Requirements for Electrical Installations, IET Wiring Regulations, 18th Edition, BS 7671:2018

Home>>BS EN IEC UL Standard for SPDs>>Requirements for Electrical Installations, IET Wiring Regulations, 18th Edition, BS 7671:2018

Requirements for Electrical Installations, IET Wiring Regulations, Eighteenth Edition, BS 7671:2018


Surge protection devices (SPDs) and the 18th Edition Regulations

LSP-Surge-Protection-Web-banner-p2

The arrival of the 18th Edition of the IET Wiring Regulations further reshapes the regulatory landscape for electrical contractors. Surge protection devices (SPDs) are designed to prevent electric shock and having excess voltage damaging the installation’s wiring infrastructure.

18th Edition requirements for surge protection

The arrival of the 18th Edition of the IET Wiring Regulations further reshapes the regulatory landscape for electrical contractors. A number of important areas have been scrutinised and reviewed; among them is the issue of surge protection and devices designed to mitigate any excess voltage risks. Surge protection devices (SPDs) are designed to prevent electric shock and having excess voltage damaging the installation’s wiring infrastructure. Should an over-voltage event occur, the SPD diverts the resulting excess current flow to Earth.

Regulation 443.4 requires, (except for single dwelling units where the total value of the installation and equipment therein does not justify such protection), that protection against transient over-voltages is provided where the consequence caused by over-voltage could result in serious injury, damage to culturally sensitive places, interruption of supply or affect large numbers of co-located persons or loss of life.

When should surge protection be fitted?

For all other installations a risk assessment should be carried out to determine whether SPDs should be installed. Where a risk assessment is not carried out, then SPDs should be installed. Electrical installations in single dwelling units are not required to have SPDs installed, but their use is not precluded and it may be that in discussion with a client such devices are installed, reducing significantly risks associated with transient over-voltages.

This is something contractors have not previously had to consider to any great extent, and will need to be taken account of, both in terms of time allocation for project completion as well as cost add-ons for the customer. Any electronic equipment may be vulnerable to transient over-voltages, which can be caused by lightning activity or a switching event. This creates a voltage spike increasing the wave’s magnitude to potentially several thousand volts. This could cause expensive and instant damage or significantly reduce an item of equipment’s lifespan.

The need for SPDs will depend on many differing factors. These include the level of exposure of a building to lightning-induced voltage transients, the sensitivity and value of the equipment, the type of equipment used within the installation, and whether there is equipment within the installation that could generate voltage transients. While the shift in responsibility of risk assessment falling on the contractor is likely to be a surprise to many, by accessing the correct support they can seamlessly integrate this function into their traditional work approach and ensure adherence to the new regulations.

LSP Surge Protection Devices

LSP has a range of Type 1 and 2 surge protection devices to ensure you comply with the new 18th Edition Regulations. For more information on SPDs and LSP Electrical’s range visit: www.LSP-internationa.com

Visit out 18th Edition BS 7671:2018 for free, downloadable guides on the key regulation changes of BS 76:71. Including information on RCD Selection, Arc Fault Detection, Cable Management, Electric Vehicle charging, and Surge Protection. Download these guides straight to any device so you can read them whenever and wherever.

Requirements for Electrical Installations, IET Wiring Regulations, Eighteenth Edition, BS 7671-2018Item Subjects: Electrical Regulations

Pages: 560

ISBN-10: 1-78561-170-4

ISBN-13: 978-1-78561-170-4

Weight: 1.0

Format: PBK

Requirements for Electrical Installations, IET Wiring Regulations, Eighteenth Edition, BS 7671:2018

The IET Wiring Regulations are of interest to all those concerned with the design, installation and maintenance of electric wiring in buildings. This includes electricians, electrical contractors, consultants, local authorities, surveyors and architects. This book will also be of interest to professional engineers, as well as students at university and further education colleges.

The 18th Edition of the IET Wiring Regulations published in July 2018 and came into effect in January 2019. Changes from the previous edition include requirements concerning Surge Protection Devices, Arc Fault Detection Devices and the installation of electric vehicle charging equipment as well as many other areas.

How will the 18th Edition change daily work for electrical installers

How will the 18th Edition change daily work for electrical installers?

The 18th Edition of the IET Wiring regulations has landed, bringing with it an array of new things for electrical installers to be aware of and make part of their day to day.

We are now one month in to a six-month adjustment period for electricians to make sure they have everything in place. From January 1st 2019 installations must be fully compliant to the new regulations, meaning all electrical work that takes place from December 31st 2018 must abide by the new rules.

