18th Edition Mythbuster


Guide to the New 18th Edition Wiring Regulations




Designed in the UK



18th Edition of the IET Wiring Regulations (BS7671:2018 Requirements for Electrical Installations) is now fully

effective and installations designed after January 1st 2019 must comply with these requirements. The Regulations

apply to the design, erection and verification of electrical installations, also additions and alterations to existing


In this guide we provide an overview of some of the changes including those regarding surge protection and RCD

types. This information provided is no substitute for the BS7671:2018 regulations and designers & installers should

follow the requirements & recommendations within the regulations.

Low Voltage Assemblies (421.1.201)

There has been no change in this regulation or any other regulation

which results in the way consumer units and distribution boards are

constructed. Within domestic/household premises, consumer units

and similar switchgear assemblies shall comply with BS EN 61439-3

(effective January 2016) and shall have their enclosure manufactured

from non-combustible material or be enclosed in a cabinet or

enclosure constructed of a non-combustible material.

Integration of Devices and Components (536.4.203)

In low voltage assemblies e.g. consumer units/distribution boards and incorporated devices/components shall

only be declared suitable according to the manufacturer of the assembly. Essentially this means that you must

use the board manufacturer’s approved control devices (MCBs, RCDs, etc). So, typically, the board and it’s

component devices will come from the same manufacturer. Cudis only declare Cudis devices to be suitable for

it’s boards. If a deviation from assembly manufacturer’s instructions is introduced then the person introducing the

deviation becomes the manufacturer and takes on the corresponding obligations.

Additional Requirements for Socket Outlets (411.3.3)

Additional protection by use of a 30mA RCD is now required for all socket outlets with ratings up to and including

32A and all domestic lighting circuits. Without exception, every socket outlet in a dwelling must have RCD

protection. If the installation is not a dwelling and if RCD protection may not be desirable for certain applications

(such as supply to a computer server), a documented risk assessment will be required to determine that RCD

protection is not necessary.



Caravan Parks (708.415.1)

Every socket outlet and also a final circuit intended for the fixed connection (from the

connection/metering point to the consumer) of a supply to a mobile or residential park

home shall be both individually protected by an RCD having a rated residual operating

current not exceeding 30mA. Devices selected shall disconnect all live conductors.

Arc Fault Detection Devices (421.1.7)

The use of Arc Fault Detection Devices (AFDDs) conforming to BS EN62606 is a recommended method for

providing additional protection against fire caused by arc faults in AC final circuits.

This device is specifically to detect and disconnect dangerous electrical arcs in both the fixed wiring and the

connected equipment which could be the source of a fire. If used, AFDDs should be installed in consumer units/

distribution boards at the origin of the circuit to be protected.

Following are examples of when additional AFDD protection may be want to be considered:

- Premises with sleeping accommodation

- Locations with a risk of fire due to the nature of processed or stored materials (i.e. BE2 locations such as barns,

wood-working shops, stores of combustible materials)

- Locations with combustible construction materials (i.e. CA2 locations such as wooden buildings)

- Fire propagating structures (i.e. CB2 locations)

- Locations with endangering of irreplaceable goods

In the schedule of inspections, under the section dealing with consumer units and distribution boards there is a

reference to check for the AFDD six monthly test notice, similar to what is already common practice for RCDs and


Although the use of AFDDs is not mandatory, at this time, Cudis are currently developing compact AFDD devices

which should become available during 2019.

Overvoltage Control (443.4)

Protection against transient overvoltages is now required in a number of specified situations where overvoltage

could result in:

- Serious injury or loss of life

- Interrupted public services

- Damage to cultural heritage

- Interrupted industrial and commercial activity

- Affect a large number of co-located individuals


- In any building which has lightning protection system installed or is supplied via overhead lines.

For other circumstances a risk assessment must be carried out to determine if protection against transient

overvoltage is required. Designers should note that if they do not do a risk assessment, protection against

transient overvoltage must be provided. The only exception is for individual dwellings if the total value of the

electrical installation and electrical equipment therein fails does not justify such protection.



There are different types of surge protection devices (SPDs) for different


• Type 1:

For structures equipped with an external lightning protection system or

requiring protection against effects of direct lightning (i.e. fire and electric

shock) such as structures supplied via overhead TT cables and metal

structures. The devices should be installed at the origin of the electrical

installation. Type 1 devices do not provide protection for sensitive

electrical and electronic systems.

• Type 2:

For structures not requiring protection against effects of direct lightning

but require protection against the effects of man-made transients including

switching of inductive or capacitive loads (i.e. motors, transformers, LV

generator supplies, etc). They should be installed at the origin of the

electrical installation and Cudis recommend additional type 2 devices be

installed in sub-distributions boards or close to the sensitive and critical

equipment to be protected.

• Type 3:

For additional protection of particularly sensitive equipment Class 3

devices should be installed in the fixed electrical installation close to

the equipment to be protected.

Cudis are an established supplier of Class 1, 2 and 3 surge protection

devices for both 3 phase and single phase applications. New products

and devices are being introduced and recent introductions include:

• single phase retrofit boards for existing installations

• 22 way split load consumer unit board with a surge

device pre-installed in the unit

• RCBO loaded boards with a surge device preinstalled

in the unit

• Combined Class 1+2 devices



Unwanted Tripping (531.3.2)

RCDs shall be selected and the circuits subdivided in such a way to avoid unwanted tripping by protective

conductor currents and/or earth leakage currents. The accumulation of such currents shall be not more than

30% of the rated residual operating current e,g, 30% of the RCD rating of 30mA. Another option is to have more

individual RCD/RCBO circuits or even use a RCD/RCBO for every circuit and Cudis have recently introduced a

range of RCBO loaded boards to cater to meet this need.

