Does the 17th Edition require all 13 A socket-outlets in domestic premises to be RCD-protected?

For new installations and rewires in domestic premises, all socket-outlets need to have additional protection by RCD, except perhaps for those intended to supply particular items of equipment such as freezers. Any socket-outlet not having RCD protection needs to be specifically labelled or otherwise suitably identified to indicate its intended use, such as ‘freezer only’. However, if the wiring to a dedicated socket-outlet is concealed in a wall or partition at a depth of less than 50 mm, or if the internal construction of the wall or partition includes metallic parts other than metallic fixings, the circuit will still need to be suitably protected (by RCD or other means).

The 17th Edition requires most if not all circuits in domestic premises to be RCD-protected. There have been a number of suggestions as to how the consumer unit may best be configured to comply with the Regulations, the most common being a main switch with RCBOs protecting each individual circuit. However, another suggestion favours a main switch with two RCDs protecting separate DIN rails. If careful consideration is given as to what each bar will control in the way of upstairs and downstairs lighting and power circuits, will this configuration comply?

The 17th Edition does not now define a zone 3 in a bathroom or shower room. What electrical equipment and accessories can be installed in the area between 0.6 m and 3.0 m from the edge of the bath or shower basin? Moreover, what minimum degree of IP protection are manufacturers likely to recommend?

There is no change from the 16th Edition requirements – that is, the general rules apply.  The manufacturer’s installation instructions must always be followed.

Do ‘meter tails’ concealed in walls or partitions need to be protected in accordance with Regulations 522.6.6 and/or 522.6.8?

Yes. Meter tails concealed in a wall or partition at a depth of less than 50 mm from a surface must be protected in accordance with Regulation 522.6.6. Also, irrespective of the depth from a surface, meter tails concealed in a wall or partition having internal metallic parts (except nails and screws, etc) are subject to the requirements of Regulation 522.6.8.

However, additional protection for meter tails by means of an RCD is not an acceptable option in respect of Regulation 522.6.7 (which in consequence rules out reliance on 522.6.6(v), routing in the ‘safe zones’ alone), or in respect of Regulation 522.6.8(v). Also, for TT systems, the only option remaining is to provide suitable mechanical protection (that is, to comply with Regulations 522.6.8(iv) and/or 522.6.6(iv) as appropriate).

Where the Regulations specify additional protection by RCD, does this include RCBOs?

Yes. Where the generic term ‘RCD’ is used, it refers to most types of residual current device including RCBOs (Residual current-operated circuit-breakers with integral overcurrent protection), RCCBs (Residual current-operated circuit-breakers without integral overcurrent protection) and SRCDs (Socket-outlets incorporating a residual current device). For the purposes of the Regulations, it does not include PRCDs (Portable residual current devices).

Does boiler pipework need to have additional equipotential bonding for electrical safety reasons?

There is no specific requirement in the Regulations for boiler pipework to be supplementary bonded. However, such bonding may be called for in the boiler manufacturer’s instructions, in which case BS 7671 requires those instructions to be followed (Regulation 510.2 refers). Any stated requirement for additional bonding that is considered to be unnecessary should be queried with the manufacturer concerned, and amended installation instructions requested.

Does the dispensation in Regulation 701.415.2 to omit supplementary bonding in a location containing a bath or shower apply to TT systems?

Yes

If the mains supply cable to a fixed appliance such as a flat screen TV is concealed in a wall or partition at a depth of less than 50 mm, does the 17th Edition require the lead to be RCD-protected in accordance with Regulation 522.6.7 even though it’s connected by means of a plug and socket?

Yes. The risk of penetration by a nail or screw is the same as for other concealed cable. Also, if the wall or partition has internal metallic parts (except nails and screws, etc), RCD protection in accordance with Regulation 522.6.8 is required irrespective of the depth of the cable from the surfaces.

To overcome thermal insulation issues, is it permissible to design a ring final circuit using 2.5 mm² cable protected by a 20 A protective device?

Yes, if the effective current carrying capacity of the cable is at least 12.5 A (20 A × 20/32), so as to effectively have the same ‘deemed to comply status’ as Regulation 433.1.5 gives to 30 A and 32 A ring final circuits.

Should signal amplifiers mounted at high level on television aerials where they might be struck by lightning be main bonded?

