Understanding surge protection devices (SPDs)

Helpful video reference. Artisan Electrics, a Cambridge-based renewable energy and domestic electrical installer with over 350,000 YouTube subscribers, covers the Amendment 2 SPD requirement in their video "Do we need surge protection in a house? Update to 18th edition amendment 2". They cover the practical question most homeowners have: does this apply to my property, and does my electrician need to include one? Worth watching before you get any quote for consumer unit work.

1. What is a transient overvoltage?

Your mains supply should run at around 230 V AC. Most of the time it does. But occasionally, for a fraction of a second, the voltage spikes to several thousand volts. These spikes are called transient overvoltages, and they can happen for several reasons:

• Lightning: A strike nearby, even without hitting your property directly, can induce a voltage spike on overhead supply cables serving your street.
• Switching operations: When large loads are switched on or off by the power company — or by industrial equipment nearby — brief overvoltages travel along the supply network.
• Large motors: Air conditioning units, pumps and similar equipment generate spikes when they start or stop, which can affect other equipment on the same circuit.

A spike lasting a few microseconds carries enormous energy in a very short burst. That is enough to destroy a circuit board, corrupt data on a solid-state drive, or degrade components inside expensive appliances over time.

2. What a surge protection device actually does

An SPD is connected between the live and neutral conductors (or between live and earth, or all three, depending on type) at the consumer unit. Inside the device are components — typically metal oxide varistors or gas discharge tubes — that behave normally at mains voltage but conduct very heavily when voltage exceeds a threshold, typically around 400 to 600 V for a domestic installation.

When a spike arrives, the SPD clamps the voltage: it diverts the excess energy to the earth conductor in nanoseconds. By the time the spike would have reached your television, laptop or smart heating controller, the voltage seen by those devices stays within safe limits.

The SPD itself absorbs a small amount of energy each time it operates. After a major surge (or many small ones over time), the internal components wear out. That is what the status indicator on the front of the device is for — green means still functional, red means it has been spent and needs replacing.

3. What changed with Amendment 2 in March 2022

Before March 2022, BS 7671 (the IET Wiring Regulations, 18th edition) treated SPDs as something to be considered and recommended in higher-risk situations. The decision was left largely to the designer of the installation.

Amendment 2, which became effective on 28 March 2022, changed Chapter 44 of BS 7671. The language changed from recommended to required as the default. Specifically, SPDs must now be provided unless a risk assessment demonstrates that the consequences of an overvoltage would be insignificant. In practice, for a domestic property containing modern electronics, the consequences of an overvoltage are never insignificant.

This means any consumer unit installed or replaced after March 2022 should include a Type 2 SPD unless the installing electrician has completed and retained a documented risk assessment justifying its omission. If you had a consumer unit replaced after that date and were not told about surge protection, it is worth checking what was installed.

4. Type 1, Type 2 and Type 3 SPDs

SPDs are classified into three types based on where they are fitted and what level of surge they are designed to handle:

• Type 1: Installed at the origin of the installation, typically alongside the meter, where there is a direct lightning protection system (a lightning conductor) on the building. The heavy surge from a direct strike is partially handled here before it reaches the consumer unit. Most domestic properties in the UK do not have lightning conductors, so Type 1 SPDs are uncommon in ordinary houses.
• Type 2: The most common type for domestic properties. Fitted at or near the consumer unit, a Type 2 SPD handles surges arriving from the supply network. It occupies one or two modules in the consumer unit. This is what the Amendment 2 requirement refers to for most homes.
• Type 3: Small, supplementary devices fitted at the point of use — inside sockets, plug-in adapters, or extension leads. They provide a final layer of protection directly at sensitive equipment. They must be used alongside a Type 2 SPD, not instead of one.

Combined Type 1+2 devices exist and are used in some commercial or high-risk residential installations. For a standard UK house, a Type 2 SPD at the consumer unit is the relevant device.

5. What an SPD looks like in your consumer unit

A consumer unit fitted with an SPD will have an extra module that looks different from the row of MCBs and RCBOs. It is typically about the width of one or two standard circuit breakers, with connection terminals and a prominent status indicator window on the front.

The status window shows green when the device is operational. A red indicator means the device has operated (which is what it is supposed to do during a surge event) and has reached the end of its useful life. It should be replaced, because a spent SPD provides no further protection. The rest of the consumer unit continues to work normally — a spent SPD does not trip the board or affect the circuits.

Some consumer unit manufacturers build SPDs directly into the enclosure, with the indicator visible through a window in the board cover. Others use discrete add-on modules that are wired in separately. Both approaches are acceptable.

6. Does your existing installation need one?

There is no retrospective legal requirement to retrofit an SPD to an older consumer unit that predates the Amendment 2 requirement. An existing installation that complied when it was installed does not suddenly become non-compliant because the regulations changed.

However, if your consumer unit is being replaced or if significant work is being carried out on your installation, adding an SPD at that point is straightforward. The device itself adds relatively little to the cost of a full consumer unit replacement.

Practically speaking, the number of connected devices in a typical UK home that are sensitive to voltage spikes — smart TVs, laptops, network equipment, electric vehicle chargers, heat pump controllers — has increased dramatically. The cost of a failed circuit board in any of those devices is likely to exceed the cost of an SPD many times over.

7. What to ask when getting quotes

When asking for quotes for a consumer unit replacement, ask specifically: "Does the quote include a surge protection device?" Any reputable electrician working to current standards should include one or explicitly explain why they are not including one.

The question matters because SPDs add modest cost to the job. Some quotes may omit them to appear competitive. If a quote arrives without mention of surge protection, it is a reasonable question to raise before accepting it.

Also ask: what brand and type is the SPD, and what is the replacement cost if the indicator shows red in future? Some SPDs are proprietary and can only be replaced by the original manufacturer at higher cost; others use standard modules available from multiple suppliers.

When to call us

If you are considering a consumer unit upgrade, or if you want to check whether your existing board includes surge protection, Richard can carry out an assessment in Sandwich and across east Kent. Any consumer unit replacement carried out by Sandwich Electrical is done to current 18th edition standards, including Amendment 2.