What is an SPD
on a Consumer Unit?
A Surge Protection Device sits inside your consumer unit plus diverts transient overvoltage to earth before it reaches your appliances. What it does, why it became mandatory under the 18th Edition plus how Type 1, Type 2 plus Type 3 SPDs differ.
An SPD (Surge Protection Device) is a sacrificial component fitted to a consumer unit that clamps transient overvoltage spikes, then diverts the surge current to earth. The 18th Edition of BS 7671 published in 2018 plus Amendment 2 in 2022 require risk assessment for SPDs on most domestic installations. Three types exist: Type 1 for direct lightning strikes, Type 2 for indirect lightning plus switching surges (the most common domestic SPD) plus Type 3 for sensitive equipment downstream. A modern UK consumer unit fitted in 2026 will normally include a Type 2 SPD as standard.
Three figures
worth knowing
18th Edition Trigger
BS 7671:2018 introduced Section 443 covering risk assessment for SPDs. Amendment 2 in 2022 made the assessment more prescriptive.
Typical Type 2 Rating
A typical UK domestic Type 2 SPD is rated 40kA at 8/20µs waveform. Industrial or high-risk installs use higher ratings.
BS EN Standard
Manufactured to BS EN 61643-11. Specifies the waveforms, ratings plus testing requirements for low-voltage SPDs in distribution boards.
Type 1, Type 2 plus Type 3
SPDs explained
SPDs are classified by where they sit in the surge protection chain plus how big a surge they can handle. The three types are designed to layer.
Direct Lightning Protection
Designed to handle the partial lightning current that flows through the supply conductors after a direct strike on a building's external lightning protection system (LPS).
Type 1 SPDs are tested with the harshest 10/350 microsecond waveform that simulates a real lightning impulse. Most UK domestic properties do not need a Type 1 device.
Indirect Surge Protection
The most common domestic SPD. Handles indirect lightning surges (induced from nearby strikes) plus switching transients from the supply network.
Type 2 SPDs are tested with an 8/20 microsecond waveform representing typical induced surge events. A modern UK consumer unit fitted in 2026 will normally include a Type 2 SPD as standard.
Sensitive Equipment Protection
Smaller fine-protection devices fitted close to sensitive equipment. Mops up the residual voltage that gets past the upstream Type 2 SPD.
Type 3 SPDs are normally point-of-use devices: built into surge-protected sockets, plug-in adaptors or dedicated wall outlets near the equipment they protect.
A sacrificial device that absorbs the spike before it reaches you
The mains supply that arrives at your property does not sit at a flat 230V. Real-world voltage on a UK domestic supply varies between roughly 216V plus 253V within the statutory tolerance. Brief excursions above 253V also happen many times per day in normal operation. Most are tens of volts plus last microseconds. Connected appliances ride them out without a problem.
A transient overvoltage is something different. It is a sudden voltage spike that can reach thousands of volts for a few microseconds. The two main causes are:
- Lightning activity. A direct lightning strike on or near the supply network induces a fast voltage spike that propagates through the lines. Indirect strikes (on a tree, building or substation a few hundred metres away) cause a smaller but still significant induced spike.
- Switching events. Large industrial loads switching on or off, capacitor bank operations on the network plus heavy motor inrush events all generate switching surges that travel along the network.
Either source can deliver a peak voltage that exceeds the impulse withstand rating of your appliances. UK domestic equipment is typically rated to BS EN 60664-1 category II which means it can withstand transient peaks of around 2.5kV without damage. A spike above that level can degrade or destroy:
- Switched-mode power supplies in TVs, computers, laptop chargers plus modern lighting drivers.
- The control electronics inside boilers, heat pumps plus solar inverters.
- Smart home hubs, networked devices plus EV charger control boards.
How an SPD intercepts the spike
An SPD is built around a non-linear component (typically a metal-oxide varistor, MOV) that has very high resistance at normal supply voltage but switches to very low resistance the instant the voltage rises above its clamping threshold. When a transient spike hits the consumer unit:
- The voltage rises rapidly above the SPD's clamping point (typically 1.3kV to 1.5kV for a UK Type 2 device).
- The SPD switches into low-resistance mode within nanoseconds.