In line with the latest technology advances and updated technical data, the new regulations aim to make installations safer for both electricians and the end user, as well as impact on energy efficiency.

All changes are important, however we have picked out four key points that we think are particularly interesting:

1: Metal Cable Supports

Regulations currently outline that only cable located on fire escape routes must be supported against early collapse in the event of a fire. The new regulations now demand that metal fixings, rather than plastic ones, be used to support all cables throughout installations, to reduce risk to occupants or fire fighters from falling cables as a result of failed cable fixings.

2: Installation of Arc Fault Detection Devices

Considering that UK buildings now have more electrical equipment in them than ever before, and electrical fires are occurring at roughly the same rate year-on-year, the installation of Arc Fault Detection Devices (AFDDs) to moderate fire risk in some circuits has been introduced.

Electrical fires caused by arc faults usually occur at poor terminations, loose connections, though old and failing insulation or in damaged cable. These sensitive AFDDs can lower the likelihood of electrical fires resulting from arcs by early detection and isolation.

Installation of AFDDs started in the US several years ago, and there has been a reduction in related fires by about 10%.

3.All AC sockets rated up to 32A now require RCD protection

Residual Current Devices (RCDs) constantly monitor the electric current in the circuits they protect and trips the circuit if flow through an unintended path to earth is detected—such as a person.

These are life safety devices and potentially a life-saving update. Previously, all sockets rated up to 20A required RCD protection, but this has been extended in an effort to reduce electric shocks to installers working with live AC socket outlets. It will also protect the end user in cases where a cable is damaged or cut and the live conductors could be accidentally touched, causing current to flow to earth.

To prevent the RCD being overwhelmed by the current wave form, however, care must be taken to ensure the appropriate RCD is used.

4: Energy efficiency

The draft of the 18th Edition update featured a clause on the energy efficiency of electrical fixings. In the final version published, this has been changed to full recommendations, found in Appendix 17. This recognises the nationwide need to reduce energy consumption overall.

The new recommendations encourage us to make the most of overall use of electricity, in the most efficient way.

Overall, the revised installation processes may call for investments in new equipment, and of course further training. Most importantly though, if working on a new build project, for example, electricians may now have opportunities to take on more leading roles in the design process of a building, to ensure the whole project complies to the new regulations

The 18th Edition brings new progress toward safer installation and safer spaces for end users. We know that electricians across the UK are working hard to prepare for these changes and we want to know what you think will affect you most and what you are doing to make the transition as smooth as possible.

Requirements for Electrical Installations

BS 7671

Ensure that your work meets the requirements of the Electricity at Work Regulations 1989.

BS 7671 (IET Wiring Regulations) sets the standards for electrical installation in the UK and many other countries. The IET co-publishes BS 7671 with the British Standards Institution (BSI) and is the authority on electrical installation.

About BS 7671

The IET runs the JPEL/64 committee, (the national Wiring Regulations committee), with representatives from a wide range of industry organisations. The committee takes on board information from international committees and UK specific requirements, to ensure consistency and improve safety throughout the UK electrical industry.

The 18th Edition

The 18th Edition IET Wiring Regulations (BS 7671:2018) published in July 2018. All new electrical installations will need to comply with BS 7671:2018 from 1st January 2019.

To help industry apply the requirements of BS 7671, and to get up to date with the 18th Edition, the IET provides a wealth of resources, from guidance materials, events and training, to free information such as Wiring Matters online magazine. See the boxes below for more information on our range of resources.

18th edition changes

The following list provides an overview of the main changes within the 18th Edition IET Wiring Regulations (publishing 2nd July 2018). This list is not exhaustive as there are many smaller changes throughout the book not included here.

BS 7671:2018 Requirements for Electrical Installations will be issued on 2nd July 2018 and is intended to come into effect on 1st January 2019.

Installations designed after 31st December 2018 will have to comply with BS 7671:2018.

The Regulations apply to the design, erection and verification of electrical installations, also additions and alterations to existing installations. Existing installations that have been installed in accordance with earlier editions of the Regulations may not comply with this edition in every respect. This does not necessarily mean that they are unsafe for continued use or require upgrading.

A summary of the main changes is given below. (This is not an exhaustive list).

Part 1 Scope, object and fundamental principles

Regulation 133.1.3 (Selection of equipment) has been modified and now requires a statement on the Electrical Installation Certificate.

Part 2 Definitions

Definitions have been expanded and modified.

