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)

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.