- Surge current flows through the SPD to earth, bypassing the rest of the installation.
- The voltage on the protected side is held below the appliance impulse rating.
- Once the spike passes, the SPD returns to its high-resistance state automatically.
Each clamping event slightly degrades the MOV. Over many years plus many surges the device gradually loses capacity. Modern SPDs include a status indicator (a small window or LED) that shows green when functional plus red when the device has reached end of life. End-of-life SPDs are replaced as discrete cartridges without disturbing the rest of the consumer unit.
When an SPD is required by BS 7671
Section 443 of BS 7671:2018+A2:2022 requires a risk assessment whenever an installation could be exposed to transient overvoltages. Amendment 2 made this assessment more prescriptive. In practice for UK domestic installs the risk assessment lands on one of two outcomes:
- SPD required. The installation contains equipment whose disruption would be inconvenient or expensive (most modern homes), the supply is overhead in a rural area or the property has an external lightning protection system. This covers the majority of UK homes today.
- SPD not required. The installation is a small outbuilding with no sensitive electronics, in an urban environment with a buried supply, with no external LPS. Increasingly rare on a domestic install.
The Section 443 risk assessment must be documented on the EIC for any new consumer unit install plus on the EICR for existing ones. Where an SPD has been omitted the certificate must record the justification.
Six factors that
shape the decision
The Section 443 risk assessment looks at supply type, location plus what is connected. These six factors are the ones the assessment leans on most.
Overhead supply in a rural area
Overhead supply lines are exposed to direct plus indirect lightning. Rural areas typically have longer line runs, fewer surge-protected substations plus higher induced-surge risk.
External lightning protection system fitted
Buildings with a structural LPS (lightning rod plus down conductors) need Type 1 plus Type 2 protection layered together. The LPS itself diverts strikes. The SPD protects the supply path.
Suburban property with sensitive electronics
The majority of UK homes. Smart TVs, computers, networked hubs plus modern boilers all have surge-vulnerable electronics. The cost of a Type 2 SPD is low compared with replacement of those items.
Property with EV charger or solar PV
Both add a long external cable that increases coupling exposure. Inverters plus charger control boards are also among the most expensive electronic items in a modern home.
Dense urban property with buried supply
Buried supply cables provide a degree of shielding from induced surges. Dense urban substations typically include their own surge protection. Risk profile is lower but not zero.
Small outbuilding with no sensitive load
Garden shed, garage or workshop with only basic lighting plus power. Section 443 may permit omission with documented justification. Even here Type 2 cost is small.
Four things every homeowner
should know
SPDs are sacrificial
An SPD absorbs surge energy until its varistor degrades. Replacement after a major event or every 10 to 15 years is normal.
The status window matters
Check the indicator window every six months. Green is functional. Red means end of life plus needs replacement to restore protection.
Section 443 is now the default
BS 7671 risk assessment now lands on SPD-required for the vast majority of UK domestic installs. Omission requires documented justification.
Type 2 is the home default
Most UK domestic installs need a Type 2 SPD only. Type 1 is for buildings with a structural LPS. Type 3 is point-of-use protection.
SPD Installation in Milton Keynes
C-Lec Electrical fits BS EN 61643 compliant Type 2 SPDs as part of every consumer unit upgrade across Milton Keynes, Bedford plus the surrounding Bedfordshire area. Section 443 risk assessment plus EIC documentation handled in full.
For the wider context on consumer units, RCBOs, AFDDs plus the regulations behind UK distribution boards, head back to our full guide to consumer units where every common question is answered in one place.
Back to the Consumer Units Guide
This article sits inside our complete Consumer Units knowledge base. The hub covers everything from board types plus RCBOs through to landlord requirements plus BS 7671 wiring regulations.
More on consumer
unit protection
To understand the BS 7671 framework that triggers the SPD requirement (plus every other modern protection device) head to consumer unit wiring regulations. To see the wider 18th Edition context that introduced Section 443 alongside arc fault protection, see what is an AFDD board. To work out whether your existing board needs the modern protection layered onto it, head to do I need to upgrade my consumer unit. If you need a new consumer unit with a Type 2 SPD fitted in Milton Keynes or Bedford, our consumer unit upgrades service page is the fastest route to a quote.