to earth of a set nominal value, they will trip to cut off the supply to the circuit. Whilst they have a nominal tripping current (IΔn), they may trip at a value below the nominal value. For example, a 30mA RCD may trip at currents of around 18mA to 22mA.
BS7671 (Reference 1.) requires the installation of RCDs for 3 types of protection. Each of the 3 types of circuit protection are listed below.
Additional Protection is often required when there is an additional risk of shock. Examples include electricity being used outside of a building to supply portable appliances where the user is exposed to the additional risk of water or that the user may not be wearing shoes and/or their body is exposed in places such as bathrooms, swimming pools and saunas. Also, most of the special locations listed in Part 7 of BS7671 require Additional Protection by the use of an RCD due to the additional risk of shock.
For an RCD to provide Additional Protection the tripping current (IΔn) must not exceed 30mA AND operate at a residual current of 5 x IΔn at a time, not exceeding 40ms. See Regulation 415.1.1. An RCD with a tripping current greater than 30mA will not provide Additional Protection.
Where the earth fault loop impedance of a circuit (Zs) is too high for an overcurrent device to provide Fault Protection, such as a fuse or a circuit breaker, an RCD may be installed to provide the Fault Protection. This is a common situation in installations with a TT earthing system where the earth fault path resistance is too high for a fuse or circuit breaker to operate in the required disconnection time for an earth fault.
For an RCD to provide Fault Protection the disconnection time (tripping time for the RCD) required for the circuit, as set out in Regulations 4220.127.116.11 and 418.104.22.168, must be achieved AND RA x IΔn ≤ 50V, where RA = sum of resistance of the earth electrode and the resistance of the protective conductor connecting the exposed conductive parts. The maximum values of Zs are set out in Table 41.5 of BS7671, reproduced below.
|Rated RCD Operating Current (mA)||Maximum Earth Fault Loop Impedance (Zs)|
(*) There is a footnote to the table to say earth resistances should be as low as possible and a value exceeding 200Ω may not be stable.
Where there is an increased risk of fire BS7671 requires installations to have RCD protection by the use of an RCD with a tripping current (IΔn) not exceeding 300mA.
The use of RCD protection is required for Fire Protection in buildings that are mostly constructed of combustible materials such as wood, or where combustible materials are stored, or where they may be additional fire hazards from dust or fibres. See Regulations 422.4, 422.3.9.
In agricultural and horticultural premises there is a requirement for all circuits to have RCD protection with a tripping current not exceeding 300mA for Fire Protection. This is required as it is likely that there will be stored combustible materials in the form of hay and straw and also the presence of rodents who are prone to chewing cable insulation. See Regulation 705.422.7.
Installed electronic equipment under normal operating conditions will leak some current to earth. This is due to the electronics in the equipment having noise filters inside which have capacitors connected to earth. These leakage currents may be in the region of 1-2mA per item of equipment or as high as 3.5mA. Leakage currents will be cumulative in the CPCs for multiple items of equipment and are known as ‘protective conductor currents’.
BS 7671 Regulations 314.1(iv) and 531.2.4 require installations to be subdivided into circuits to prevent accumulated protective conductor currents causing unnecessary tripping of RCDs. For instance, where sockets are installed in a school computer room or an office and the computers have a leakage current of 2mA each, it would be sensible not to have more than 6 or 8 computers on each circuit protected by a 30mA RCD. This assumes that a 30mA RCD will trip at around 20mA so a cumulative protective conductor current of 12-16mA will not trip the RCD unnecessarily.
Where an installation has an RCD for Fault Protection or Fire Protection protecting a whole installation, or a substantial part of a large installation, and there are RCDs in series for Additional Protection then Selectivity must be provided to comply with Regulation 536.3. This can be provided by the use of ‘Type S’ RCDs which will provide a time delay to allow for a lower rated RCD downstream of the main RCD to operate first under fault conditions. For example, a TT installation may have a 100mA Type S RCD as a main switch and 30mA RCDs for additional protection of sockets. A fault to earth on an RCD protected socket will cause the 30mA RCD to operate first, preventing the loss of supply to the whole installation.