Chapter 41 Protection against electric shock

Section 411 contains a number of significant changes. Some of the main ones are mentioned below:

Metallic pipes entering the building having an insulating section at their point of entry need not be connected to the protective equipotential bonding (Regulation 411.3.1.2).

The maximum disconnection times stated in Table 41.1 now apply for final circuits up to 63 A with one or more socket-outlets and 32 A for final circuits supplying only fixed connected current-using equipment (Regulation 411.3.2.2).

Regulation 411.3.3 has been revised and now applies to socket-outlets with a rated current not exceeding 32A. There is an exception to omit RCD protection where, other than a dwelling, a documented risk assessment determines that RCD protection is not necessary.

A new Regulation 411.3.4 requires that, within domestic (household) premises, additional protection by an RCD with a rated residual operating current not exceeding 30 mA shall be provided for AC final circuits supplying luminaires.

Regulation 411.4.3 has been modified to include that no switching or isolating device shall be inserted in a PEN conductor.

Regulations 411.4.4 and 411.4.5 have been redrafted.

The regulations concerning IT systems (411.6) have been reorganized. Regulations 411.6.3.1 and 411.6.3.2 have been deleted and 411.6.4 redrafted and a new Regulation 411.6.5 inserted.

A new Regulation group (419) has been inserted where automatic disconnection according to Regulation 411.3.2 is not feasible, such as electronic equipment with limited short-circuit current.

Chapter 42 Protection against thermal effects

A new Regulation 421.1.7 has been introduced recommending the installation of arc fault detection devices (AFDDs) to mitigate the risk of fire in AC final circuits of a fixed installation due to the effects of arc fault currents.

Regulation 422.2.1 has been redrafted. Reference to conditions BD2, BD3 and BD4 has been deleted. A note has been added stating that cables need to satisfy the requirements of the CPR in respect of their reaction to fire and making reference to Appendix 2, item 17. Requirements have also been included for cables that are supplying safety circuits.

Chapter 44 Protection against voltage disturbances and electromagnetic disturbances

Section 443, which deals with protection against overvoltages of atmospheric origin or due to switching, has been redrafted.

The AQ criteria (conditions of external influence for lightning) for determining if protection against transient overvoltages is needed are no longer included in BS 7671. Instead, protection against transient overvoltages has to be provided where the consequence caused by overvoltage (see Regulation 443.4)

(a) results in serious injury to, or loss of, human life, or(b) results in interruption of public services/or damage to and cultural heritage, or
(c) results in interruption of commercial or industrial activity, or
(d) affects a large number of co-located individuals.

For all other cases, a risk assessment has to be performed in order to determine if protection against transient overvoltage is required.

There is an exception not to provide protection for single dwelling units in certain situations.

Chapter 46 Devices for isolation and switching – A new Chapter 46 has been introduced.

This deals with non-automatic local and remote isolation and switching measures for the prevention or removal of dangers associated with electrical installations or electrically powered equipment. Also, switching for the control of circuits or equipment. Where electrically powered equipment is within the scope of BS EN 60204, only the requirements of that standard apply.

Chapter 52 Selection and erection of wiring systems

Regulation 521.11.201 which give requirements for the methods of support of wiring systems in escape routes, has been replaced by a new Regulation 521.10.202. This is a significant change.

Regulation 521.10.202 requires cables to be adequately supported against their premature collapse in the event of a fire. This applies throughout the installation and not just in escape routes.

Regulation 522.8.10 concerning buried cables has been modified to include an exception for SELV cables.

Regulation 527.1.3 has also been modified, and a note added stating that cables also need to satisfy the requirements of the CPR in respect of their reaction to fire.

Chapter 53 Protection, isolation, switching, control and monitoring

This chapter has been completely revised and deals with general requirements for protection, isolation, switching, control and monitoring and with the requirements for selection and erection of the devices provided to fulfil such functions.

Section 534 Devices for protection against overvoltage

This section focuses mainly on the requirements for the selection and erection of SPDs for protection against transient overvoltages where required by Section 443, the BS EN 62305 series, or as otherwise stated.

Section 534 has been completely revised and the most significant technical change refers to the selection requirements for the voltage protection level.

Chapter 54 Earthing arrangements and protective conductors

Two new regulations (542.2.3 and 542.2.8) have been introduced concerning earth electrodes.

Two further new regulations (543.3.3.101 and 543.3.3.102) have been introduced. These give requirements for the insertion of a switching device in a protective conductor, the latter regulation relating to situations where an installation is supplied from more than one source of energy.

