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18th Edition

18th Edition Wiring Regulations

ELECTRICIANS GUIDE TO THE SELECTION OF MAINS SURGE PROTECTION DEVICES ACCORDING TO THE 18TH WIRING REGULATIONS (BS 7671:2018) . . .

Surge Protection requirements according to BS 7671:2018 18th Edition Wiring Regulations . . .

Previously, the 17th Edition Wiring Regulations stated that the need for surge protection was determined by risk assessment.

Since 1 January 2019, according to BS 7671:2018 section 443.3 (reproduced in full on page 4), protection against transient overvoltages shall be provided where the consequence of damage caused by overvoltages meets certain criteria.

If these criteria are not met, a risk assessment must be carried out to determine if surge protection is required. If a risk assessment is not carried out, surge protection must be installed.

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“Surge Protection is designed to protect electronics from the effects of lightning and other transient overvoltages”

The probability of electronic systems being damaged by lightning is far greater than that of the building itself being struck, as lightning strikes up to 1km away from a building can damage electronics inside it. This is a far larger target area than the size of the building itself.

Damaging transient overvoltages are not only a result of lightning activity. Electrical switching, a common event, can also cause overvoltages which impair electrical components.

“The probability of electronic systems being damaged by lightning is far greater than that of the building itself being struck, as lightning strikes up to 1km away from a building can damage electronics inside it.”

As per BS 7671:2018, there is now a requirement to consider protection from damage caused by transient overvoltages.

Even in domestic settings, the effects of overvoltages can cause costly damage to equipment and fittings. Because of this risk, surge protection is a recommended step in adequately protecting a building and its contents, as the potential cost of replacing damaged installations and equipment far outweighs the cost of installing surge protection.

surge protection applications
surge protection applications
surge protection applications

Given that electronic systems are a vital part of everyday life and have become increasingly miniaturised over the past decades, protecting them from surges is more important than ever. If they were left unprotected and damaged by a transient overvoltage this could result in major inconvenience, lack of service and potentially life-threatening situations.

Surge Protection Devices

Type 1

Lightning Current Arrester
Install at MDBs to protect against lightning currents

Kingsmill supply combined Type 1+2 protectors, instead of Type 1 protectors, to ensure that the voltage protection level is below the withstand voltage for electrical/electronic equipment

Type 2

Surge Arrester
Install at MDBs and SDBs to protect against surge voltages and indirect lightning currents

Type 3

Fine Protection
Install less than 5m from critical, or high value, equipment for fine protection

Risk assessment according to section 443.4 of the 18th edition UK wiring regulations (BS 7671:2018) . . .

“Protection against transient overvoltages shall be provided where the consequence caused by overvoltage could:
(i) result in serious injury to, or loss of, human life, or
(ii) result in interruption of public services and/or damage to cultural heritage, or
(iii) result in interruption of commercial or industrial activity, or
(iv) affect a large number of co-located individuals.

For all other cases, a risk assessment according to Regulation 443.5 shall be performed in order to determine if protection against transient overvoltages is required. If the risk assessment is not performed, the electrical installation shall be provided with protection against transient overvoltages, except for single dwelling units where the total value of the installation and equipment therein does not justify such protection.

Protection against switching overvoltages shall be considered in the case of equipment likely to produce switching overvoltages or disturbances exceeding the values according to the overvoltage category of the installation eg where an LV generator supplies the installation or where inductive or capacitive loads (eg motors, transformers, capacitor banks), storage units or high-current loads are installed.”

BS 7671:2018/BS:EN 62305 Surge Protection Devices Selection Charts

Does the building have overhead power or does the Main Distribution Board (MDB) feed appliances outside the building eg gates, CCTV, outside lighting or electric vehicle charging?

YES

Earthing System
Three Phase Systems
Single Phase Systems
TN-C
KM1+2-12.5-3+0 SC
KM1+2-12.5-1+0 SC
TN-S & TN-C-S
KM1+2-12.5-4+0 SC
KM1+2-12.5-2+0 SC
TT
KM1+2-12.5-3+1 SC
KM1+2-12.5-1+1 SC

MAIN DISTRIBUTION BOARD

NO

Earthing System
Three Phase Systems
Single Phase Systems
TN-C
KM2-20-3+0 SC
KM2-20-1+0 SC
TN-S & TN-C-S
KM2-20-4+0 SC
KM2-20-2+0 SC
TT
KM2-20-3+1 SC
KM2-20-1+1 SC

Is the Sub-Distribution Board (SDB) >50m from the MDB or does it feed appliances outside the building eg gates, CCTV, outside lighting or electric vehicle charging?

