Buyer's guide

Type 1 SPD against Type 2 and Type 3: where each belongs in an installation

12 August 2026 10 minute read

A surge protective device is wired in parallel with the circuit it protects, ready to divert surge current to earth and hold down the transient overvoltage that reaches the load. This covers the SPDs fitted inside a switchboard or distribution board — not plug-in surge strips on a desk. Standards recognise three types, split by where each sits in the installation and which surge event it is built to survive.

Type 1: absorbing a direct strike at the origin

A Type 1 SPD is fitted at or near the service entrance, where a direct lightning strike to the structure or its incoming service can inject a share of partial lightning current straight into the installation. It is tested against a 10/350 µs current waveform, the shape used to represent that kind of direct-strike energy.

Standards treat this as a distinct function from overvoltage limiting further downstream: the equipotential-bonding stage at the origin is one job, and the transient-overvoltage stage placed further into the installation is another. A Type 1 device's required discharge current rating is not picked arbitrarily — it follows the structure's assigned Lightning Protection Level. A structure classified at the highest protection level is designed around a higher assumed maximum strike current than one at a lower level, so its Type 1 SPD needs a correspondingly higher per-conductor rating.

Where Type 1 stops: bonding is not the same as protecting electronics

A Type 1 device's purpose is narrower than it sounds. Its job is to stop dangerous sparking between the structure's lightning protection system and the metal services running into the building — pipework, cable trays, conduit — by holding them all near the same potential during a strike. That is a safety function, not an electronics one. On its own, a Type 1 SPD gives no meaningful protection to the electronics and low-voltage control gear further into the installation; the standard is explicit that bonding and equipment protection are separate problems, and solving the first does not solve the second.

Type 2: main protection at the distribution board

A Type 2 SPD is the main protective device, fitted at distribution boards downstream of the origin rather than at the service entrance itself. It is tested to an 8/20 µs current waveform, and it is the stage that handles switching transients and indirect or coupled lightning effects, rather than a direct strike.

The indirect route matters here because it explains what Type 2 is actually built for. Lightning reaches an installation two ways: a direct hit to an incoming service, which is Type 1 territory, or indirect coupling — resistive, inductive or capacitive — where a strike some distance away raises the local earth potential and pushes current along whatever path offers least resistance, including buildings that were never struck directly. That resistive route is the more common cause of transient overvoltage from a nearby strike. Type 2 is what stands between that coupled energy and the equipment on the board.

Type 3: a supplement near sensitive equipment, not a replacement

A Type 3 SPD is fitted close to the load it protects — near the sensitive equipment itself, not at a distribution board. Its discharge capacity is low by design, and the standard series defines it as a supplement to a Type 2 device, installed alongside one, never as something that stands in for it. Two independent professional sources — a UK trade association's SPD guide and an institution's technical article — state the same relationship separately: Type 3 adds protection close to the load; it does not replace the stage upstream of it.

The three types at a glance

The types are cumulative and cascaded, not alternatives

Type 1, 2 and 3 are not three options a specifier chooses between. Where more than one is needed, they form a coordinated set, cascaded so that each stage hands off what it cannot handle to the next. Standards treat this cumulative arrangement, not any single type on its own, as what actually protects electrical and electronic systems from both lightning and switching events.

Coordination between the stages has to be established, not assumed. An SPD that receives more surge energy than its position in the cascade is designed to absorb is put at risk along with the equipment behind it, and the physics of how energy divides between two cascaded devices depends on the same connecting details discussed below — separation distance and cable inductance both affect how much of a surge ends up at the downstream device rather than the upstream one.

An uncoordinated cascade is wasteful more often than it is dangerous

A specifier should not treat an uncoordinated cascade as safe to leave to chance, even though the usual outcome is forgiving rather than damaging. Which of the two devices ends up absorbing most of a given surge comes down to which one happens to clamp at the lower voltage — not to any design choice that was actually verified. When the downstream unit is the one that clamps lower, it draws and absorbs the bulk of the surge itself, and the upstream device is left doing little useful work rather than being overwhelmed. In that common case the result is wasted capacity upstream, not a damaged device — but a specifier who relies on that outcome without checking coordination is trusting an accident of clamping voltages, not a verified design.

Parameters a specification has to state

A written SPD specification needs several parameters, not just a type number:

• Impulse discharge current — the 10/350 µs current rating relevant to Type 1 devices.
• Nominal and maximum discharge current — the 8/20 µs current ratings that apply to Type 2 devices.
• Voltage protection level — the figure that characterises how well the device limits the transient overvoltage reaching its terminals. Lower is better for the equipment behind it.
• Maximum continuous operating voltage — the highest rms voltage the device can sit under continuously without conducting or degrading. It has to be selected above the system's normal operating voltage, or the device conducts continuously, overheats and fails.

