Surge protective devices (SPDs): Types 1, 2 and 3

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.

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 0_A is the fully exposed outside: a direct strike is possible here and the full electromagnetic field is present. LPZ 0_B 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.

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.

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, U_p, 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 U_p 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.

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.

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.