Guide to Surge Protection Devices (SPDs): Selection, Application and Theory

Introduction

Electronic systems now touch nearly every part of daily life, from the workplace to the local supermarket, and society depends on them running continuously and reliably. Computer, process control, and telecommunications use has grown enormously over the last couple of decades — and at the same time, the physical size of the electronics involved has shrunk, meaning less energy is now needed to damage them. Lightning activity and electrical switching events can both cause very short but severe voltage increases on mains power and data lines — surges, or transient overvoltages — with potentially serious consequences. The BEAMA Guide to Surge Protection Devices (SPDs) helps designers, specifiers, and installers meet the surge protection requirements in BS 7671:2018, and this article summarises its key concepts.

What Is an SPD?

A Surge Protection Device (SPD) is connected in parallel with the circuits it protects, and can be applied at any level of the power supply network — making it the most common and most practical form of overvoltage protection. SPDs limit transient overvoltages from lightning or switching and divert the associated surge current safely to earth, keeping overvoltages below the level likely to damage equipment or the installation.

It's worth being clear that circuit breakers and fuses don't provide this kind of protection. Those Overcurrent Protective Devices (OCPDs) are designed for events like short circuits or overloads — an entirely different failure mode from the extremely short-duration high-voltage spikes an SPD is built to handle.

Key Terminology

A few terms come up constantly in SPD specifications: Nominal Voltage (U₀) is the line-to-earth AC voltage of the mains system the SPD is designed for. Maximum Continuous Operating Voltage (Uc) is the highest RMS voltage that can be continuously applied to the SPD's mode of protection. Voltage Protection Level (Up) is the key performance figure — how well the SPD limits transient overvoltage — and a low Up value is critical for effectively protecting electronic equipment. Impulse Current (I_imp) is a current peak simulated with a 10/350µs waveform representing partial lightning current, used for Type 1 SPDs. Nominal Discharge Current (In) is a defined peak current with an 8/20µs waveform, used for Type 2 SPDs. Maximum Discharge Current (I_max) is the peak current through the SPD with that same 8/20µs waveform.

Types of SPD

International standards define three types:

• Type 1 — protection against direct lightning strokes. Recommended for protecting installations against partial lightning currents from a direct strike, discharging voltage that spreads from the earth conductor into network conductors. Characterised by a 10/350µs current wave.
• Type 2 — protection against switching and indirect lightning strokes. The main protection system for low voltage installations, installed at each electrical switchboard to stop overvoltages spreading through the installation and protect connected loads. Characterised by an 8/20µs current wave.
• Type 3 — local protection for sensitive loads. Lower discharge capacity, and must only be installed as a supplement to Type 2 protection, positioned close to the sensitive load itself.

Combined SPD types are also available as a single unit.

What BS 7671 Requires

BS 7671 sets out the requirements for a safe installation, including adequate protection for people and equipment against transient overvoltages of atmospheric origin (via the supply system) and against switching overvoltages. Two sections cover SPDs specifically: Section 443 (protection against transient overvoltages of atmospheric origin or due to switching) and Section 534 (devices for protection against overvoltage).

How Installation Affects Protection

Connecting lead length matters enormously. The residual voltage reaching protected equipment is the SPD's voltage protection level (Up) plus the inductive voltage drop along its connecting leads — and that inductance scales directly with lead length, roughly 1µH per metre. With an impulse current rising at 1kA/µs, that works out to roughly 1kV of extra voltage per metre of lead. For optimum protection, total connecting lead length (both legs combined) should be as short as possible and preferably not exceed 0.5m; given this additive effect, it may even be necessary to choose an SPD with a lower Up to compensate for unavoidable lead length. Leads should also be bound tightly together over as much of their length as possible, which is genuinely effective at cancelling inductance. The consequence of getting this wrong is stark: a protector with 2m of unbound leads can see overall voltage rise to 2,300V — far past a safe 700V.

Protective distance matters too. If an SPD installed in parallel sits too far from the equipment it protects, oscillation effects can push the voltage at the equipment's terminals up to double the SPD's own protection level (Up). This effect can generally be disregarded for cable runs under 10m from the SPD.

Common and Differential Mode Surges

"Mode" describes which conductors a surge occurs between. Lightning-related transients typically start as common mode disturbances (with respect to earth), while switching transients typically start as differential mode disturbances (between line and neutral) — and both can damage equipment. Common mode surges tend to be larger and can cause flashover and insulation breakdown if they exceed the connected equipment's rated impulse voltage; equipotential bonding or Type 1 lightning current SPDs protect against these. Differential mode surges are more likely to directly damage terminal equipment. Downstream overvoltage SPDs protect against both modes at once — a real advantage over protection measures like shielding that only address one.

Protecting the SPD Itself

BS 7671 Section 534 requires installed SPDs to be protected against short-circuit via an appropriate OCPD, and reputable SPD manufacturers provide clear guidance on selecting the right OCPD ratings in their installation instructions. The SPD — either alone or as an assembly with its OCPD — must be able to withstand the short-circuit current rating (I_SCCR) the manufacturer states, defined and tested to BS EN 61643-11.

Conclusion

The BEAMA Guide to Surge Protection Devices gives designers, specifiers, and installers what they need to meet BS 7671:2018's requirements — covering SPD types, selection, and the installation details that make or break real-world performance. Understanding why short, tightly bound connecting leads matter, and the difference between common and differential mode surges, is what actually determines whether modern electronic equipment survives a transient overvoltage event.

This article summarises key concepts from the "BEAMA Guide to Surge Protection Devices (SPDs) – Selection, Application and Theory." The full guide is available as a free download from BEAMA.