Residual Current Devices (RCDs) and Residual Current Circuit Breakers with Overload (RCBOs)

Residual Current Devices (RCDs)

  • Operate by monitoring current in line and neutral conductors.

  • Balanced current indicates a healthy circuit; imbalance indicates an earth fault.

  • If imbalance exceeds device setting, it trips by inducing current to a solenoid.

Residual Current Circuit Breakers with Overload (RCBOs)

  • Combine overload protection with RCD functionality.

  • Include a test button for regular mechanical checks.

  • Mechanical issues in RCDs/RCBOs pose risks due to lower energy levels required to trip compared to typical circuit breakers.

Applications of RCDs & RCBOs

  • Sensitive to earth fault currents as low as 30 mA with response times of 20-40 ms.

  • Essential for protecting portable electrical equipment against electric shocks.

  • BS 7671:2018 allows wider RCD use for indoor circuits and outdoor supplies.

Testing RCD’s & Resources for RCD Information in BS7671

  • Find RCD operations in the On-Site Guide, Section 11.

  • Section 7: Final Circuits details specific scenarios needing RCD protection.

  • Chapter 41 (regulation 415) outlines additional protection requirements, termed 'Additional protection' in the industry.Furthermore, the IET Wiring Regulations also emphasize the risk of electric shock and fire, which mandates the installation of RCDs in specific locations such as swimming pools, outdoor circuits, and locations where equipment is likely to be used by unskilled persons.

Testing RCDs

  • Regular User Testing: Utilise the test button on the device for a mechanical check. This confirms the tripping mechanism is functional but does not test earth fault protection.

  • Professional Functional Testing: Requires specialised RCD test equipment to simulate an earth fault current (e.g., IΔn) and measure the trip time. This ensures the RCD trips within the specified time (e.g., 300ms for IΔn and 40ms for 5IΔn).

  • Frequency: BS 7671 recommends periodic inspection and testing, often annually for commercial/industrial settings and every 5-10 years for domestic, alongside user checks.

RCD Ratings and Selection

  • Rated Residual Operating Current (IΔnIΔn​): This is the current imbalance that will cause the RCD to trip.

  • Common RCD Ratings:

    • 30mA30mA RCDs:

      • Purpose: Primarily for additional protection against electric shock in cases of direct contact (human protection).

      • Applications: Required for most socket outlets up to 32A32A in domestic installations, and for circuits supplying portable equipment outdoors or in other high-risk environments.

      • BS 7671 Requirements: Mandated for many circuits where increased safety is needed, especially for circuits liable to be used by ordinary persons (e.g., general purpose socket outlets).

    • 100mA100mA RCDs:

      • Purpose: Primarily for protection against fire and indirect contact (automatic disconnection of supply).

      • Applications: Often used for the main incoming supply to an installation or for circuits supplying fixed equipment, especially where earth fault loop impedance is higher than that required for a 30mA30mA device, or where nuisance tripping is a concern with 30mA30mA devices for non-personal safety critical loads. Also used for whole-house protection as an upstream device to provide fire protection, with 30mA30mA RCDs providing local person protection.

      • Selection: Chosen when the earth fault current is deemed sufficient to cause rapid disconnection to prevent fire, but a lower 30mA30mA rating is not strictly required for direct contact protection or would lead to nuisance tripping in certain systems.

    • 300mA300mA and 500mA500mA RCDs:

      • Purpose: Primarily fire protection and sometimes for discrimination in larger industrial installations.

      • Selection: Employed when greater discrimination is needed (e.g., to prevent a single fault from tripping the entire installation), or when the earth fault loop impedance is very high, making lower current RCDs impractical. Generally used upstream from lower rated RCDs.

  • Selection Criteria:

    • Risk Assessment: Identify the level of risk (e.g., direct contact, fire, specific environments like bathrooms or construction sites).

    • Regulatory Requirements (BS 7671): Adhere to specific regulations for different circuit types and locations. For example, Section 701 (Bathing and Shower Rooms) or Section 704 (Construction Sites) have specific RCD requirements.

    • Discrimination: In installations with multiple RCDs, higher-rated RCDs are often installed upstream of lower-rated RCDs to ensure that only the RCD nearest to the fault trips, minimizing downtime.

    • Nuisance Tripping: Consider the typical leakage current of the circuit's equipment to avoid unnecessary tripping, especially with lower-rated RCDs.

Testing RCD

  • Find RCD operations in the On-Site Guide, Section 11.

  • Section 7: Final Circuits details specific scenarios needing RCD protection.

  • Chapter 41 (regulation 415) outlines additional protection requirements, termed 'Additional protection' in the industry.Furthermore, the IET Wiring Regulations also emphasize the risk of electric shock and fire, which mandates the installation of RCDs in specific locations such as swimming pools, outdoor circuits, and locations where equipment is likely to be used by unskilled persons.

Testing RCDs

  • Regular User Testing: Utilise the test button on the device for a mechanical check. This confirms the tripping mechanism is functional but does not test earth fault protection.

  • Professional Functional Testing: Requires specialised RCD test equipment to simulate an earth fault current (e.g., IΔnIΔn​)
    ) and measure the trip time. This ensures the RCD trips within the specified time (e.g., 300ms300ms

    for IΔnIΔn​)

    and 40ms40ms

    for 5IΔn5IΔn​)

  • Frequency: BS 7671 recommends periodic inspection and testing, often annually for commercial/industrial settings and every 5-10 years for domestic, alongside user checks.


RCD Ratings and Selection

  • Rated Residual Operating Current (IΔnIΔn​): This is the current imbalance that will cause the RCD to trip.

