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UK Lightning Protection Standard BSEN 62305

UK Electrical Regulation BS7671 18th Edition


Surge Guidance Notes

These notes are only for general guidance and do not constitute a design, The reader should seek independent qualified advice.


  Design Guidance

Lightning Protection Standard BSEN 62305 & IET Wiring Regulations BS 7671

   Lightning current surge protection Pt2 R1 risk designed/installed as per Pt3 Iimp 10/350μs

   SPM (Surge Protection Measures) over voltage surge protection Pt2 R2 risk designed/installed as per Pt4 Imax 8/20μs & combined 1.2/50μs wave forms

NOTE:Electrical wiring regulations BS7671 17th Edition 2008 Amendment 1 (2011) & new BS7671 18th Edition Section 534 (Eleven pages) clarifies the requirement for SPDs harmonized to the Lightning standard BSEN 62305. Section 534 contains detailed information for the selection and installation of SPDs to protect against direct Lightning strikes and indirect Lightning strikes following the BSEN 62305 code for the prevention of risk to loss of life, damage to property, loss of services and equipment failure.

BS7671 (AQ) SPD assessment is not required were a structures assessed outcome under BSEN 62305 requires SPDs as part of the installation of a Lightning Protection System. Also see guide note 2 BS7671 (2011) regulation 443.2.4 list of structure’s that require SPDs without further assessment.

BS7671 18th edition regulation 443, which deals with protection against over voltage’s of atmospheric origin or due to switching, has been redrafted and removing regulation 443.2

The AQ criteria (conditions of external influence for lightning) for determining if protection against transient over voltage’s is needed are no longer included in BS 7671. Instead, protection against transient over voltage’s shall be provided where the consequence caused by over voltage (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 over voltage is required.

There is an exception not to provide protection for single dwelling units in certain situations based on the value of potential loss being less than the cost of implementing surge protection measures.

Where there is a requirement for Protection against Lightning BSEN 62305 Pt3 – Physical damage to structures and life hazard – Surge protection must in all cases be provided as close as practically possible to cables entering or exiting the building Pt3 6.2.5 – also refer to the UK wiring regulations BS7671 18th edition revised regulation 443 and regulation 534
Prevention measures inclusive of SPDs conforming to BSEN 62305 Pt3 and Pt2 (R1) risk assessment –
prevention of loss of human life – Pt3 6.2.1, 6.2.3, 6.2.5 must be completed. Also see – BIP 2118 –, 5.4.1, 5.4.3, 10.5; BS7671 regulation 443, 534. and BS7671 18th edition ON-SITE Guide 3.7
All insulated copper cable cores no matter what the voltage (other than HV) entering exiting the Faraday cage Lightning protection system under a BSEN 62305 design LPZ0a↔LPZ1 This requirement is also found within BS7671 and must be protected against high yield current produced by atmospheric origins from entering the structure.
The R1 risk – prevention of loss of human life is achieved by inserting appropriate Lightning current type 1 10/350μs wave form units selected at the appropriate KA rating for the outcome level(class) of risk via the risk assessment.(Pt2 R1 Risk)

Electrical equipment directly wall mounted with cables entering directly through the wall i.e. bulkhead lights, door bells and entry systems mounted to the external wall, camera pods, fire alarm / intruder alarm bell boxes.


Type 1 SPDs, commonly referred to as ‘Service Entry’ or ‘Equipotential Bonding’ SPDs are required for structural protection as well as prevention of loss of life hazard and are installed as close to the cable entry/exit point as practically possible – BSEN 62305 – Pt3 6.2.3 also see – BIP 2118 – 5.4.3; BS7671 – 443, 534 and on site guide 3.7
The main electrical supply cable (normally 4 cores) and incoming telecom lines, normally multi pairs arranged in banks of ten pair krone strips are obvious service entry lines which in the absence of any other metallic services present must be assumed to be exposed to 50% of the total high yield current produced by a lightning strike. This current must be safely dispersed to earth by means of appropriately rated type 1 SPDs. It is important to consider let through over voltage resulting from the Lightning strike. Type 1 SPDs cannot limit the over voltage and it is good practice to select a combined T1&T2 SPD at the service entry position. The SPD should have a let through voltage no greater than 1.5KV. Enhanced SPDs will have <1.5KV let through voltage Lightning current (Type 1)10/350 Iimp KA rating is determined by the level (class) of protection required via the risk assessment outcome (BSEN 62305 Pt2 R1 Risk)

A level 1 (class I) external system will be arranged in such a way as to withstand a 200KA lightning strike of which only 50% is dissipated through the external tapes and rods etc. the remaining 50%enters via the service lines. (All cables entering/exiting the property)

A level 2 (class II) system should be designed to withstand a 150KA lightning strike and levels 3&4 (class III IV) a 100KA lightning strike; again the external system is only designed to protect the fabric of the structure, 50% of the lightning strike magnitude will pass through the external Faraday cage protection grid and into the structure via connected services.