Chapter 55 Other equipment

Regulation 550.1 introduces a new scope.

New Regulation 559.10 refers to ground-recessed luminaires, the selection and erection of which shall take account of the guidance given in Table A.1 of BS EN 60598-2-13.

Part 6 Inspection and testing

Part 6 has been completely restructured, including the regulation numbering to align with the CENELEC standard.

Chapters 61, 62 and 63 have been deleted and the content of these chapters now form two new Chapters 64 and 65.

Section 704 Construction and demolition site installations

This section contains a number of small changes, including requirements for external influences (Regulation 704.512.2), and a modification to Regulation 704.410.3.6 concerning the protective measure of electrical separation.

Section 708 Electrical installations in caravan/camping parks and similar locations

This section contains a number of changes including requirements for socket-outlets, RCD protection, and operational conditions and external influences.

Section 710 Medical locations

This section contains a number of small changes including the removal of Table 710, and changes to Regulations 710.415.2.1 to 710.415.2.3 concerning equipotential bonding.

In addition, a new Regulation 710.421.1.201 states requirements regarding the installation of AFDDs.

Section 715 Extra-low voltage lighting installations

This section contains only minor changes including modifications to Regulation 715.524.201.

Section 721 Electrical installations in caravans and motor caravans

This section contains a number of changes including requirements electrical separation, RCDs, proximity to non-electrical services and protective bonding conductors.

Section 722 Electric vehicle charging installations

This section contains significant changes to Regulation 722.411.4.1 concerning the use of a PME supply.

The exception concerning reasonably practicable has been deleted.

Changes have also been made to requirements for external influences, RCDs, socket-outlets and connectors.

Section 730 Onshore units of electrical shore connections for inland navigation vessels

This is an entirely new section and applies to onshore installations dedicated to the supply of inland navigation vessels for commercial and administrative purposes, berthed in ports and berths.

Most, if not all, of the measures used to reduce the risks in marinas apply equally to electrical shore connections for inland navigation vessels. One of the major differences between supplies to vessels in a typical marina and electrical shore connections for inland navigation vessels is the size of the supply needed.

Section 753 Floor and ceiling heating systems

This section has been completely revised.

The scope of Section 753 has been extended to apply to embedded electric heating systems for surface heating.

The requirements also apply to electric heating systems for de-icing or frost prevention or similar applications, and cover both indoor and outdoor systems.

Heating systems for industrial and commercial applications complying with IEC 60519, IEC 62395 and IEC 60079 are not covered.

Appendices

The following main changes have been made within the appendices

Appendix 1 British Standards to which reference is made in the Regulations includes minor changes, and additions.

Appendix 3 Time/current characteristics of overcurrent protective devices and RCDs

The previous contents of Appendix 14 concerning earth fault loop impedance have been moved into Appendix 3.

Appendix 6 Model forms for certification and reporting

This appendix includes minor changes to the certificates, changes to the inspections (for new installation work only) for domestic and similar premises with up to 100 A supply, and examples of items requiring inspection for an electrical installation condition report.

Appendix 7 (informative) Harmonized cable core colours

This appendix includes only minor changes.

Appendix 8 Current-carrying capacity and voltage drop

This appendix includes changes regarding rating factors for current-carrying capacity.

Appendix 14 Determination of prospective fault current

The contents of Appendix 14 concerning earth fault loop impedance have been moved into Appendix 3. Appendix 14 now contains information on determination of prospective fault current.

Appendix 17 Energy efficiency

This is a new appendix that provides recommendations for the design and erection of electrical installations including installations having local production and storage of energy for optimizing the overall efficient use of electricity.

The recommendations within the scope of this appendix apply for new electrical installations and modification of existing electrical installations. Much of this appendix will not apply to domestic and similar installations.

It is intended that this appendix is read in conjunction with BS IEC 60364-8-1, when published in 2018

The IET Wiring Regulations require all new electrical system designs and installations, as well as alterations and additions to existing installations, to be assessed against transient overvoltage risk and, where necessary, protected using appropriate surge protection measures (in the form of Surge Protection Devices SPDs).

Transient overvoltage protection introduction
Based on the IEC 60364 series, the 18th Edition of BS 7671 Wiring regulations covers the electrical installation of buildings including the use of surge protection.

The 18th Edition of BS 7671 applies to the design, erection and verification of electrical installations, and also to additions and alterations to existing installations. Existing installations that have been installed in accordance with earlier editions of BS 7671 may not comply with the 18th edition in every respect. This does not necessarily mean that they are unsafe for continued use or require upgrading.

