Surge protective devices (SPDs): Types 1, 2 and 3
A lightning strike can destroy the electronics inside a building without ever touching it. Surge protective devices are how IEC 62305 stops that. This guide explains the three SPD types, the lightning protection zones they sit between, what coordinated protection means, and how SPDs lower the calculated risk of internal-system failure.
Type 1, 2 and 3 SPDs are three classes of surge protective device, each built for a different position in the installation. A Type 1 SPD sits at the service entrance and withstands a share of the direct lightning current, so it is tested with a 10/350 impulse current called Iimp. A Type 2 SPD sits at distribution boards and handles the induced surges that travel along the wiring, tested with an 8/20 nominal discharge current called In. A Type 3 SPD sits close to sensitive equipment and gives fine protection against whatever surge gets past the earlier stages. They are designed to work as a coordinated set, not as interchangeable parts.
That last point is the one most often missed. The value of surge protection comes from the devices acting together, each handing the residual surge to the next at a level the next can handle. A strike does not have to hit a building to wreck the electronics inside it: the electromagnetic field it radiates induces surges on the wiring from a distance. This guide explains how that happens, the lightning protection zones that frame the problem, what each SPD type is for, what coordinated protection means, and how SPDs lower the calculated risk of internal-system failure in the IEC 62305-2 method. New to the standard? Start with what is IEC 62305.
What is LEMP, and why electronics fail without a direct strike
The reason surge protection exists as a separate discipline is that lightning harms electronics at a distance. A strike carries a current of tens of thousands of amperes that rises to its peak in microseconds. A current changing that fast radiates a strong, fast-changing electromagnetic field, which the standard calls the lightning electromagnetic impulse, or LEMP. That field is the carrier of the damage, and it reaches far beyond the point of strike.
A changing magnetic field induces a voltage in any conductor it passes through, and a building is full of conductors: power cables, data cables, and the loops that wiring forms as it runs from board to socket to device. When the LEMP washes over them, a surge appears on those conductors with no physical contact at all. A flash to the ground a short distance away, or to an overhead power line some way off, can drive that surge straight into the equipment along the incoming cable. This is why a building with no direct hit, and even with a sound external lightning protection system, can still lose its electronics. The external system leads a direct strike safely to earth, but it does nothing about the induced surge arriving on the wiring.
This is the gap SPDs fill. The lightning protection system protects the structure from a direct strike; surge protective devices protect the systems inside from the surge that a strike induces, whether the strike lands on the building, nearby, or on a line some distance away. The two are complementary, and the risk method treats them as separate measures acting on different kinds of damage.
Lightning protection zones, and why SPDs sit at boundaries
IEC 62305 organises the inside of a building into lightning protection zones, or LPZ. A zone is defined by how severe the lightning threat is within it, both the surge that can reach it on conductors and the strength of the electromagnetic field. The idea is to step the threat down as you move from the outside towards the most sensitive equipment, treating each step as a boundary where something is done to reduce it.
The outer zones. LPZ 0A is the fully exposed outside: a direct strike is possible here and the full electromagnetic field is present. LPZ 0B is also outside but shielded from a direct strike, for example under the protective cover of an air-termination system, though it still sees the field. These are the zones a cable runs through before it enters the building.
The inner zones. LPZ 1 is the first protected zone inside the structure, where the surge current and the field have been reduced relative to outside. LPZ 2, and any further zones, are progressively better protected regions closer to sensitive equipment, each one a step quieter than the last. A small shielded enclosure around a control system might be LPZ 2 or higher within an LPZ 1 building.
SPDs are installed at the boundaries between zones. A device at the LPZ 0 to LPZ 1 boundary, where the service enters, takes the largest surge, because that is where the cable crosses from the exposed outside into the first protected zone. A device at the LPZ 1 to LPZ 2 boundary handles the smaller residual surge that reaches it. Placing the protection at the boundaries is what makes the step-down deliberate: each SPD reduces the surge to a level the next zone, and the next device, is built to accept. This zone-and-boundary structure is the backbone of the protection measures system in Part 4 of the standard.
Type 1, Type 2 and Type 3 SPDs
Each type is rated for the surge it will see at its position. The waveform it is tested against, and the current it can carry, follow directly from where it sits in the zone structure.
Service entrance
Fitted at the LPZ 0 to LPZ 1 boundary, the point where power and lines enter. It withstands a share of the direct lightning current, so it is rated against the 10/350 impulse current, written Iimp, which carries a large charge. Required where a direct-strike path can reach the wiring, such as a building with an external lightning protection system or an overhead service.
Distribution boards
Fitted at sub-distribution boards inside the building, the workhorse of surge protection. It handles the induced surges that travel along the wiring, rated against the 8/20 nominal discharge current, written In. It clamps what gets past the Type 1 device and protects the bulk of the installation downstream of it.
At the equipment
Fitted close to sensitive equipment for fine protection, often within the socket or the device's supply. It trims the residual surge to a low let-through voltage at the point of use, and is always installed alongside an upstream Type 2 device, never on its own. It matters where equipment sits some distance from the board or needs a lower protection level than the upstream stages give.
