Understanding Your Electrics — Part 1
Surge Protection Explained: What an SPD Does and Why the Wiring Regs Require It
By Frankie · March 2026 · 5 min read
Every consumer unit I fit now includes a surge protective device as standard. Nobody asks for one by name, and most people have never heard of them — but they’re now a requirement under the wiring regulations, and there are good reasons for that.
What Is a Voltage Surge?
Your mains electricity supply runs at a nominal 230 volts. A surge is a sudden, brief spike in voltage that goes well beyond the normal range. Think of it like water pressure in a pipe. Most of the time the pressure is steady. But occasionally something causes the pressure to spike — a valve slamming shut, a pump starting. A voltage surge does the same thing to electronics: most last only microseconds, but that’s long enough to silently degrade or destroy sensitive components.
What Causes Surges?
Common surge sources
Power restoration after a cut. When the grid switches back on after an outage, the sudden reinstatement of supply often comes with a voltage spike.
Grid switching. The distribution network regularly switches between power sources. Each switch creates a transient surge that travels down the supply lines.
Lightning. Even an indirect strike nearby can induce a surge through electromagnetic effects.
Large appliances in your own home. Motors starting and stopping generate small internal surges that accumulate over time, gradually degrading sensitive components.
What Does an SPD Do?
A Surge Protective Device sits in or next to your consumer unit and acts as a pressure relief valve for your electrics. When a surge arrives, the SPD detects the voltage spike and diverts the excess energy safely to earth — all in nanoseconds, before it reaches your appliances.
The Three Types
Type 2 is fitted at the consumer unit and handles surges from the grid, power restoration, and indirect lightning. This is what the 18th Edition wiring regulations require for most homes, and what I fit as standard on every consumer unit upgrade. Type 3 plug-in surge protectors are a useful addition for sensitive equipment, but they’re not a substitute for a properly installed Type 2 SPD.
What Do the Wiring Regulations Actually Say?
The 18th Edition (BS 7671:2018+A4:2026) requires surge protection for most domestic installations — particularly those containing any modern electronics. The question of whether your home “contains equipment sensitive to voltage transients” is answered by the presence of almost anything: a smart TV, a laptop, a modern washing machine, a heat pump controller, a Wi-Fi router. This is why every consumer unit I replace now includes an SPD as part of the installation.
Frankie Sewell
NICEIC Approved Contractor • YRLA Recognised Service Provider • Bright Sparks of York
Getting a new consumer unit in York?
Every upgrade I carry out includes a Hager unit with individual RCBOs and integrated Type 2 surge protection as standard.