18th Edition BS 7671 Amd 2: effective RCD selection? |

2022-11-14 07:11:01 By : Ms. hongyuan ellen

During the years leading up to BS 7671 Amd 2 finally kicking Type AC RCDs in to touch for general domestic circuits, much has been written and discussed about the pro and cons of different Types of RCDs and where and when to use them in domestic installations – click here for article dated September 2017. 

The challenge is selecting and installing the solution that meets the Customer’s needs and the Regulations, regarding price, reliability, and safety.  Mcb Miniature Circuit Breaker

18th Edition BS 7671 Amd 2: effective RCD selection? |

Keep it simple in simple installations

Off-the-shelf consumer units should now contain Type A RCCBs or RCBOs to meet the needs of basic domestic installations covered by Amendment 2 i.e. Type AC not for use in general domestic circuits after 27th September 2022. Most manufactures have already made this change from Type AC to A for standard consumer units, but it is worth checking before leaving the wholesaler. If you purchase online, there are “offers” selling off stock with AC RCCBs installed in the consumer unit. 

Under the Electrical Equipment (Safety) Regulations 2016, the Manufacturer if UK based or the UK Company first importing the product into the UK and placing it on the market for sale, is legally responsible for providing information with the product, enabling safe installation and use. Appliances designed for permanent connection to the supply e.g. induction hobs, wall mounted HVAC, power tools, may have specific RCD requirements. For new installations, follow the Manufacturer’s installation instructions * . If you intend using an existing installation fused spur, check the RCD in the distribution board feeding the circuit, before connecting the new appliance. 

* Appliance RCD requirements in this example; given on page 22. Not that easy find in a hurry. 

Domestic installations incorporating permanently connected equipment including PV, EV, HP, Battery storage, EMS require specific training and experience to assess, design, select, install, and test - Ref BS7671 Part 3, 7, 8 and associated IET Codes of Practice (CoP).

Do not fall fowl of the old spaghetti western – see 132.16. An assessment of the existing installation work, equipment, and supply capability and how it might impact on the safety and long-term reliability of the installation will determine the best approach i.e. adding to the existing installation equipment or replacing see 134.1. Getting the basics right is prerequisite to identifying potential issues at the design stage, meeting the Regulations and completing the job within budget.

Multi-row enclosures *  associated with the manufacture’s devices, enable the provision of customised solutions for discrete installations. In some instances this is practical and safe solution when compared to installing additional enclosures alongside the existing consumer unit. However consider the isolation and switching requirements when selecting the enclosure solution - See 132.15 for general requirements and the appropriate IET CoP for the equipment.

* Enclosures mounted in, on or near the fabric of domestic properties e.g. in an attached garage must be metal - see Amd 2 Regulation 421.1.201. 

Domestic Assembly must meet BSEN 61439-3. To retain the manufacturer’s certification for DBOs assembled outside of the original manufacture’s factory (design requirement of BSEN 61439-3 1 Scope); refer to the individual manufacturer’s instructions for assembly and use: Associated devices, enclosed ratings, spacing, use of internal preformed cable links, busbars, product layouts and circuit loadings. 

The basics of effective RCD selection

There are no magic bullets for complex installations. Check the Regulations relating to your proposed design; see general installation requirements and specific requirements in Part 7, based on equipment you are intending to instal. Familiarity with these requirements and the practical methods to achieve the most cost effective but reliable and safe solution are prerequisite for a competitive quote, winning the order and a successful conclusion to the project. 

For a small domestic PV system < 4kW and the associated RCD requirements, refer to BS7671 Section 712 and the IET CoP for grid connected solar PV systems section 8.8. Most domestic PV systems are not suitable for operation with a 30 mA RCD connected to the output of the inverter. If possible, design the PV installation so that the PV AC supply cable does not require RCD protection. 

If the proposed design requires an RCD: Before selecting and purchasing anything, check the PV inverter installation manual -online, or contact the Manufacture for advice i.e. recommend sensitivity to prevent unwanted tripping and Type of RCD. Do not be surprised to find values of 300 mA. 

Regulation 712.531.3.5.1 (iii) specifies Type B RCDs for use on the out-put of PV systems, unless the manufacturer states that it is safe to use Type A or F RCDs.

Follow basic principles laid down in the Regs, keep these in mind for any design and application. Regulations 531.3.2 and 531.3.3 highlight the importance of effective installation design, regarding general layout, the number of circuits, method of cable installation / protection and the selection of appropriate devices and characteristics. 

The regulations recommend that standing or operational leakage current (leakage current flowing under normal operating conditions) should not exceed 30% of the RCD sensitivity. This allows a safety margin between the minimum tripping threshold allowed in the standards for RCCB and RCBO devises i.e. 50% of the RCD rated residual operating current (I∆n).

Regulation 531.3.3 details the limits of operation for the various Types of RCD, based on the characteristics of the residual current – summary Table 2 ref. 7/2016 V2 below.

Type A and F RCDs can provide protection with a limited amount of smooth DC superimposed on the AC waveform, but they do not detect smooth DC. Exceeding the stated value of smooth DC, compromises the protection level required in BS7671. For example, you can use Type A RCDs in conjunction with an RDC-DD for Mode 3 charging applications. The RDC-DD protects the Type A from the effects of smooth DC > 6 mA – see 722.531.3.101 (ii). For a detailed explanation relating to this subject and RDC-DDs - see Doepke Technical Publications N° 06. For a summary of the detection characteristics related to the distinct types of residual / leakage current component – see Table 2.

For a detailed explanation of AC leakage current, the types of residual current and requirements for RCD tripping characteristics - see  Doepke Technical Publications N° 27 .

Do not instal Type B and F downstream of Type A, as it would expose the Type A to smooth DC residual currents and or AC leakage currents with frequency components > 50Hz, up to the tripping threshold of the Type F or B RCD. 

Suggestion: Segregate Type B RCDs on a sperate feeder. As opposed to replacing the main incoming RCD with Type B and installing an additional Type B downstream. Same for Type F.

Incorporating these requirements into domestic and similar installations 

The minimum requirement for general purpose circuits in domestic premises, incorporating socket outlets is Type A – see 531.3.3 page 157: Single phase equipment designed for domestic use such as PCs, TVs, AV, and LED lamp drivers should be suitable for use with Type A RCDs. Single-phase kitchen appliances incorporating energy saving features, speed control, heat pumps and power tools may require Type F RCDs. If in doubt check the equipment installation data or contact the manufacturer for details.

Three phase equipment (Catering, Baking, Cleaning.) incorporating inverter controls, produce smooth dc residual currents – see 531.3.3 (iv). Equipment connected via socket-outlet  ≤  32A would require the use of 30 mA Type B RCD protection. Before choosing this method of connection, verify that the equipment is a. suitable for use with a 30 mA RCD; b. the required method of earthing – see 543.7.1.  

The risks associated with the use of Type A instead of Type F: Fault current > 50 Hz < 1 kHz will not trip the RCD – see Doepke Technical Publication N° 19 for a detailed explanation.  Additional information on Type F RCD see  Doepke Technical Publications N° 20.

To summarise, put the cart behind the horse: Check the Regs, are there specific requirements for the installation / equipment. Read any supporting CoPs. Check equipment manufacturers installation manuals – specific requirements for RCDs used with their kit. Design, Cost, Review, Quote. Purchasing from companies who provide poor technical support, costs more in time and money to sort out the issues. Not something you want to discover when installing and testing on site.

Chaz Andrews – Technical Manager, Doepke UK Ltd

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18th Edition BS 7671 Amd 2: effective RCD selection? |

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