YES

Earthing System
Three Phase Systems
Single Phase Systems
TN-C
KM1+2-12.5-3+0 SC
KM2-20-1+0 SC
TN-S & TN-C-S
KM1+2-12.5-4+0 SC*
KM2-20-2+0 SC
TT
KM1+2-12.5-3+1 SC
KM2-20-1+1 SC

SUB DISTRIBUTION BOARD

NO

Earthing System
Three Phase Systems
Single Phase Systems
TN-C
KM2-20-3+0 SC
KM2-20-1+0 SC
TN-S & TN-C-S
KM2-20-4+0 SC*
KM2-20-2+0 SC
TT
KM2-20-3+1 SC
KM2-20-1+1 SC

*If the principle concern is internally generated switching transients, change the connection type to CT2 (or 3+1) instead of CT1 (or 4+0)

TYPE 3 FINE PROTECTION (fit <5m from equipment)

Earthing System
Three Phase Systems
Single Phase Systems
TN-C/TN-S/TN-C-S
KM3-10-3+1 SC
KM3-10-1+1 SC / KM3-275-A
TT
KM3-10-3+1 SC
KM3-10-1+1 SC

If a building is fitted with a structural Lightning Protection system, the requirements of BS:EN 62305 take precedence over BS 7671:2018. If you are familiar with the requirements of BS:EN 62305, and know the Lightning Protection Level (LPL), select the appropriate Surge Protection Devices below. If in doubt, STOP NOW and contact us.

Three Phase Systems

Earthing System
Lightning Protection Level (LPL)
Main Distribution Board (MDB)
Sub-Distribution Board (SDB) (unless feeding outside circuits) (figure given is cable length, not distance)
Electronic Equipment (less than 5m cable length)
TN-C
I & II
KM1+2-25-3+0 SC
KM2-20-3+0 SC (>10m from MDB)
KM3-10-3+1 SC
KM1+2-12.5-3+0 SC (>50m from MDB)
KM1+2-25-3+0 SC (>100m from MDB)
TN-C
III & IV
KM1+2-12.5-3+0 SC
KM2-20-3+0 SC (>10m from MDB)
KM3-10-3+1 SC
KM1+2-12.5-3+0 SC (>50m from MDB)
TN-S
I & II
KM1+2-25-4+0 SC
KM2-20-4+0 SC (>10m from MDB)
KM3-10-3+1 SC
KM1+2-12.5-4+0 SC (>50m from MDB)
KM1+2-25-4+0 SC (>100m from MDB)
TN-S
III & IV
KM1+2-12.5-4+0 SC
KM2-20-4+0 SC (>10m from MDB)
KM3-10-3+1 SC
KM1+2-12.5-4+0 SC (>50m from MDB)
TN-C-S
I & II
KM1+2-25-4+0 SC
KM2-20-4+0 SC (>10m from MDB)
KM3-10-3+1 SC
KM1+2-12.5-4+0 SC (>50m from MDB)
KM1+2-25-4+0 SC (>100m from MDB)
TN-C-S
III & IV
KM1+2-12.5-4+0 SC
KM2-20-4+0 SC (>10m from MDB)
KM3-10-3+1 SC
KM1+2-12.5-4+0 SC (>50m from MDB)
TT
I & II
KM1+2-25-3+1 SC
KM2-20-3+1 SC (>10m from MDB)
KM3-10-3+1 SC
KM1+2-12.5-3+1 SC (>50m from MDB)
KM1+2-25-3+1 SC (>100m from MDB)
TT
III & IV
KM1+2-12.5-3+1 SC
KM2-20-3+1 SC (>10m from MDB)
KM3-10-3+1 SC
KM1+2-12.5-3+1 SC (>50m from MDB)