The 10/350 and 8/20 labels are engineering shorthand for the shape of the test current in microseconds: 10/350 rises in about 10 and decays to half in about 350, a long, high-charge pulse standing for a partial direct strike; 8/20 rises in about 8 and decays to half in about 20, a shorter pulse standing for an induced surge. The type follows the waveform, and the waveform follows the position.
What coordinated SPD protection means
Fitting an SPD at each board is not the same as protecting the installation. The devices have to be coordinated, which means chosen and placed so they share the surge energy in the right proportions and hand off to each other in sequence. The Type 1 device at the entrance takes the large partial lightning current and brings it down to a level the Type 2 device can absorb; the Type 2 device clamps the residual surge to a level the Type 3 device, and the equipment, can tolerate. Each stage does a defined part of the work, and what it lets through is the input the next stage is sized for.
Coordination can fail in two ways. If the devices are too close together electrically, the upstream one may not see enough voltage to start conducting before the downstream one is already taking the surge, so the small device is hit with energy meant for the large one. If they are mismatched, the let-through of one stage can exceed what the next can handle. The remedy is the separation, the device ratings, and sometimes a decoupling length of cable or element between stages, all chosen so the energy lands where it should. The standard treats coordination, not the mere presence of devices, as the thing that delivers protection.
This is why a surge protection design is read as a chain rather than a parts list. A correctly coordinated set with two devices can outperform an uncoordinated set with three, because what protects the equipment is the residual surge at the end of the chain, not how many devices are counted on the schedule.
How SPDs are chosen, in plain terms
Selection follows a small number of principles, and they all trace back to the zones and the threat at each point. The aim is to match each device to its position and to confirm that what reaches the equipment is genuinely below what the equipment can survive.
Match the SPD type to the zone boundary. The boundary a device sits on sets the surge it must handle and therefore its type. A device at the entrance, crossing from the exposed outside, needs to withstand the partial lightning current, which is a Type 1 duty. A device deeper inside handles induced surges, a Type 2 or Type 3 duty. Putting a Type 2 device where a Type 1 belongs leaves it facing energy it was never rated for.
Coordinate the energy let-through between stages. The let-through of each stage has to be something the next stage can absorb, with enough electrical separation between devices that each conducts in the right order. Coordination is checked across the chain, not device by device, because it is the interaction between stages that decides whether the downstream device is overstressed.
Compare the voltage protection level to the equipment withstand. Every SPD has a voltage protection level, Up, the highest voltage it lets through during a surge. That figure, plus the voltage added by the connecting leads, must sit safely below the impulse withstand voltage of the equipment it protects. If Up is too close to the equipment's limit, the surge that gets past can still cause damage, which is often why a Type 3 device is added near equipment to bring the let-through down further.
The standard sets out the detailed parameters and the values behind each of these choices. The point here is the shape of the decision: type to boundary, coordination across the chain, let-through against withstand.
How SPDs lower the calculated risk of internal failure
Surge protection is not just good practice; it is a measure the risk method gives explicit credit for. In the IEC 62305-2 risk assessment, one of the types of damage a strike can cause is the failure of internal electrical and electronic systems, driven by the LEMP. Each risk is built from components, and each component is how often a dangerous event happens, multiplied by the probability it causes that damage, multiplied by the loss when it does. Surge protection acts on the middle term.
Coordinated SPDs lower the probability that an induced surge actually causes an internal-system failure. The better the coordinated set, the lower that probability, and because the probability multiplies through the component, reducing it reduces the component and so the risk. This is precise: the method credits coordinated SPDs against the internal-failure components and nothing else. They do not lower the probability of physical damage from a direct strike, which is the lightning protection system's job, and they do not reduce injury to people from touch and step voltages. Keeping the credit tied to the right components is what lets the method recommend the right combination of measures instead of over-specifying one to cover risks it cannot touch.
The detail that catches people out is that the credit depends on coordination, not on counting devices. The method recognises a coordinated SPD system as the measure that lowers the internal-failure probability. An uncoordinated collection of SPDs does not earn the same credit, because it does not deliver the same protection, which is exactly the engineering reality the risk model reflects.
Where Part 4 fits, and how the pieces connect
SPDs are one element of a wider scheme. IEC 62305-4 is the part that defines that scheme, and the other measures in it work alongside the devices rather than instead of them.
From surge protection to a filed report
Coordinated SPDs are one of the measures that move an IEC 62305-2 result. Lumex runs the full Part 2 method on the building you describe, computes the internal-system failure components, and shows how crediting a coordinated SPD system lowers the probability of failure and brings a failing risk into line, traceable clause by clause. Because the method keeps the credit tied to the right components, the report shows exactly what the SPDs did and did not change. To see the method end to end, read the IEC 62305-2 risk assessment, or explore the platform.
New to the standard? Start with what is IEC 62305, then read how the risk method turns a building, its surroundings and its measures into a defensible verdict.