A key update in the 18th Edition relates to Sections 443 and 534, which concern protection of electrical and electronic systems against transient over-voltages, either as a result of atmospheric origin (lightning) or electrical switching events. Essentially, the 18th Edition requires all new electrical system designs and installations, as well as alterations and additions to existing installations, to be assessed against transient overvoltage risk and, where necessary, protected using appropriate protection measures (in the form of SPDs).

Within BS 7671:
Section 443: defines the criteria for risk assessment against transient over-voltages, considering the supply to the structure, risk factors and rated impulse voltages of equipment

Section 534: details the selection and installation of SPDs for effective transient overvoltage protection, including SPD Type, performance and co-ordination

Readers of this guide should be mindful of the need to protect all incoming metallic service lines against the risk of transient over-voltages.

BS 7671 provides focused guidance for the assessment and protection of electrical and electronic equipment intended to be installed on AC mains power supplies.

In order to observe the Lightning Protection Zone LPZ concept within BS 7671 and BS EN 62305, all other incoming metallic service lines, such as data, signal and telecommunications lines, are also a potential route through which transient over-voltages to damage equipment. As such all such lines will require appropriate SPDs.

BS 7671 clearly points the reader back to BS EN 62305 and BS EN 61643 for specific guidance. This is covered extensively in the LSP guide to BS EN 62305 Protection Against Lightning.

IMPORTANT: Equipment is ONLY protected against transient over-voltages if all incoming / outgoing mains and data lines have protection fitted.

Transient overvoltage protection Safeguarding your electrical systems

Transient overvoltage protection Safeguarding your electrical systems

Why is transient overvoltage protection so important?

Transient over-voltages are short duration surges in voltage between two or more conductors (L-PE, L-N or N-PE), which can reach up to 6 kV on 230 Vac power lines, and generally result from:

  • Atmospheric origin (lightning activity through resistive or inductive coupling, and/or Electrical switching of inductive loads
  • Transient over-voltages significantly damage and degrade electronic systems. Outright damage to sensitive electronic systems, such as

computers etc, occurs when transient over-voltages between L-PE or N-PE exceed the withstand voltage of the electrical equipment (i.e. above 1.5 kV for Category I equipment to BS 7671 Table 443.2). Equipment damage leads to unexpected failures and expensive downtime, or risk of fire/electric shock due to flashover, if insulation breaks down. Degradation of electronic systems, however, begins at much lower overvoltage levels and can cause data losses, intermittent outages and shorter equipment lifetimes. Where continuous operation of electronic systems is critical, for example in hospitals, banking and most public services, degradation must be avoided by ensuring these transient over-voltages, which occur between L-N, are limited below the impulse immunity of equipment. This can be calculated as twice the peak operating voltage of the electrical system, if unknown (i.e. approximately 715 V for 230 V systems). Protection against transient over-voltages can be achieved through installation of a coordinated set of SPDs at appropriate points in the electrical system, in line with BS 7671 Section 534 and the guidance provided in this publication. Selecting SPDs with lower (i.e. better) voltage protection levels (UP) is a critical factor, especially where continuous usage of electronic equipment is essential.

Examples of overvoltage protection requirements to BS 7671Examples of overvoltage protection requirements to BS 7671

Risk assessment
As far as Section 443 is concerned, the full BS EN 62305-2 risk assessment method must be used for high risk installations such as nuclear or chemical sites where the consequences of transient over-voltages could lead to explosions, harmful chemical or radioactive emissions thus affecting the environment.

Outside of such high risk installations, if there is a risk of a direct lightning strike to the structure itself or to overhead lines to the structure SPDs will be required in accordance with BS EN 62305.

Section 443 takes a direct approach for protection against transient over-voltages which is determined based on the consequence caused by overvoltage as per Table 1 above.

Calculated Risk Level CRL – BS 7671
BS 7671 clause 443.5 adopts a simplified version of risk assessment derived from the complete and complex risk assessment of BS EN 62305-2. A simple formula is used to determine a Calculated Risk Level CRL.

The CRL is best seen as a probability or chance of an installation being affected by transient over-voltages and is therefore used to determine if SPD protection is required.

If the CRL value is less than 1000 (or less than a 1 in 1000 chance) then SPD protection shall be installed. Similarly if the CRL value is 1000 or higher (or greater than a 1 in 1000 chance) then SPD protection is not required for the installation.