Single Phase Systems

Earthing System
Lightning Protection Level (LPL)
Main Distribution Board (MDB)
Sub-Distribution Board (SDB) (unless feeding outside circuits) (figure given is cable length, not distance)
Electronic Equipment (less than 5m cable length)
TN-C
I & II
KM1+2-25-1+0 SC
KM2-20-1+0 SC (>10m from MDB)
KM3-10-1+1 SC
KM3-275-A
KM1+2-12.5-1+0 SC (>50m from MDB)
KM1+2-25-1+0 SC (>100m from MDB)
TN-C
III & IV
KM1+2-12.5-1+0 SC
KM2-20-1+0 SC (>10m from MDB)
KM3-10-1+1 SC
KM3-275-A
KM1+2-12.5-1+0 SC (>50m from MDB)
TN-S
I & II
KM1+2-25-2+0 SC
KM2-20-2+0 SC (>10m from MDB)
KM3-10-1+1 SC
KM3-275-A
KM1+2-12.5-2+0 SC (>50m from MDB)
KM1+2-25-2+0 SC (>100m from MDB)
TN-S
III & IV
KM1+2-12.5-2+0 SC
KM2-20-2+0 SC (>10m from MDB)
KM3-10-1+1 SC
KM3-275-A
KM1+2-12.5-2+0 SC (>50m from MDB)
TN-C-S
I & II
KM1+2-25-2+0 SC
KM2-20-2+0 SC (>10m from MDB)
KM3-10-1+1 SC
KM3-275-A
KM1+2-12.5-2+0 SC (>50m from MDB)
KM1+2-25-2+0 SC (>100m from MDB)
TN-C-S
III & IV
KM1+2-12.5-2+0 SC
KM2-20-2+0 SC (>10m from MDB)
KM3-10-1+1 SC
KM3-275-A
KM1+2-12.5-2+0 SC (>50m from MDB)
TT
I & II
KM1+2-25-1+1 SC
KM2-20-1+1 SC (>10m from MDB)
KM3-10-1+1 SC
KM1+2-12.5-1+1 SC (>50m from MDB)
KM1+2-25-1+1 SC (>100m from MDB)
TT
III & IV
KM1+2-12.5-1+1 SC
KM2-20-1+1 SC (>10m from MDB)
KM3-10-1+1 SC
KM1+2-12.5-1+1 SC (>50m from MDB)
  1. What should I do if I have no information, or the risk assessment has not been completed?
    If there is little or no information available to guide you through the product selection process, or the risk assessment cannot be completed, surge protection must be installed as per BS 7671:2018 443.4.
    Kingsmill would recommend installing a Type 1+2 combined Surge Protection Device (SPD) with a minimum discharge current of 12.5kA per pole.
  2. The building to be protected has a structural Lightning Protection System (LPS) but the Lightning Protection Level (LPL) is unknown. What do I have to do?
    If there is an LPS installed but the LPL is unknown, a Type 1+2 SPD is required to be installed at the main service entry (MDB) to the building. The device should have a minimum of 25kA discharge current per pole (see page 6 ‘BS EN 62305 Products’).
  3. What is a transient overvoltage?
    Also referred to as a spike, surge or transient, a transient overvoltage is a form of power disturbance. They are caused by the secondary effects of lightning and electrical switching events. They only last for microseconds but can reach 6,000 volts in magnitude.
  4. In a single domestic dwelling, how would I determine if SPD installation is worthwhile?
    The main consideration here is the effects of a transient overvoltage and the possible damage to the electrical installation. For example, not only can appliances (TVs, computers etc) be damaged, but the internal wiring may also be damaged and require replacement. It is not uncommon for houses to undergo a full “re-wire” as a result of transient overvoltage damage, which can cost thousands of pounds. This is certainly more than the cost of SPD installation.
  5. Do I need SPDs if I already have circuit breakers and/or fuses?
    SPDs and circuit breakers serve different purposes, protecting electronics from different events. Circuit breakers protect against overcurrents caused by a short circuit. SPDs protect against transient overvoltages. Circuit breakers and fuses do not provide protection against transient overvoltages.
  6. Do I need to protect the SPD with a fuse?
    Overcurrent protection (OCPD) or fuse installation to protect the SPD specifically is only required if the mains supply fuse exceeds the recommended back up fuse of the SPD. Otherwise, the SPD is protected by the mains fuse/overcurrent protection. Details are provided on page 11 and in the installation instructions supplied with each SPD.
  7. Can I protect the SPD with an OCPD rated lower than the recommended?
    Kingsmill would always recommend the maximum back up OCPD for the SPD; installing a lower rated OCPD may lead to ineffective protection as the SPD may be disconnected by the OCPD before discharging the transient overvoltage safely to earth.
  8. Do transient overvoltages only occur on mains power cables?
    No. Even though mains power cables are the most common coupling method associated with transient overvoltages, they can occur on generator or battery supplies, UPS outlets and data, signal and telephone lines.
  9. How can I tell if an SPD has been damaged by transient overvoltages?
    Kingsmill SPD modules include lifetime status indication, clearly displaying if they have reached the end of their lifetimes, and hence need to be replaced to ensure continued protection. The modules can be easily and individually replaced.
  10. How far away can my device be from an SPD and still be protected?
    Different types of SPD have different protective distances. SPDs should be installed at the service entrance and as close to the equipment to be protected as possible. A full explanation is provided on page 7 but, in general, a Type 1+2 device should be installed no further than 10m from the electronic equipment to be protected and a Type 3 device should be no further than 5m from sensitive equipment.
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