Lightning risk assessment for hospitals

IEC 62305 produces a risk of injury to people, R, and a separate frequency of damage, F, for the availability of the internal systems, and it recognises three kinds of loss, L1 to L3 (injury to human beings, physical damage to the structure and its contents, and failure of internal systems). The 2010 edition had a fourth, purely economic loss, which the 2024 edition removed. For most occupied buildings the risk R leads, because the worst outcome of a strike is harm to the people inside. A hospital takes that to its limit. The people inside are not only present in large numbers, they include patients who cannot move on their own and who depend on equipment that cannot safely stop.

So the risk of injury to people R leads a hospital study and carries weight an ordinary building does not place on it. The method accounts for the difficulty of evacuation and the presence of people at special risk, which tightens the margin the life-safety risk has to meet. A strike that injures people through touch and step voltages, or starts a fire in an occupied ward, is the outcome the assessment is built to drive down. This is close to the reverse of an unmanned data centre, where the availability of the internal systems leads and life safety is rarely the deciding risk, and it sets the whole tone of the hospital assessment.

New to how R and the frequency of damage F are built? The IEC 62305-2 risk method sets out how each is assembled from its components, and what is IEC 62305 covers the damage and loss model the risks rest on.

Life safety leads, but it is not the only risk a hospital has to answer for. A hospital is critical public infrastructure for the area it serves: emergency departments, surgery, diagnostics and inpatient care that the surrounding population relies on and that cannot simply pause. A lightning event that takes part of that capability offline, even briefly, affects far more people than the ones inside the building at the time.

That is the loss of service to the public that a hospital study has to weigh. An outage in power, in the clinical network, or in a system a ward depends on interrupts a service the public needs, and the frequency of damage F to those internal systems captures how often a strike would cause it. It rarely overtakes the life-safety risk as the deciding one, but it raises the consequence of an internal-system failure beyond the immediate clinical danger, and it is part of why a hospital is held to a higher standard of continuity than an ordinary occupied building.

The standard groups what a strike can do into three types of damage: injury to people from touch and step voltages (D1), physical damage such as fire or explosion (D2), and the failure of electrical and electronic systems caused by the lightning electromagnetic pulse, or LEMP (D3). In a hospital all three carry real weight. The first two are the direct threats to life that the assessment always weighs, but the third, D3, the loss of internal systems, is unusually important because so much of the building's safety depends on electronics that fail easily.

LEMP is the mechanism. A strike radiates an electromagnetic field that induces surges on the wiring inside the building, and a surge arriving over an incoming line travels straight toward the equipment. Patient monitors, ventilators, infusion pumps, imaging and laboratory analysers fail at low surge levels, far below what a strike can induce. In a theatre or an intensive care unit the failure of one of these at the wrong moment is immediately dangerous, which is why the protection of internal systems sits alongside the protection of life, not below it.

This is where the assessment credits the protection measures. Coordinated surge protective devices on the incoming lines clamp the surge in stages before it reaches the equipment. A thorough equipotential bonding network ties the building steel, cable trays and metallic services together so dangerous voltage differences do not build up around patients and apparatus. Shielding of the most sensitive cable routes reduces how much of the field couples onto the wiring at all. And a lightning protection zone (LPZ) scheme divides the building into zones that step the surge environment down inward, so the intensive care units and theatres sit in the most protected zones. Part 4 of the standard sets out how these measures work together against LEMP, and the risk method gives the engineer credit for them by lowering the probability of internal-system damage.

For how the surge protective devices are graded and coordinated, see SPD types under IEC 62305, and for the zone scheme itself see lightning protection zones (LPZ).

Few buildings are connected to as many conductive services as a hospital. Beyond its power feeds it takes in multiple data and telecom lines, nurse-call and fire alarm wiring, building management cabling, and in many cases metallic medical gas pipework running between a central plant and the wards and theatres. Every conductive service entering the building is a potential surge path, which is why the services, rather than the roof alone, are usually where the assessment finds much of the line-related risk.

Two properties of each line drive its contribution. The length sets how much of the surrounding ground a strike near it can couple into the cable, so a long external run collects more than a short one. The routing, whether the line runs overhead or buried, screened or unscreened, changes how much of a nearby strike actually reaches the building. Together these set the line-related risk components. The metallic services deserve particular attention: a medical gas pipe or a long signal run that is not properly bonded and protected at entry can carry a surge straight into a clinical area, so it has to be modelled as the conductive path it is.

This is the part of the IEC 62305 method that a single occupied building rarely stresses and that a hospital stresses hard. Getting the count, the lengths and the routing of the connected services right, the bonding of the metallic ones, and the SPDs that protect each entry, is much of what decides the answer for this kind of building.

Hospitals are built to keep running through a fault. Standby generation, dual power paths and protected distribution to clinical areas exist so that care does not stop when something fails. Lightning is one threat the standby and redundancy do not fully answer, because an induced surge can reach equipment on more than one power path at once and can disrupt the low-voltage signal and data systems the generators do nothing for.

An IEC 62305 assessment supports the continuity the building is designed for by putting a number on the lightning part of it. It estimates how often a strike-related event could injure someone or take a clinical system offline, and shows that the external protection, surge protection, bonding, shielding and zoning bring those frequencies below an acceptable level. It does not replace the standby power and protected distribution that clinical resilience is built on. What it gives is the recognised evidence that the lightning threat to both safety and continuity has been assessed on the numbers and controlled, rather than left as an assumption that the building is probably fine.

Lumex models a hospital as a structure with its zones, its many incoming services and its protection measures, then runs the IEC 62305-2 method across them. It shows how an external protection system, coordinated SPDs, bonding, shielding and the zone scheme bring the risk to life and the risk of internal-system loss below the tolerable level, and it lets an engineer set out the clinical zones, add each service with its own length and routing, and see the risk move as the protection changes. Every figure traces back to the clause behind it, so an authority, accreditation body or insurer can follow the reasoning rather than take a single number on trust.

New to the standard? Start with what is IEC 62305, then see the Lumex platform for how a building is assessed end to end.