The CRL is found by the following formula:
CRL = fenv / (LP x Ng)

Where:

  • fenv is an environmental factor and the value of fenv shall be selected according to Table 443.1
  • LP is the risk assessment length in km
  • Ng is the lightning ground flash density (flashes per km2 per year) relevant to the location of the power line and connected structure

The fenv value is based on the structure’s environment or location. In rural or suburban environments, structures are more isolated and therefore more exposed to over-voltages of atmospheric origin compared to structures in built up urban locations.

Determination of fenv value based on environement (Table 443.1 BS 7671)

Risk assessment length LP
The risk assessment length LP is calculated as follows:
LP = 2 LPAL + LPCL + 0.4 LPAH + 0.2 LPCH (km)

Where:

  • LPAL is the length (km) of low-voltage overhead line
  • LPCL is the length (km) of low-voltage under­ground cable
  • LPAH is the length (km) of high-voltage overhead line
  • LPCH is the length (km) of high-voltage under­ground cable

The total length (LPAL + LPCL + LPAH + LPCH) is limited to 1 km, or by the distance from the first overvoltage protective device installed in the HV power network (see Figure) to the origin of the electrical installation, whichever is the smaller.

If the distribution network’s lengths are totally or partially unknown then LPAL shall be taken as equal to the remaining distance to reach a total length of 1 km. For example, if only the distance of underground cable is known (e.g. 100 m), the most onerous factor LPAL shall be taken as equal to 900 m. An illustration of an installation showing the lengths to consider is shown in Figure 04 (Figure 443.3 of BS 7671). Ground flash density value Ng

The ground flash density value Ng can be taken from the UK lightning flash density map in Figure 05 (Figure 443.1 of BS 7671) – simply determine where the location of the structure is and choose the value of Ng using the key. For example, central Nottingham has an Ng value of 1. Together with the environmental factor fenv, the risk assessment length LP, the Ng value can be used to complete the formula data for calculation of the CRL value and determine if overvoltage protection is required or not.

Surge arrestor (overvoltage protective device) on the overhead HV system

The UK lightning flash density map (Figure 05) and a summary flowchart (Figure 06) to aid the decision making process for the application of Section 443 (with guidance to the Types of SPD guide to Section 534) follows. Some risk calculation examples are also provided.

UK FLASH DENSITY MAP

IET WIRING REGULATIONS BS 7671 18TH EDITION

Risk assessment SPD decision flow chart for installations within the scope of this BS 7671 18th Edition

Examples of calculated risk level CRL for the use of SPDs (BS 7671 informative Annex A443).

Example 1 – Building in rural environment in Notts with power supplied by overhead lines of which 0.4 km is LV line and 0.6 km is HV line Ground flash density Ng for central Notts = 1 (from Figure 05 UK flash density map).

Environmental factor fenv = 85 (for rural environment – see Table 2) Risk assessment length LP

  • LP = 2 LPAL + LPCL + 0.4 LPAH + 0.2 LPCH
  • LP = (2 × 0.4) + (0.4 × 0.6)
  • LP  = 1.04

Where:

  • LPAL is the length (km) of low-voltage overhead line = 0.4
  • LPAH is the length (km) of high-voltage overhead line = 0.6
  • LPCL is the length (km) of low-voltage underground cable = 0
  • LPCH is the length (km) of high-voltage underground cable = 0

Calculated Risk Level (CRL)

  • CRL = fenv / (LP × Ng)
  • CRL = 85 / (1.04 × 1)
  • CRL = 81.7

In this case, SPD protection shall be installed as the CRL value is less than 1000.

Example 2 – Building in suburban environment located in north Cumbria supplied by HV underground cable Ground flash density Ng for north Cumbria = 0.1 (from Figure 05 UK flash density map) Environmental factor fenv = 85 (for suburban environment – see Table 2)

Risk assessment length LP

  • LP = 2 LPAL + LPCL + 0.4 LPAH + 0.2 LPCH
  • LP = 0.2 x 1
  • LP = 0.2

Where:

  • LPAL is the length (km) of low-voltage overhead line = 0
  • LPAH is the length (km) of high-voltage overhead line = 0
  • LPCL is the length (km) of low-voltage underground cable = 0
  • LPCH is the length (km) of high-voltage underground cable = 1

Calculated Risk Level (CRL)

  • CRL = fenv / (LP × Ng)
  • CRL = 85 / (0.2 × 0.1)
  • CRL = 4250

In this case, SPD protection is not a requirement as CRL value is greater than 1000.