  • Common RCD Ratings:

    • 30mA30mA RCDs:

      • Purpose: Primarily for additional protection against electric shock in cases of direct contact (human protection).

      • Applications: Required for most socket outlets up to 32A32A in domestic installations, and for circuits supplying portable equipment outdoors or in other high-risk environments.

      • BS 7671 Requirements: Mandated for many circuits where increased safety is needed, especially for circuits liable to be used by ordinary persons (e.g., general purpose socket outlets).

    • 100mA100mA RCDs:

      • Purpose: Primarily for protection against fire and indirect contact (automatic disconnection of supply).

      • Applications: Often used for the main incoming supply to an installation or for circuits supplying fixed equipment, especially where earth fault loop impedance is higher than that required for a 30mA30mA device, or where nuisance tripping is a concern with 30mA30mA devices for non-personal safety critical loads. Also used for whole-house protection as an upstream device to provide fire protection, with 30mA30mA RCDs providing local person protection.

      • Selection: Chosen when the earth fault current is deemed sufficient to cause rapid disconnection to prevent fire, but a lower 30mA30mA rating is not strictly required for direct contact protection or would lead to nuisance tripping in certain systems.

    • 300mA300mA and 500mA500mA RCDs:

      • Purpose: Primarily fire protection and sometimes for discrimination in larger industrial installations.

      • Selection: Employed when greater discrimination is needed (e.g., to prevent a single fault from tripping the entire installation), or when the earth fault loop impedance is very high, making lower current RCDs impractical. Generally used upstream from lower rated RCDs.

  • Selection Criteria:

    • Risk Assessment: Identify the level of risk (e.g., direct contact, fire, specific environments like bathrooms or construction sites).

    • Regulatory Requirements (BS 7671): Adhere to specific regulations for different circuit types and locations. For example, Section 701 (Bathing and Shower Rooms) or Section 704 (Construction Sites) have specific RCD requirements.

    • Discrimination: In installations with multiple RCDs, higher-rated RCDs are often installed upstream of lower-rated RCDs to ensure that only the RCD nearest to the fault trips, minimizing downtime.

    • Nuisance Tripping: Consider the typical leakage current of the circuit's equipment to avoid unnecessary tripping, especially with lower-rated RCDs.

Testing RCD

  • Find RCD operations in the On-Site Guide, Section 11.

  • Section 7: Final Circuits details specific scenarios needing RCD protection.

  • Chapter 41 (regulation 415) outlines additional protection requirements, termed 'Additional protection' in the industry.Furthermore, the IET Wiring Regulations also emphasize the risk of electric shock and fire, which mandates the installation of RCDs in specific locations such as swimming pools, outdoor circuits, and locations where equipment is likely to be used by unskilled persons.

Testing RCDs

  • Regular User Testing: Utilise the test button on the device for a mechanical check. This confirms the tripping mechanism is functional but does not test earth fault protection.

  • Professional Functional Testing: Requires specialised RCD test equipment to simulate an earth fault current (e.g., IΔnIΔn​)
    ) and measure the trip time. This ensures the RCD trips within the specified time (e.g., 300ms300ms

    for IΔnIΔn​)

    and 40ms40ms

    for 5IΔn5IΔn​)

  • Frequency: BS 7671 recommends periodic inspection and testing, often annually for commercial/industrial settings and every 5-10 years for domestic, alongside user checks.


RCD Ratings and Selection

  • Rated Residual Operating Current (IΔnIΔn​): This is the current imbalance that will cause the RCD to trip.

  • Common RCD Ratings:

    • 30mA RCDs:

      • Purpose: Primarily for additional protection against electric shock in cases of direct contact (human protection).

      • Applications: Required for most socket outlets up to 32A in domestic installations, and for circuits supplying portable equipment outdoors or in other high-risk environments.

      • BS 7671 Requirements: Mandated for many circuits where increased safety is needed, especially for circuits liable to be used by ordinary persons (e.g., general purpose socket outlets).

    • 100mA RCDs:

      • Purpose: Primarily for protection against fire and indirect contact (automatic disconnection of supply).

      • Applications: Often used for the main incoming supply to an installation or for circuits supplying fixed equipment, especially where earth fault loop impedance is higher than that required for a 30mA device, or where nuisance tripping is a concern with 30mA devices for non-personal safety critical loads. Also used for whole-house protection as an upstream device to provide fire protection, with 30mA RCDs providing local person protection.

      • Selection: Chosen when the earth fault current is deemed sufficient to cause rapid disconnection to prevent fire, but a lower 30mA rating is not strictly required for direct contact protection or would lead to nuisance tripping in certain systems.

    • 300mA and 500mA RCDs:

      • Purpose: Primarily fire protection and sometimes for discrimination in larger industrial installations.

      • Selection: Employed when greater discrimination is needed (e.g., to prevent a single fault from tripping the entire installation), or when the earth fault loop impedance is very high, making lower current RCDs impractical. Generally used upstream from lower rated RCDs.

  • Selection Criteria:

    • Risk Assessment: Identify the level of risk (e.g., direct contact, fire, specific environments like bathrooms or construction sites).

    • Regulatory Requirements (BS 7671): Adhere to specific regulations for different circuit types and locations. For example, Section 701 (Bathing and Shower Rooms) or Section 704 (Construction Sites) have specific RCD requirements.

    • Discrimination: In installations with multiple RCDs, higher-rated RCDs are often installed upstream of lower-rated RCDs to ensure that only the RCD nearest to the fault trips, minimizing downtime.

    • Nuisance Tripping: Consider the typical leakage current of the circuit's equipment to avoid unnecessary tripping, especially with lower-rated RCDs.