GUIDE TO THE NEW
18 TH EDITION
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.
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.
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).
RCD similar to Type A but for composite residual currents and pulsating DC currents up to
Suitable for equipment with frequency controlled speed drives.
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
(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
(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
(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
(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
email@example.com - Power House, Parker Street, Bury, Lancs BL9 0RJ