Example 3 – Building in urban environment located in southern Shropshire – supply details unknown Ground flash density Ng for southern Shropshire = 0.5 (from Figure 05 UK flash density map). Environmental factor fenv = 850 (for urban environment – see Table 2) Risk assessment length LP

  • LP = 2 LPAL + LPCL + 0.4 LPAH + 0.2 LPCH
  • LP = (2 x 1)
  • LP = 2

Where:

  • LPAL is the length (km) of low-voltage overhead line = 1 (details of supply feed unknown – maximum 1 km)
  • LPAH is the length (km) of high-voltage overhead line = 0
  • LPCL is the length (km) of low-voltage underground cable = 0
  • LPCH is the length (km) of high-voltage underground cable = 0

Calculated Risk Level CRL

  • CRL = fenv / (LP × Ng)
  • CRL = 850 / (2 × 0.5)
  • CRL = 850

In this case, SPD protection shall be installed as the CRL value is less than 1000. Example 4 – Building in urban environment located in London supplied by LV underground cable Ground flash density Ng for London = 0.8 (from Figure 05 UK flash density map) Environmental factor fenv = 850 (for urban environment – see Table 2) Risk assessment length LP

  • LP = 2 LPAL + LPCL + 0.4 LPAH + 0.2 LPCH
  • LP = 1

Where:

  • LPAL is the length (km) of low-voltage overhead line = 0
  • LPAH is the length (km) of high-voltage overhead line = 0
  • LPCL is the length (km) of low-voltage underground cable = 1
  • LPCH is the length (km) of high-voltage underground cable = 0

Calculated Risk Level (CRL)

  • CRL = fenv / (LP × Ng)
  • CRL = 850 / (1 × 0.8)
  • CRL = 1062.5

In this case, SPD protection is not a requirement as the CRL value is greater than 1000.

Transient overvoltage protection Selection of SPDs to BS 7671

Selection of SPDs to BS 7671
The scope of Section 534 of BS 7671 is to achieve overvoltage limitation within AC power systems to obtain insulation co-ordination, in line with Section 443, and other standards, including BS EN 62305-4.

Overvoltage limitiation is achieved through installation of SPDs as per the recommendations in Section 534 (for AC power systems), and BS EN 62305-4 (for other power and data, signal or telecommunications lines).

Selection of SPDs should achieve the limitation of transient overvoltages of atmospheric origin, and protection against transient overvoltages caused by direct lightning strikes or lightning strikes in the vicinity of a building protected by a structural Lightning Protection System LPS.

SPD selection
SPDs should be selected according to the following requirements:

  • Voltage protection level (UP)
  • Continuous operating voltage (UC)
  • Temporary overvoltages (UTOV)
  • Nominal discharge current (In) and impulse current (Iimp)
  • Prospective fault current and the follow current interrupt rating

The most important aspect in SPD selection is its voltage protection level (UP). The SPD’s voltage protection level (UP) must be lower than the rated impulse voltage (UW) of protected electrical equipment (defined within Table 443.2), or for continuous operation of critical equipment, its impulse immunity.

Where unknown, impulse immunity can be calculated as twice the peak operating voltage of the electrical system (i.e. approximately 715 V for 230 V systems). Non-critical equipment connected to a 230/400 V fixed electrical installation (e.g. a UPS system) would require protection by an SPD with a UP lower than Category II rated impulse voltage (2.5 kV). Sensitive equipment, such as laptops and PCs, would require additional SPD protection to Category I rated impulse voltage (1.5 kV).

These figures should be considered as achieving a minimal level of protection. SPDs with lower voltage protection levels (UP) offer much better protection, by:

  • Reducing risk from additive inductive voltages on the SPD’s connecting leads
  • Reducing risk from voltage oscillations downstream which could reach up to twice the SPD’s UP at the equipment terminals
  • Keeping equipment stress to a minimum, as well as improving operating lifetime

In essence, an enhanced SPD (SPD* to BS EN 62305) would best meet the selection criteria, as such SPDs offer voltage protection levels (UP) considerably lower than equipment’s damage thresholds and thereby are more effective in achieving a protective state. As per BS EN 62305, all SPDs installed to meet the requirements of BS 7671 shall conform to the product and testing standards (BS EN 61643 series).