There is no specific requirement in BS 7671 but, if there is considered to be a significant risk of a lightning strike, the advice of a lightning protection specialist should be sought.

I am still working on an electrical installation that was designed to the 16th Edition. To which Edition should the installation be inspected, tested, verified and certificated?

An installation designed and installed to the 16th Edition should be inspected, tested, verified and certificated to that Edition.

What types of mechanical protection provide sufficient protection against penetration by nails, screws and the like?

As an example, steel of 3 mm minimum thickness is generally considered to provide sufficient mechanical protection, except where shot-fired nails are likely to be used.

Five flats are fed individually from five ‘sub-mains’ originating at the main intake position. The five gas and water meters are main bonded at the main intake position. Do we still need to provide main bonding in each of the flats?

Whilst main protective bonding is required at the main intake position, it is good practice also to provide main protective bonding in each flat even though, in this case, the origin of each installation could be considered to be at the main intake position.

Is an RCD main switch (such as a 100 mA time-delayed device) still required in the consumer unit of a new domestic installation forming part of a TT system?

For a domestic installation complying with the 17th Edition where all the final circuits are RCD-protected, an RCD main switch is no longer required, provided that the consumer unit is of all insulated construction.

Does the device that has to be provided for switching off a bathroom extract fan for mechanical maintenance need to be located adjacent to the fan?

Can accessories and electrical equipment such as socket-outlets and under-cupboard lighting be fixed to fitted kitchen units?

Yes, provided that they are securely fixed to rigid parts of the units that are not demountable or otherwise liable to be disturbed in normal service. However, care must be taken to comply with all the relevant requirements of BS 7671, including accessibility for inspection, testing and maintenance, and provision of adequate protection against damage (by impact or water for example) for the accessories, equipment and associated wiring. Regulation 530.4.2 is not intended to preclude such common practice. This is intended to be clarified in the 17th Edition in due course.

When a Grade D fire detection and alarm system to BS 5839-6: 2004 is protected by an RCD, should the RCD operate independently of any circuits supplying socket-outlets or portable equipment?

There are no particular requirements or limitations in BS 5839-6 concerning the use of RCDs with Grade D fire detection and alarm systems.

For Grade D systems, which comprise one or more mains-powered smoke alarms each with an integral standby power (such as typical domestic smoke alarm with battery), BS 5839-6 simply recommends that the mains supply to the smoke and/or heat alarms should take the form of either:
i) an independent circuit at the dwelling’s main distribution board, in which case no other electrical equipment should be connected to the circuit (other than a dedicated monitoring device installed to indicate failure of the mains supply to the smoke and/or heat alarms), or
ii) a separately electrically protected, regularly used local lighting circuit.

We recommend option ii) on the grounds that disconnection of the supply to the fire detection and alarm system will be more readily noticed. In our opinion ‘separately electrically protected’ does not preclude sharing an RCD with several other circuits where this is permitted by BS 7671.

Does the 17th Edition permit connection of smoke alarms to an adjacent lighting circuit taking into account Regulation 560.7.1, which states that circuits of safety services shall be independent of other circuits?

Does the R₁ + R₂ test confirm the correct polarity of a radial circuit?

Appendix 15 of BS 7671: 2008 gives advice on ring final circuits and sharing/spreading the load around the circuit. Item (iii) suggests that cookers, ovens and hobs over 2 kW should be on their own dedicated circuit. Why can’t ovens of less than 3 kW be connected to a ring final circuit via a suitable connection point such as a socket-outlet or fused connection unit?

Are Regulations 522.6.6, 522.6.7 and 522.6.8, which require additional protective measures for cables concealed in walls and partitions, intended to apply to cables of SELV, PELV and/or FELV (other than to the cables supplying the sources at Low Voltage)?

In locations containing a bath or shower, is Regulation 701.411.3.3, which requires additional protection by RCD, intended to apply to circuits of SELV, PELVand/or FELV (other than to the cables supplying the sources at Low Voltage if installed within the location)?

In ‘smith construction’ or ‘truss’ dwellings, metal frame parts in the loft area are likely to be bolted to structural metalwork embedded in the ground and hence are likely to be extraneous-conductive-parts. Do they require main protective bonding?

Is Regulation 701.411.3.3, which requires RCD protection for all circuits of a bathroom or shower room, intended to apply to ELV circuits?

