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Frequently Asked Questions: Consumer units and protective devices

How often does my RCD need testing?

Operating the RCD test button provides assurance that it is operating correctly. If RCDs are regularly tested by operating the test button, problems can be detected.

Previous versions of BS 7671 required a label to be installed advising the user to press the test button quarterly, this has changed to six months in the current version of BS 7671:2018, as indicated in Regulation 514.12.2.  

A good way of remembering to test your RCD, do it when you change the clocks, that way it will be done twice a year.

This IET Wiring Matters article ‘Which RCD Type’ provides further guidance on RCDs.

Should I use a torque screwdriver?

If a connection is not tight enough, it can create hotspots and arcing. If you over tighten the connection, you could crush the conductor causing a hotspot and increasing the risk of it breaking. Regulation 526.1 of BS 7671:2018 requires every electrical connection to provide durable electrical continuity and adequate mechanical strength and protection.

These high-resistance terminations can generate high amounts of heat where the connections can actually glow red. This over-heating can potentially cause the consumer unit to reach a high enough temperature for a plastic enclosure to ignite.

Regulation 134.1.1 of BS 7671:2018, states that the installation of electrical equipment shall take account of manufacturer’s instructions. Most consumer units now come with a torque setting sticker as standard, otherwise, the information is available from the manufacturer.

This BEAMA technical guide provides guidance.

Do I need to install a metal consumer unit in domestic premises?

The requirements for non-combustible consumer units were introduced in the 17th Edition of the IET Wiring Regulations (BS 7671:2008+A3:2015).

Regulation 421.1.20 of BS 7671:2018 states, for consumer units and similar switchgear assemblies in domestic premises, to be manufactured from non-combustible materials or be enclosed in a cabinet constructed from non-combustible materials.

This BEAMA technical update provides guidance.

How to fire seal a consumer unit?

There are requirements for consumer units and similar switchgear assemblies in domestic premises to be manufactured from non-combustible materials or to be enclosed in a cabinet constructed from non-combustible materials.

The requirements are identified in BS 7671:2018 Regulation 421.1.201, but there are no requirements for fire-rated glands or internal and/or external intumescent seal, pad, strip or similar to be used to seal a consumer unit.

Manufacturer’s instructions should be followed. 

This BEAMA technical update provides guidance.

Do I need to install an RCD on a lighting circuit?

Most circuits within a domestic installation now require RCD protection. Regulation 411.3.4 of BS 7671:2018 requires that all luminaires within domestic (household) premises, are to have additional protection by a 30 mA RCD when installed in an AC final circuit.

This new Regulation was introduced in the 18th Edition of the IET Wiring Regulations, it means that electrical installers have to provide RCD protection for all new lighting circuits, and for existing circuits which are significantly altered in domestic (household) premises.

Can I install a metal consumer unit with a TT earthing system?

High earth fault loop impedance values associated with TT earthing systems mean the earth fault current level is unlikely to be sufficient to operate the distributor’s protective device within the permitted time of 1 second, as required in Regulation 411.2.4, or at all in some cases, depending on Earth resistance values.

There is nothing to prevent a metal consumer unit being installed with a TT earthing system, but it’s important to ensure that an earth fault cannot develop between the meter tails and the metallic enclosure before the RCD.

Methods of reducing an earth faults include keeping the meter tails as short as possible, the installation of proprietary clamps and glands to secure the cables and prevent strain on the terminations, minimising the risk of damage to the meter tails and a subsequent earth fault.

Can I install a consumer unit remote from the distributor’s cut-out?

If it’s required to install a consumer unit in a different location to the distributor’s cut-out, it may be necessary to install a sub-main cable, protected with a switch fuse disconnector.

BS 7671:2018, Regulations 433.2.2 and 434.2.1 states that unless the conductors are installed in such a manner as to reduce any fault to a minimum, they should be no longer than three metres.

The installer should contact the Distribution Network Operator (DNO) to verify their requirements, it’s their cut-out fuse which is protecting the meter tails along with their equipment.

ENA Engineering Recommendation G87 provides guidance on meter tails in Section 7.4.2.

Is a Type AC RCD suitable for all installations?

Different types of RCD exist, the appropriate type of RCD shall be selected, depending on the type of residual direct current which is produced from the equipment it’s protecting.

If any element of residual direct current is present such as produced from electronic equipment, this can ‘blind’ a Type AC RCD and prevent it from operating.

Regulation 531.3.3 of BS 7671:2018 states that Type AC RCDs are only suitable for sinusoidal residual current.

It is important to select the appropriate device for the type of equipment which is likely to be used in the installation. Manufacturers of equipment should provide guidance on which type of RCD is required.

This IET Wiring Matters article ‘Which RCD Type’ provides guidance.

Do RCDs provide overcurrent protection?

Residual Current Device (RCD) is a generic term to cover a wide range of residual current detecting devices.

Unless it is a Residual Current Breaker with Overcurrent protection (RCBO), Residual Current Circuit Breakers (RCCBs) do not provide overcurrent protection.

There are two different ratings on an RCCB - the tripping current rated in milliamps, and the current carrying capacity rating. It’s important to remember that the device is not designed to trip at the current capacity rating value and additional protection against overload is required.

It’s no longer acceptable to rely on diversity alone, BS 7671:2018, Regulation 536.4.202 states that overload protection shall not solely be based on the use of diversity factors of the downstream circuits.

In order to achieve overload protection of the RCD circuits, the rating shall be selected to coordinate with the upstream fuse, which is likely to be the distributors fuse.

To achieve overload protection of RCDs, the rated current of the overcurrent protective device shall be selected according to the RCD manufacturer’s instructions.

This BEAMA technical bulletin provides guidance.

Do I have to install SPDs in a domestic installation?

Lightning strikes are the most common source of extreme transient overvoltage’s, where total outage of an unprotected system can occur, with significant damage to equipment and cable insulation through flashover, potentially resulting in loss of life or injury due to fire or electric shock.

Switching transients may also occur, for example, due to interruption of short circuit currents, such as a fuse blowing. Whilst this may be of a lower magnitude than a lightning strike, it is likely to happen more frequently.

BS 7671:2018, Regulation 443.4 states that protection against transient overvoltages shall be provided within single dwellings unless the value of the installation and equipment therein does not justify such protection.

Surge Protective Devices (SPDs) are relatively inexpensive when compared to the cost of an electrical installation and expensive electronic equipment such as PCs and TVs, it is difficult not to justify the installation of SPDs. 

This IET webinar on SPDs and AFDDs provides further guidance.