The designers SPD selection must take the resulting Lightning current type 1 10/350μs into account. The minimum mains incoming service protection must be 12.5KA/line. The designer once thoroughly assessed may choose to apply “current division” sharing across multiple services to meet the total KA expectation. (Example: – A level 1 outcome design 200KA strike has a remaining 50% “100KA” shared across one 3Ph&N mains voltage service entry. This would require 25KA protection per line; if the structure had two 3Ph&N mains service lines the KA would only require 12.5KA per line)

Particular attention must be made when designing high rise flatted developments, or multi metered developments such as student accommodation due to the inherent nature of the mains wiring infrastructure. These developments tend to be wired with the utility company’s own equipment to final locations within each dwelling. The domestic side (after the meter) within each dwelling is therefore as close to the cable entry/exit point as practically possible due to ownership of equipment and as such will require multiple 230V SPDs with 10/350Us type 1 capability (1 per dwelling) – BSEN 62305 – Pt3 6.2.3, also see – BIP 2118 – 5.4.3; BS7671 443, 543


Local mains service lines i.e. ones that are from plant adjacent to or roof mounted – supplies to air-conditioning, chillers, lights, barrier gates, camera poles etc. shall be type 1 10/350μs rated. (In rare occasions type II 8/20μS would be used – if the external equipment has an adequate separation distance from the LC system and both it and its cables are placed within Zone 0b) see BSEN 62305 Pt1 fig 2, Pt3 fig E.33

The selection of the type II device if protecting electronic equipment, critical equipment must be Differential mode configuration L-N (The SPD will be marked 1+1 for 1Ph&N 230V and 3+1 for 3Ph&N 400V) Common mode mains voltage SPDs do not limit voltage across L-N. Common mode SPDs only protect against current/over voltage L-PE (SPDs marked 2+0 and 4+0) so will not prevent damage to electronic systems or prevent data corruption. It is imperative to protect Fire alarm control panels, Disabled refuge panels, Sprinkler system control activation etc. These must be protected as a requirement under the R1 risk (BSEN 62305 Pt1 page 18 R1 risk “failure of internal systems where this failure may immediately endanger human life”) Other specific systems, such as Nurse Call etc, would come under this requirement. Also see BS7671 17th edition page 146 regulation 534.2.2 note 1, sensitive electronic equipment must be protected with differential mode ‘CT2’ (1+1,3+1) connection devices. BS7671 18th edition page 164 regulation 534.4.4.2 note 1 & regulation 534 .4.3.2

Where external equipment is present, the control wiring is not normally mains voltage and will also require surge protection at the entry points. Most types of arrestor for these applications are designed to protect against current and over voltage and are therefore able to protect electrical/ electronics. There are many configurations of these arrestors due to the specific applications and terminal connections. The designer must select the correct voltage range, or Hz signal range etc so that the SPD selected does not have a detrimental effect on the circuit in which it is placed; i.e. BMS wiring, fire alarm loop monitoring, radio or video frequencies etc. BSEN 62305 Pt4 Protection against Lightning – Electrical and Electronic systems within structures (Pt2 R2) risk assessment (loss of services to the public) by prevention of failure of internal systems. This is a mandatory compliance requirement under the outcome of an “R1” risk assessment for Lightning protection designs that include emergency service providers i.e. Hospitals, Ambulance, Fire, Police, Coast Guard, NHS and emergency public service call centers as well as transport infrastructures such as airports, road; rail and air traffic control. There are many more situations a designer will come across a need to consider the consequence of the loss such as Prisons, Military installations etc. (BSEN 62305 Pt1 page 18 R1 risk “failure of internal systems where this failure may immediately endanger human life”) Also see BS7671 17th edition regulation 534.2.2 Note 1 & 18th edition 534.4.2 note 1

