Energy coordination between Type 1 and Type 2 SPDs — the '10-metre rule' and decoupling impedance

The guide on surge protection (SPD, §443) covers when an SPD is mandatory and the difference between Type 1, 2 and 3. This article covers a follow-on question that becomes relevant when combining multiple SPDs in the same installation: why a Type 1 SPD (at the origin, intended to divert direct lightning current) and a Type 2 SPD (further down the distribution, intended to limit the residual voltage to a safe level) cannot simply be placed next to each other without accounting for their mutual energy coordination.

The problem: both SPDs can start conducting at the same time

A Type 1 SPD is designed to divert a large part of a direct or nearby lightning current (the 10/350 µs waveform), while a Type 2 SPD further into the installation must limit the remaining, faster switching and induced surges (the 8/20 µs waveform) to a level safe for equipment. These two SPDs must therefore operate sequentially: first the Type 1 (which diverts the largest energy), only then the Type 2 (which handles the remaining, smaller energy). If both SPDs are placed too close together — with too little cable length, and therefore too little self-inductance, between them — they can conduct almost simultaneously. The Type 2 SPD then has to handle a share of the energy it isn't sized for, with a risk of premature wear-out or even destructive failure of that device.

The '10-metre rule' as a practical rule of thumb

An installation cable has a self-inductance of roughly 1 µH per metre. At around 10 metres of cable length between a Type 1 and a Type 2 SPD, that yields about 10 µH of self-inductance — enough voltage drop (via L·di/dt) during the steep current edge of a surge to make the Type 1 SPD conduct before the Type 2. This rule of thumb is, however, informative, taken from the application guide IEC 61643-12, and not a normative requirement in itself — the actual requirement is energy coordination between the two SPDs, which can also be met another way.

Alternative: a decoupling inductor

If 10 metres of cable between the two SPDs isn't achievable in practice (for example in a compact distribution board), the same effect can be achieved with a separate decoupling inductor of typically around 10-20 µH in the connection between the two SPDs. With factory-coordinated SPD sets — where the manufacturer explicitly specifies the Type 1 and Type 2 as a coordinated pair — a considerably smaller decoupling impedance (on the order of about 1.5 µH) can already be sufficient, because the manufacturer has then already demonstrated the coordination with test data rather than relying on a generic rule of thumb.

The simplest solution: a combined Type 1+2 device

A factory-coordinated combined SPD (a single enclosure with both Type 1 and Type 2 protection components, internally matched to each other) makes both the 10-metre rule of thumb and a separate decoupling inductor unnecessary: the energy coordination is then already inherent to the device's own design, validated by the manufacturer.

Practical relevance

When designing an installation with both a Type 1 and a Type 2 SPD (for example a building with its own lightning protection system, see the guide on LPS separation distance), it must always be checked whether the energy-coordination requirement is met — via sufficient cable length, a separate decoupling inductor, or a combined device. Skipping this can leave the Type 2 SPD failing prematurely during a surge despite an at-first-glance correct selection.

Common mistakes

Placing a Type 1 and a Type 2 SPD next to each other in a compact distribution board without checking energy coordination — too little cable length without a decoupling inductor leaves the coordination risk unresolved.
Treating the 10-metre rule of thumb as a hard normative requirement instead of a practical indication — the actual requirement is energy coordination, which can also be achieved via a decoupling inductor or a factory-coordinated device.
Applying a generic decoupling inductor without using the value specified and tested by the manufacturer for that specific SPD pair — coordination is validated differently per manufacturer and per product combination.
Assuming that any combination of a Type 1 and Type 2 SPD from different manufacturers is automatically coordinated — this must be explicitly demonstrated (via test data or one of the measures above), not assumed.