No. This is intended to be clarified in the 17th Edition in due course.

However, RCD protection would be required for the circuits supplying ELV sources (such as transformers) if those sources are in a bathroom or shower room.

Is it necessary to verify voltage drop during initial verification?

Considering Regulations 134.1.1 and 510.2 which require equipment to be installed in accordance with instructions provided by the manufacturer, are installers now required to check torque settings for connection tightness at consumer units where these are manufacturers instructions.

In a location containing a shower, what is the horizontal limit of zone 1 for showers without a basin?

Regulation 560.7.7 requires cables for safety circuits, other than metallic screened fire-resistant cables, to be adequately and reliably separated from other circuit cables. In addition to mineral insulated cables, what cables would be exempted from this separation requirement?

Is it acceptable to use an unfixed connector block to join the line or neutral conductors at a light switch?

As the designer of an installation, am I allowed to rely on the RCD element of an RCBO to provide for fault protection in order to allow for loop impedance values greater than given in Table 41.3?

Are there any particular requirements relating to the mounting height or location of consumer units for electrical installations in new dwellings?

The provision of access to consumer units is not specifically covered by Building Regulations or BS 7671. However, consumer units need to be so located as to enable reasonable access by the users, including for the purpose of testing the RCDs at regular intervals, and in case of emergency.

BSI Draft for development DD 266: 2007 – Design of accessible housing: Lifetime homes – Code of practice, explains how, by following the principles of inclusive design, general needs housing can be made sufficiently flexible and convenient to meet the existing and changing needs of most households, and so give disabled and older people more choice over where they live.

Amongst other things, the code of practice recommends that meters and consumer units should be mounted 1200 mm to 1400 mm from the floor so that the readings and switches can be viewed by a person standing or sitting, and should be positioned to be accessible.

Is the space within an airing cupboard in a bathroom or shower room that would otherwise be in a particular zone were it not for the cupboard door, considered to be within that zone?

What is the correct sequence for testing RCDs?

Preferably, RCDs should be tested in the sequence of: x1I_Δn, x5I_Δn (if required for additional protection), followed by x0.5 I_Δn and then finally the test button trip.

However, some automated test instruments test in the sequence of: x0.5 I_Δn followed by the x1I_Δn test, and then x5I_Δn test (if required for additional protection).

In any case, the test button should be operated last in the sequence.

Is it acceptable to mount a cooker switch in a kitchen cupboard that is adjacent to the cooking appliance?

Yes, provided the switch is located so as to be readily accessible.

Is a cooker switch intended to be used as an emergency switch?

No.

Where the earth fault loop impedance for a circuit exceeds the maximum Z_s for the overcurrent device, is it permissible to use an RCD?

Yes.

In the model Schedule of Inspections in BS 7671, ‘Presence of supplementary bonding conductors’ is included under both ‘Automatic disconnection of supply’ and ‘Additional protection’. Why are there two boxes with the same meaning and where should one be ticked rather than the other?

Supplementary bonding may be provided for either reason. The appropriate box(es) should be completed.

In the model Schedule of Inspections in BS 7671, what needs to verified in order to confirm the ‘Presence of earthing arrangements for combined protective and functional purposes’?

To verify that the requirements of Regulation 543.5.1 relating to combined protective and functional earthing, and Regulation 543.7 relating to high protective conductor currents, have been met as appropriate.

In the model Schedule of Inspections in BS 7671, what has 'Presence of adequate arrangements for alternative sources' to do with automatic disconnection of supply?

To confirm that the fault protection arrangements for alternative sources of supply have been met.

In the model Schedule of Inspections in BS 7671, under ‘Automatic disconnection of supply’, where applicable, what needs to be verified to confirm the requirements for FELV have been met?

It needs to be verified that the supplementary provisions described in Regulations 411.7.2 and 411.7.3 have been applied to ensure basic protection and fault protection.

The model Schedule of Inspections in BS 7671 would be used as part of an Electrical installation certificate where a consumer unit was installed, but no circuit cables were replaced. All boxes have to be a tick or n/a. What then do you need to do with the boxes to do with cable routes in prescribed zones?

As the installer of the consumer unit has insufficient knowledge of the cable routes, the box ‘Routing of cables in prescribed zones’ should be marked ‘n/a’.