Other non-life threatening loss of service to the public R2 r i sks may include financial institutes , public broadcasts , government buildings etc. The R2 risk assessment maybe specified by a client which otherwise would not normally be assessed. This may be due to the client advising a desire to protect significant high cost equipment within the building or loss of production time, stored information on private data base server systems etc. The designer, once appraised of the specific services within the structure will follow the guidance contained within BSEN 62305 Pt3 & Pt4. BS7671: 2011 version onward also has an SPD mandatory compliance requirement for premises which have consequences of loss defined within section 443.2.4 Note 2 (i–iii) safety services; medical equipment in hospitals; loss of public services, information technology centers, museums; hotels, banks, industries, commercial markets; farms. BS7671: 2011 443 note 3 also provides guidance on consequences to structures described in (iv–v) BS7671 2018 regulation 443.4 updates and strengthens this now mandatory requirement.

Once Pt2 risk assessment (R1) service cables have been identified and designed to be surge protected under Pt3 section 6; Pt4 is relatively straight forward. All the low volt controls, coaxial, data lines etc. entering/exiting the structure from LPZ1 through to LPZ0a or are LPZ1 through toLPZ0b positioned equipment bonded to the LP system must have (type I) 10/350μs wave form protection but also (type II) 8/20μs wave form, over voltage protection and dependent on the specific unit, (type III) 1.2/50μs wave form protection. If external equipment and connected cables are isolated from the LP system without exposure to LPZ0a, SPDs are still required but do not need to have (type I) 10/350μs wave form protection. Where the R2 assessment is required, the mains voltage incoming supply will require surge equipment that has multi functionality of both current arresting and prevention of voltage L-N of <1.5KV If the main board contains sensitive electronics or feeds equipment having sensitive electronics. In the specific case of life dependent services the power supply line must be protected against TOV (temporary over voltage) immunity at the highest voltage for the longest duration possible without disruption to the supply.

Multi functioning SPDs must be Differential mode, BS7671: 534.4.4.3 ‘CT2’ (1+1 or 3+1 L-N/N-PE) configured when used within the context of BSEN 62305 pt4. Internal wiring to significant technologies may require additional local surge equipment dependent on the placement of these relative to distance from the main board multi functioning SPD. These SPDs are designed to work in co-ordination; typically placed where the equipment being protected is more than 10Mtrs from the upstream SPD provision; see BS7671 534.4.4.2. Surge units downstream of the main board unit willbe typically type II and or type III combined ‘CT2’(1+1, 3+1 L-N /N-PE) to protect the final equipment from 8/20μs over voltage and 1.2/50μs voltage/current residuals. Final critical equipment locations may require a type III SPD dependent on the type and placement of the upstream SPD device. If sub boards have any circuits leaving the structure the SPD selected at this location must also have type I current capability in addition to the Type II & III differential configuration. Also see BSEN 62305 Pt4 4.1, 7 and figures 2a-d, Annex C SPD coordination, Annex D selection and installation of coordinated SPDs

Under the BSEN 62305 standard it is non-compliant to leave cables penetrating the Faraday cage unprotected, Mains, RF, CCTV, Telecom or signal & control. ALL conductive cable cores under Pt2 (R1) risk of loss of human life as described within Pt3 protection against life hazard, or increased under Pt4 Protection against Lightning – Electrical and Electronic systems within structures (Pt2 R2) risk of loss of services to the public dependent on specific needs regardless of individual work packages they are in shall be protected against sparking between cores .

The responsibility lies with the designer / installation contractor ; i.e. a main contractor may have split several packages through various sub-contractors. The sub-contractor who wins the package including Lightning protection has taken on this element under contract to comply with the spec and or code of practice referred. The code of practice is BSEN 62305 and does not stop at inferred work packages. M&E contractors should follow BS7671 regulation 534.4.1.6. Co-ordination of design elements should be with a Lightning protection specialist suitably trained, qualified and experienced in UK electrical installations. (See BSEN 62305 Pt4 page 33 Table 2)

LSS Ltd are a fully independent surge protection specialist company and Raycap® partner with expert knowledge of both BSEN 62305 and BS7671 requirements as per this guidance document. This guidance does not replace the standards referred too which must be followed for all UK installations. It is important that the reader is aware of these standards and are able to access the entries these guide note reference.

For all your surge protection requirements contact us and we will be happy to provide our Raycap® products, technical support and consultancy services.

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