Compared to standard SPDs, enhanced SPDs offer both technical and economic advantages:

  • Combined equipotential bonding and transient overvoltage protection (Type 1+2 & Type 1+2+3)
  • Full mode (common and differential mode) protection, essential to safeguard sensitive electronic equipment from all types of transient overvoltage – lightning & switching and
  • Effective SPD co-ordination within a single unit versus installation of multiple standard Type SPDs to protect terminal equipment

Compliance to BS EN 62305/BS 7671, BS 7671 Section 534 focuses guidance on selection and installation of SPDs to limit transient overvoltages on the AC power supply. BS 7671 Section 443 states that‚ transient overvoltages transmitted by the supply distribution system are not significantly attenuated downstream in most installations BS 7671 Section 534 therefore recommends that SPDs are installed at key locations in the electrical system:

  • As close as practicable to the origin of the installation (usually in the main distribution board after the meter)
  • As close as practicable to sensitive equipment (sub-distribution level), and local to critical equipment

Installation on a 230/400 V TN-C-S/TN-S system using LSP SPDs, to meet the requirements of BS 7671.

How effective protection comprises a service entrance SPD to divert high energy lightning currents to earth, followed by coordinated downstream SPDs at appropriate points to protect sensitive and critical equipment.

Selecting appropriate SPDs
SPDs are classified by Type within BS 7671 following the criteria established in BS EN 62305.

Where a building includes a structural LPS, or connected overhead metallic services at risk from a direct lightning strike, equipotential bonding SPDs (Type 1 or Combined Type 1+2) must be installed at the service entrance, to remove risk of flashover.

Installation of Type 1 SPDs alone however does not provide protection to electronic systems. Transient overvoltage SPDs (Type 2 and Type 3, or Combined Type 1+2+3 and Type 2+3) should therefore be installed downstream of the service entrance. These SPDs further protect against those transient overvoltages caused by indirect lightning (via resistive or inductive coupling) and electrical switching of inductive loads.

Combined Type SPDs (such as the LSP FLP25-275 series) significantly simplify the SPD selection process, whether installing at the service entrance or downstream in the electrical system.

LSP range of SPDs enhanced solutions to BS EN 62305/BS 7671.
The LSP range of SPDs (power, data and telecom) are widely specified in all applications to ensure the continuous operation of critical electronic systems. They form part of a complete lightning protection solution to BS EN 62305. LSP FLP12,5 and FLP25 power SPD products are Type 1+2 devices, making them suitable for installation at the service entrance, whilst giving superior voltage protection levels (enhanced to BS EN 62305) between all conductors or modes. The active status indication informs the user of:

  • Loss of power
  • Loss of phase
  • Excessive N-E voltage
  • Reduced protection

The SPD and supply status can also be monitored remotely via the volt-free contact.

Protection for 230-400 V TN-S or TN-C-S supplies

LSP SLP40 power SPDs Cost effective protection to BS 7671

LSP SLP40 range of SPDs compliment DIN rail product solutions offering cost effective protection for commercial, industrial and domestic installations.

  • When one component is damaged, the mechanical indicator will switch green to red, triggering the volt-free contact
  • At this stage the product should be replaced, but the user still has protection during the ordering and installation process
  • When both components are damaged, the end of life indicator will become completely red

Installation of SPDs Section 534, BS 7671
Critical length of connecting conductors
An installed SPD will always present a higher let through voltage to equipment compared with the voltage protection level (UP) stated on a manufacturer’s data sheet, due to additive inductive voltage drops across the conductors on the SPD’s connecting leads.

Therefore, for maximum transient overvoltage protection the SPD’s connecting conductors must be kept as short as possible. BS 7671 defines that for SPDs installed in parallel (shunt), the total lead length between line conductors, protective conductor and SPD preferably should not exceed 0.5 m and never exceed 1 m. See Figure 08 (overleaf) for example. For SPDs installed in-line (series), the lead length between the protective conductor and SPD preferably should not exceed 0.5 m and never exceed 1 m.

Best practice
Poor installation can significantly reduce effectiveness of SPDs. Therefore, keeping connecting leads as short as possible is vital to maximise performance, and minimise additive inductive voltages.

Best practice cabling techniques, such as binding together connecting leads over as much of their length as possible, using cable ties or spiral wrap, is highly effective in cancelling inductance.

The combination of an SPD with low voltage protection level (UP), and short, tightly bound connecting leads ensure optimised installation to the requirements of BS 7671.

Cross-sectional area of connecting conductors
For SPDs connected at the origin of the installation (service entrance) BS 7671 requires the minimum cross-sectional area size of SPDs connecting leads (copper or equivalent) to PEve conductors respectively to be:
16 mm2/6 mm2 for Type 1 SPDs
16 mm2/6 mm2 for Type 1 SPDs
shares
back to top