None of these figures can be chosen in isolation. Equipment at different points in an installation is built to withstand different transient voltages depending on how far it sits from the origin — equipment near the service entrance is rated for a higher withstand voltage than equipment at a socket outlet, following four recognised overvoltage categories. Surge exposure itself falls the same way, moving from the service entrance through sub-distribution and into final circuits, each broadly associated with a lower order-of-magnitude surge current the further into the building it sits. An SPD's protection level at a given point has to sit comfortably below whatever withstand voltage applies there — checked against the actual location, not picked from a generic table.

The connecting lead that undoes a compliant device

An SPD's rated protection level is measured at its own terminals, not at the equipment it protects. The wire connecting the device to the busbar — line-in and earth-out — adds its own voltage drop during a surge, because a fast-rising current develops a voltage across the lead's inductance, U = L·di/dt. That drop is additive. A device that is correctly specified on paper can still deliver more voltage to the protected equipment than its datasheet figure implies, purely because of how it is wired in.

Worked example: what the lead adds to a 1.5 kV device

Take a Type 2 SPD rated at a voltage protection level of 1.5 kV, connected with a combined line-in and earth-out lead of 1.2 metres — deliberately longer than the 0.5 m guidance figure discussed below — during a surge with a stated rate of rise of 1 kA/µs. Using the standard assumption of roughly 1 µH of lead inductance per metre:

• Lead inductance: 1.2 m × 1 µH/m ≈ 1.2 µH
• Inductive voltage drop: U = L × di/dt ≈ 1.2 µH × 1 kA/µs ≈ 1.2 kV
• Total voltage at the protected equipment: 1.5 kV + 1.2 kV ≈ 2.7 kV

That is nearly double the device's rated protection level, and the shortfall comes entirely from wiring, before any oscillation effect from a long downstream cable run is even considered. A device that passed every test on the bench can still under-protect the equipment it was installed to guard.

Separation between cascaded stages matters as much as any one lead

This is also why the physical separation between cascaded SPD stages matters, not just the length of any one device's own leads: a shorter distance and a slower-rising surge push more of the total surge energy toward whichever device is downstream, changing how the cascade actually shares the load.

Keeping the lead short, bundled and positioned correctly

The worked example points directly to the installation practice that limits it. Total connecting-lead length — line-in plus earth-out together — should be kept to around 0.5 metres. Where the separate conductors have to run any distance, binding or twisting them together over as much of their length as possible reduces the effective inductive drop, because the outgoing and return current paths' opposing magnetic fields partially cancel when the conductors run close together.

Distance from the protected equipment matters too, separately from lead length. If an SPD sits too far from the load it protects, oscillation between the intervening cable's inductance and the equipment's own capacitance can double the voltage that actually reaches the equipment's terminals compared with the voltage measured at the SPD itself. Lead length, bundling and physical position are three different variables, and all three have to be right at once for a correctly rated device to deliver its rated protection.

The overcurrent device ahead of the SPD, and knowing when it has failed

Wiring is not the only connection detail that decides whether a correctly chosen SPD actually works. Every installed SPD needs a dedicated overcurrent protective device in its own connecting leads, separate from the overcurrent protection already fitted for the circuit it protects — see the types of circuit breaker available for that role and what each is actually rated to interrupt. That dedicated device has to be selected so it does not nuisance-trip against either the SPD's own normal surge-current handling or the overcurrent device further upstream. Taking a maximum permitted rating from a datasheet without separately checking that it discriminates correctly against the upstream device is a common installation error.

SPDs are also required to indicate when they have reached end of life — typically a visible fault flag that appears once an internal thermal limit has tripped after sustained overload. Periodic inspection of any installation carrying SPDs should include checking that flag, because a tripped device that still looks connected is no longer providing the protection the specification assumed.

Questions buyers ask

Can a Type 3 SPD be used instead of a Type 2?

No. The standard series defines Type 3 as a supplement to Type 2, with low discharge capacity — it is meant to sit alongside a Type 2 device near the load, not replace it.

Does every installation need a Type 1 SPD?

Only where a direct strike could inject partial lightning current into the installation — typically where the structure carries external lightning protection or has overhead service conductors. Underground services on a structure without external lightning protection are not subject to that direct-injection path in the same way, so a Type 2 stage alone can be sufficient there.

What decides a Type 1 SPD's current rating?

The structure's Lightning Protection Level. A higher level assumes a higher maximum strike current, which in turn requires a higher discharge current rating per conductor.

Does a circuit breaker protect against surges?

No. Overcurrent protective devices guard against short-circuit and overload current, a different failure mode from a transient overvoltage. Neither device substitutes for the other.

Why does a correctly rated SPD still let a high voltage through?

Its own connecting leads add an inductive voltage drop on top of its rated protection level during a fast-rising surge — the worked example above shows this can add roughly as much voltage as the device's own rating, from wiring alone.