BS EN 62305 explained

BS EN 62305 is the standard UK lightning protection is designed, installed and tested to. It is long, technical and sold by BSI, so most building owners only meet it as a line on a test certificate. This page sets out what it actually asks for, in plain terms.

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Replaced: BS 6651 (withdrawn 2008)
Parts: 4
Classes: I, II, III and IV
Earth target: 10 ohms or less

The four parts

Part 1
General principles
How lightning behaves, the current and energy a strike carries, and the terms used in the other three parts.
Part 2
Risk management
The calculation that decides whether a structure needs protection and to which class, by weighing the chance of a strike against the losses it could cause: loss of life, loss of service to the public, loss of cultural heritage and economic loss.
Part 3
Physical damage to structures and life hazard
The lightning protection system itself: air termination, down conductors, earth termination, bonding, materials, and the inspection and maintenance that keep it working.
Part 4
Electrical and electronic systems within structures
Protection of what is inside: zoning the building, bonding, shielding and coordinated surge protection devices.

What the class changes

The rolling sphere is the clever bit. Imagine a ball of the given radius rolled over and around the building. Anywhere it touches can be struck, so needs air termination. Anywhere it cannot reach is in the shadow of something that is protected. A smaller sphere, for a higher class, reaches more of the building.

Down conductor spacing and mesh size tighten with the class, which is why a Class I system costs considerably more than Class IV on the same roof.

Class	Rolling sphere	Mesh	Down conductors
I	20 m	5 × 5 m	typically every 10 m
II	30 m	10 × 10 m	typically every 10 m
III	45 m	15 × 15 m	typically every 15 m
IV	60 m	20 × 20 m	typically every 20 m

How often it must be inspected

Part 3 sets the longest a system may go between inspections. A visual inspection looks for damage and changes; a complete inspection adds the continuity and earth resistance tests. Critical situations include offices, commercial buildings, buildings with sensitive internal systems and places where many people may be present, so in practice schools, hospitals and most workplaces fall into the yearly column.

Many UK specialists test every 11 months so readings are taken in a different season each time, and the DfE asks schools to test at least annually.

Class	Visual	Complete	Complete, critical
I	1 year	2 years	1 year
II	1 year	2 years	1 year
III	2 years	4 years	1 year
IV	2 years	4 years	1 year

Structures with explosive risks: visual every 6 months, electrical testing yearly. BS EN 62305-3, Table E.2.

The risk assessment in part 2

Part 2 asks how likely a building is to be struck, from its size, height, surroundings and the local lightning density, and what a strike would cost in four kinds of loss: loss of human life, loss of service to the public, loss of cultural heritage, and economic loss. Each is compared with a tolerable level. If the risk is above it, protection measures are added, and the class is chosen, until the risk falls below.

The calculation needs software and a fair amount of data about the building, which is why a full assessment costs £1,500 to £3,500 on published prices. It is the normal starting point for a new building or a major change, and it gives you a written reason for the class you are paying for.

What an older system means

Systems fitted to the withdrawn BS 6651 often have a single down conductor and earth, or wider spacing than BS EN 62305 expects. They are not unsafe by default, and they can be tested and certified. The report may record them as a conditional pass and recommend an upgrade, which usually means extra down conductors and earths rather than a new system. See repairs and upgrades.

Where our information comes from

BS EN 62305-1 to 4:2011, Protection against lightning, BSI (classes from part 3 Tables 2 and 4, inspection intervals from Table E.2)
Historic England, Lightning Protection (HEAG182), April 2019
Department for Education, Good estate management for schools: health and safety, updated July 2026

Questions about BS EN 62305

What is BS EN 62305?

The British adoption of the European and international standard for protection against lightning (IEC 62305). It replaced BS 6651 in the UK and covers risk assessment, the design and installation of lightning protection systems, and protection of electrical and electronic systems.

When did BS EN 62305 replace BS 6651?

BS EN 62305 was first published in 2006, and BS 6651 was withdrawn in 2008 after a period running side by side. The second edition followed in 2011. Systems installed to BS 6651 can still be tested, but the report will often say they do not meet the current standard.

What does Class I to IV mean?

The class of lightning protection system, set by the part 2 risk assessment. Class I is the most demanding, with the smallest rolling sphere, tightest roof mesh and closest down conductors; Class IV the least. Most ordinary buildings that need protection come out at Class III or IV.

How often does BS EN 62305 say a system should be inspected?

Table E.2 of part 3 sets maximum gaps: a visual inspection every year and a complete inspection every 2 years for Classes I and II, and every 2 and 4 years for Classes III and IV. For critical situations, including offices, commercial buildings and places where many people gather, the complete inspection should be yearly.

What earth resistance does BS EN 62305 ask for?

It recommends an overall earth termination resistance of 10 ohms or less, as Historic England notes. Where the soil makes that hard, the standard allows other measures based on the length and layout of the electrodes.

Is a BS EN 62305 risk assessment always needed?

For new buildings and significant changes, it is the normal way to decide whether protection is needed and to which class. It typically costs £1,500 to £3,500. Historic England considers it disproportionate for most parish churches, where a simpler judgement is usual.

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