There are different types of RCD, that respond to varying types of earth fault current, which need to be selected depending on the type of installation. A brief description for each type is listed below:-
1. Type AC
Trips in response to AC sinusoidal residual current either suddenly applied or smoothly increasing. This is the most common type of RCD fitted in installations.
The RCD casing for a Type AC RCD will be marked with the symbol shown below.
2. Type A
Trips in response to AC sinusoidal residual current and on pulsating direct current either suddenly applied or smoothly increasing. May be fitted where there is a DC component that may flow in the residual current from electronic devices (see Regulation 722.531.2.101). The DC component could lock up the RCD preventing it from operating under fault conditions.
The RCD casing for a Type A RCD will be marked with the symbol shown below.
3. Type B
Trips in response to AC sinusoidal currents, pulsating DC currents and smooth DC currents either suddenly applied or slowly rising. This type of RCD may be required for Solar Photovoltaic (PV) Power Supply Systems (see Regulation 712.422.214.171.124.1) and in Electric Vehicle Charging Installations (see Regulation 722.531.2.101).
The RCD casing for a Type B RCD will be marked with the symbol shown below.
4. Type F
Trips in response to the same conditions as the Type A RCD and for composite residual currents, whether suddenly applied or slowly rising intended for circuit supplies between phase and neutral or phase and earthed middle conductor and residual pulsating direct currents superimposed on smooth direct current. The Type F RCD will have the same symbol on the casing as a Type B.
This is an ordinary RCD without overcurrent protection (Residual Current operated Circuit Breaker). These devices are manufactured to BS EN 61008. They were also manufactured to an obsolete standard BS 4293.
In addition to the RCD function, protective devices may be fitted with overcurrent protection. These devices are known as RCBOs and are manufactured to BS EN 61009.
These are socket outlets fitted with an RCD (Socket Outlet fitted with Residual Current Device).
These are fused connection units fitted with an RCD (Fused Connection Unit incorporating a Residual Current Device).
These are obsolete devices commonly fitted to TT installations to provide fault protection (Voltage Operated Earth Leakage Circuit Breaker). They were manufactured to BS842. They rely on a rise in voltage on earthed conductive parts of the installation, causing current flowing to earth through the device to operate. They can be identified as they have terminals for the earthing conductor (E) and to the installation (F). They cannot be safely tested to ensure they will operate satisfactorily under fault conditions. They should be removed and replaced with RCD(s) for fault protection.
BS7671 Chapter 61 has a requirement to test RCDs but it is not specific as to how this should be carried out. The Institution of Engineering and Technology (IET) Guidance Note 3 (Reference 2.) provides further guidance but again is not definitive.
Where an RCD is provided for Additional Protection it must have a tripping current (IΔn) of 30mA or less and trip at 5 x IΔn in 40ms or less. Therefore, for a 30mA RCD a test current of 150mA needs to be applied to ensure the 40ms disconnection time is achieved. It is unnecessary and inappropriate to test RCDs with nominal tripping currents above 30mA, such as 100mA RCDs, at 5 x IΔn as they do not provide Additional Protection.
Where an RCD is installed for Fault Protection or Fire Protection they should be tested at the nominal tripping current, 1 x IΔn, to ensure that the disconnection time is achieved for the type and rating of the circuit and the earthing system employed. For example, where provided for fault protection on a TT earthing system for final circuits up to 32A the RCD should trip at less than 0.2s (200ms).
Test equipment instruments commonly have settings for different nominal test currents and multiples of nominal tripping currents. They also have a test facility to test on both halves of the AC cycle, 00 and 1800. They will usually have a facility to test at ½ IΔn to prove that the RCD will not trip at an unacceptably low level of residual current.
