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Surge protection
and SPDs

Protecting what is inside, under BS EN 62305-4.

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Where the damage comes from

A large proportion of lightning damage never involves a strike on the building at all. The current arrives as a surge through the electrical supply or along metallic services from a strike somewhere nearby, and it destroys electronics rather than brickwork. A building can have a sound external lightning protection system and still lose its controls, its comms and its plant to a single event.

That is why BS EN IEC 62305 devotes a whole part, Part 4, to protecting electrical and electronic systems inside the structure, and why surge protective devices now form part of the risk assessment rather than being an optional extra bolted on at the end.

Where SPDs go

  • Type 1 at the origin of the installation, where the structure has an external lightning protection system.
  • Type 2 at distribution boards, for the residual surge that gets past Type 1.
  • Type 3 local to sensitive equipment as a final stage.
  • On incoming telecoms and data lines, which are as likely a route in as the power supply.
  • On roof mounted plant, which sits in the most exposed part of the building.

Coordinated, not assorted

Surge protective devices work as a coordinated set. A Type 1 device at the intake and a Type 3 at the equipment are doing different jobs, and fitting one without the other leaves a gap. Getting the coordination right depends on knowing the protection class of the external system and the surge current the standard expects at that class.

Since we do the risk assessment and the external system, we can specify the internal protection against the same set of numbers rather than estimating them.

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Common questions

Why do I need surge protection as well as a lightning conductor?

Because most lightning damage is not caused by the strike hitting the building. It arrives as a surge through the incoming electrical supply or along metallic services, from a strike some distance away. The external system protects the structure. Surge protective devices protect what is plugged into it.

What is BS EN 62305-4?

Part 4 of the lightning protection standard covers protection of electrical and electronic systems inside a structure against the lightning electromagnetic impulse. It deals with surge protective device selection and coordination, bonding, screening and zoning, and it is where the requirement for SPDs is set out.

What is the difference between Type 1, 2 and 3 SPDs?

Type 1 devices handle the partial lightning current at the origin of the installation and are used where a structure has an external lightning protection system. Type 2 devices protect distribution boards from residual surges. Type 3 devices sit close to sensitive equipment as a final stage. They work as a coordinated set, not as alternatives.

Can my electrician fit the SPDs?

Yes. Surge protective devices inside the building are installed under BS 7671 and a competent electrical contractor can fit them. What the electrician usually does not have is the BS EN 62305-2 risk assessment that says which devices are needed and where, and the coordination with the external system. We provide that part.

Need a quote?

Send us the address, a roof plan if you have one, and what you already know about the building. We will tell you what is needed and what it costs. Call 01892 667258 or use the form.

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01892 667258