Lightning Protection Zones (LPZ) — why an SPD's location is determined by a zone division, not by the installation alone

The guide on separation distance and the external lightning protection system covers external lightning protection (air-termination system, down conductors, earthing). The [guide on SPD energy coordination](/guides/nen-1010/spd-energiecoordinatie-type-1-2-iec-61643) covers how Type 1 and Type 2 surge protective devices are coordinated with one another. This article covers the underlying concept that connects both guides: the Lightning Protection Zones (LPZ) of IEC 62305-4, which determine where in an installation which kind of protection is needed.

The concept: the electromagnetic lightning climate decreases zone by zone

IEC 62305-4 describes the protection of electrical and electronic systems against the lightning electromagnetic impulse (LEMP) via a division of a building and its surroundings into successive zones, where each subsequent zone has a substantially lower threat level than the previous one — both for the direct lightning strike current and for the electromagnetic field and the surges it induces:

LPZ 0A: the unprotected outdoor area, directly exposed to a possible direct lightning strike and to the full, undamped electromagnetic field of a lightning strike.
LPZ 0B: also outdoor area, but protected against a direct strike by the air-termination system of the external lightning protection system (LPS) — see the related guide — while the full electromagnetic field is still present here.
LPZ 1: typically the building interior, where the building envelope and the deliberate, continuous bonding of all metal parts and conductors crossing the zone boundary (see below) already substantially reduce both the current component and the electromagnetic field.
LPZ 2 and higher: zones located further inward (for example a cabled, metal enclosure or a specifically shielded room) where additional shielding, bonding and SPDs reduce the level still further — intended for the most sensitive electronic equipment.

Why every zone boundary is a bonding and/or SPD point

The core principle of the LPZ division is that every conductor passing from a higher-threat zone into a lower-threat zone — a supply cable, a data cable, a metal pipe — forms a potential route along which the lightning current or the induced surge from the higher zone can still penetrate the lower, better-protected zone, unless bonding (directly connecting metal, non-current-carrying parts) and, for current-carrying conductors, an SPD is applied at that zone boundary. This is precisely the underlying logic of the SPD type coordination covered in the guide on SPD energy coordination: a Type 1 SPD belongs at the transition from LPZ 0 to LPZ 1 (for example the main distribution board where the supply cable enters, where part of the direct lightning current can still occur), a Type 2 SPD at a further zone boundary (for example LPZ 1 to LPZ 2, such as a distribution board further into the installation), and possibly a Type 3 SPD even closer to sensitive end equipment.

Why this is more than just SPD placement

Although SPD coordination is the most visible application of the LPZ concept, the zone approach of IEC 62305-4 explicitly also includes non-SPD measures: the shielding effect of the building structure itself (for example reinforced concrete or a metal façade), the routing of cables (preferably not right along the outer envelope of a higher zone), and the bonding of all metal pipes and cable trays that cross a zone boundary — not just the current-carrying conductors. An installation that only places SPDs without having the remaining zone-boundary bonding in order does not achieve the full degree of protection the LPZ concept intends.

Note: the exact number of zones, the location of the zone boundaries and the required SPD and bonding measures per boundary follow from the risk assessment of the specific installation (see also the guide on lightning protection risk analysis) and the system design per IEC 62305-4; this article covers the underlying zone concept, not a ready-made zone design for every installation.

Practical relevance

When assessing the surge protection of an installation, it is useful to first identify the building's LPZ boundaries — where does a cable pass from outside (LPZ 0) to inside (LPZ 1), and are there further zone transitions further into the installation towards sensitive equipment (LPZ 2)? — before specifying SPDs. An SPD placed at the wrong zone boundary, or a zone boundary where an SPD is present but consistent bonding of the remaining metal parts is not, does not provide the protection the zone division assumes.

Common mistakes

Specifying SPDs without first identifying the installation's LPZ zone boundaries — the correct SPD type and location follow from the zone transition being protected, not from a fixed rule of thumb.
Bonding or fitting an SPD only on the current-carrying conductors at a zone boundary, and forgetting metal pipes/cable trays crossing the same boundary — these form an equally real route for a conducted surge or a coupled field.
Confusing LPZ 0B with a fully protected zone — the air-termination system protects against a direct strike, but the electromagnetic field of a lightning strike elsewhere is still fully present in LPZ 0B.
Assuming that placing SPDs alone is sufficient, without applying the remaining, non-SPD measures of the LPZ concept (shielding, cable routing, bonding).