Before carrying out an RCD test any equipment connected to the installation or circuit should be disconnected or isolated. An earth loop impedance test should be carried out before the RCD test to ensure that the circuit or installation is connected to a satisfactory means of earthing. The RCD can be tested at any point on the circuit either at the terminals of the RCD or somewhere on the circuit as it makes no difference where the test is carried out. Some RCBOs cannot be tested at the terminals as the sensing coil for the RCD is positioned in the device after the terminal. Below is a table of acceptable test results for different types of RCD. On completion of the instrument tests the test button should be operated to ensure the RCD trips. A warning label should also be present to advise the user to operate the test button quarterly. See Regulation 514.12.2.
|Type||Test Current (IΔn) mA||Maximum tripping time ms|
|BS EN 61008||X 1||≤300*|
|BS 4293||X 1||≤200|
|All types for Additional Protection ≤30mA||X 5||≤40|
|Type S||X 1||130-500|
(*) Less than 200ms for final circuit ≤ 32A on TT installations
|Sockets in dwellings up to 20A||411.3.3||All sockets up to 20A rating must be 30mA RCD protected unless labelled for a particular item of equipment. The provision for omission of RCD protection where there is a Risk Assessment does NOT apply to dwellings.|
|Sockets in commercial premises||411.3.3||All sockets up to 20A rating must be 30mA RCD protected unless labelled for a particular item of equipment. Or if there is a written risk assessment to say RCD protection is not required.|
|Mobile equipment outdoors with a rating of up to 32A||411.3.3||Includes 3 phase supplies. No exceptions.|
|All low voltage circuits in a bathroom||701.411 .3.3, 701.415.2||None|
|Circuits passing through bathrooms Zones 1 and/or 2||701.411 .3.3||Only applies to Zones 1 and 2, not to cables outside the Zones.|
|Cables without mechanical protection at a depth of less than 50mm in a wall||522.6.202, 522.6.204.||Cables should be in Safe Zones and 30mA RCD protected.|
|Cables in walls with internal metal construction unless protected||522.6.203, 522.6.204.||Unprotected cables such as twin and earth in metal studwork walls.|
|Swimming pools||702.410.3.4.2, 702.4126.96.36.199,702.53702.55.1||Supplies to equipment in the Zones.|
|Saunas||703.411.3.3||All circuits in the sauna.|
|Construction sites||704.410.3.10||230/400V Sockets up to 32A.|
|Agricultural and Horticultural Premises||705.411.1||Sockets up to 32A.|
|Caravan and camping sites||708.553.1.13||All socket outlets. Each socket outlet to have individual RCD.|
|Caravans||721.411||All circuits and must interrupt all live conductors.|
|Marinas||709.531.2||Socket outlet. Device must disconnect all poles including the neutral. Sockets to be individually protected by an RCD.|
|Medical locations||710.531.2.4, 710.411.4||Socket outlets and all circuits up to 63A in Group 1 locations.|
|Exhibitions, shows and stands||711.411.3.3||All socket outlets. All circuits other than those supplying emergency lighting.|
|Solar Photovoltaic (PV) power supplies||712.4188.8.131.52.2||May be required for supply cable and RCD needs Type B.|
|Outdoor lighting installations||714.411.3.3.||Lighting in telephone kiosks, bus shelters, advertising panels and town plans.|
|Mobile or transportable units||717.515.1||LV circuits supplying equipment outside the unit.|
|Electric vehicle charging installations||722.531.2.101||Every charging point to have individual RCD that disconnects all live conductors.|
|Temporary installations for structures, amusement devices, fairgrounds etc||740.415.1||All lighting circuits, sockets up to 32A and mobile equipment up to 32A.|
|Floor and ceiling heating systems||753.415.1||Circuit supplying heating system.|
1. BS7671:2008 Incorporating Amendment 3 2015. Requirements for Electrical Installations.
2. The Institution of Engineering and Technology Guidance Note 3 Inspection and Testing.
Should you require any further guidance, please contact our team on 0845 621 11 11.
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