Current Ratings (536.4.202)

The rated current of a switch or RCD shall be based upon one of the following:

- Sum of final circuit current demand after any applicable load diversity factors or

- As above together with allowances for diversity between final circuits or

- The sum of the OCPD’s rated current multiplied by a diversity factor

Cudis recommend a diversity factor of 60% for it’s OCPDs and also recommend that designers conduct proper

electrical design assessments rather than standardising on 100A RCDs in consumer units, as currently suggested

by one OCPD supplier, as this may result in inappropriate device selection.

Type of RCD (531.3.3)

Differing types of RCDs behave differently in the presence of DC components or different

frequencies. The appropriate RCD should be selected for the specific application:

Cudis have decided to retain their high integrity dual RCD boards with Type AC RCDs but

have available Type A RCDs and Type A miniature RCBOs which the installer can buy, and

separately change, for less common applications.

RCDs are marked with a different symbol according to their type and the most common types

are shown below.

Type AC:

RCD can detect & respond to sinusoidal residual AC currents. May be used for general

purposes and suitable for the majority of applications. Other RCD types, described below, use

more expensive electronic tripping technology and are used in special applications.

Type A:

RCD can detect & respond to sinusoidal residual AC currents and residual pulsating DC

currents up to 6mA.

Suitable for electric vehicle charging.

(722.531.2.101: each charging point shall be protected by it’s own RCD if at least Type A having

a rated residual current not exceeding 30mA).

Type F:

RCD similar to Type A but for composite residual currents and pulsating DC currents up to


Suitable for equipment with frequency controlled speed drives.

Type B:

RCD can detect & respond to sinusoidal residual AC currents and pulsating DC currents and

smooth DC currents up to 20kHz.

Suitable for electric vehicle charging, photo voltaic and wind generators.


Let Cudis shine some light on the 18th Edition.

Do I need to install an 18th Edition Consumer Unit?

No. There has been no change to the design of a metal consumer unit since the 17th Edition,

Amendment 3. However the installation itself and circuit protection included must comply with

the current regulations.

Do I need to install 100A RCD’s in split load boards?

No. The use of diversity factors to determine maximum demand when designing an installation

is still an accepted practice in the 18th Edition (following the manufacturers guidelines). In

most cases a 63A or 80A RCD would be appropriate, in cases where there is a possibility of

overloading the RCD protecting the downstream circuits, then it may be more suitable to install

100A RCD’s.

(BS 7671 Chapter 314.1)

Do I need to install Type A RCCB’s as standard?

No. For general purposes, Type AC RCD’s may be used.

Certain installations require the use of Type A, Type B or Type F RCD’s to protect against DC

fault current which may become present on the installation. Such installations include Solar

PV and Car Charging Equipment. These devices can be installed at the location of the DC

installation (at the AC side) and will prevent any DC earth leakage current entering the rest of

the installation.

(BS 7671 Chapter 531.3.3)

Do I need to install a board with RCBO’s to overcome unwanted tripping of


No. The total earth leakage of the circuits protected by an RCD should not be more than 30%

of the rated residual operating current (eg. a 30mA RCD, regulation state no more than 9mA of

earth leakage detected on the downstream circuits).

Bearing in mind that some circuits will naturally have more protective conductor current / earth

leakage eg, circuits powering power supply units (like in PC’s) or transformers like LED Drivers.

Careful consideration of subdivision of circuits between the 2 protective RCD’s may be enough

to combat the accumulation of fault leakage.

Although installing all RCBO’s will give more flexibility in the design of installation and reduce

the consequences from tripping as only individual circuits will be affected.

(BS 7671 Chapter 531.3.2)



Can I use another brands circuit protection in a Cudis consumer unit?

Cudis, as the original manufacturer, do not recommend the use of another manufacturers circuit

protection product be used in our enclosures, as they have not been designed or tested for use

in our enclosures, therefore we cannot guarantee compatibility.

As per the 18th Edition, if incompatible products are installed inside a Cudis consumer

unit, then the person introducing the product becomes the original manufacturer with the

corresponding obligations.

(BS 7671 Chapter 536.4.203)

Do I need to install AFDD’s?

No. Arc Fault Detection Device’s at the moment are only recommended as a means of providing

additional protection against fire caused by arc faults in AC final circuits.

Although they may be specified in an electrical installation design, if so they should be

installed at the origin of the final circuit to be protected and in AC single phase circuits not

exceeding 230V.

(BS 7671 Chapter 421.1.7 & 532.6)

Do the provided clip in plastic blanks, to cover unused ways in a consumer

unit, comply with the 18th Edition Regulations?

Yes. All of our consumer units conform to BS61439-3 and we ensure that the blanks are firmly

secured in place with sufficient stability and durability to maintain the required degrees of

protection and appropriate separation from live parts under normal service conditions, taking

account of relevant external influences.

However, we also offer an MCB Blank which clips into the din rail offering greater levels of

stability and durability which may be more suitable in some circumstances.

(BS 7671 Chapter 416.2.3)

Do I have to install an SPD in a domestic single dwelling?

No. Not always. If a risk assessment is carried out and there are NO unacceptable

consequences that exist and there is not an increased risk of lightning present. Then an SPD

only has to be installed where the total value of the installation and equipment therein, justify

having an SPD installed.

This will involve a conversation with the customer to determine whether an SPD is required,

however it should be considered that the cost of an SPD is usually far less than even one

electronic device that may be attached to the installation.

(BS 7671 Chapter 443.4)


- Lumo® Consumer Units - Control Gear - Rotary Isolators - Circuit Protection -

- 3 Phase Distribution - Surge Protection - Voltage Optimisation -

Designed in the UK

www.cudis.co.uk - 0161 765 3000

sales@cudis.co.uk - Power House, Parker Street, Bury, Lancs BL9 0RJ

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