Impulse current Iimp
1. Where there are no national regulations or specific regulations for the type of building to be protected:
the impulse current Iimp shall be at least 12.5 kA (10/350 µs wave) per branch in accordance with IEC 60364-5-534.
2. Where regulations exist:
standard IEC 62305-2 defines 4 levels: I, II, III and IV
The table in Figure 2 shows the different levels of Iimp in the regulatory case.
Fig. 1 – Basic example of balanced Iimp current distribution in 3 phase system
Fig. 2 – Table of Iimp values according to the building’s voltage protection level (based on IEC/EN 62305-2)
| Protection level as per EN 62305-2 | External lightning protection system designed to handle direct flash of: | Minimum required Iimp for Type 1 SPD for line-neutral network |
| I | 200 kA | 25 kA/pole |
| II | 150 kA | 18.75 kA/pole |
| III / IV | 100 kA | 12.5 kA/pole |
Autoextinguish follow current Ifi
This characteristic is applicable only for SPDs with spark gap technology. The autoextinguish follow current Ifi must always be greater than the prospective short-circuit current Isc at the point of installation.
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