impulse discharge current for class I test  imp

The impulse discharge current passing through the device under test (SPD) is defined by the crest value Iimp, the charge Q and the specific energy W/R. The impulse current shall show no polarity reversal and shall reach Iimp within 50 µs. The transfer of the charge Q shall occur within 5 ms and the specific energy W/R shall be dissipated within 5 ms.

The impulse duration shall not exceed 5 ms

Preferred values of impulse discharge current Iimp for class I SPD  Iimp：1，2，5，10， 12,5，20 and 25 kA

specific energy for class I test  W/R

energy dissipated by a unit resistance of 1 Ώ with the impulse discharge current Iimp

NOTE：This is equal to the time integral of the square of the current (W/R = ∫ i 2 d t).

Q (As) and W/R (kJ/W) for given values of Iimp (kA).

Q = Iimp × a，  where a = 5 × 10-4 s

W/R = Iimp2 x b，  where b = 2.5 × 10-4 s

The IEC 62305 standard, defines “protection levels” as specific categories of lightning protection systems, each designed to provide a different degree of safety and effectiveness.

lightning protection level  LPL

number related to a set of lightning current parameters values relevant to the probability that the associated maximum and minimum design values will not be exceeded in naturally occurring lightning.

IEC 62305-2 defines 4 levels: I, II, III and IV

The probability of occurrence of lightning with minimum or maximum current parameters outside the range of values defined for LPL I is less than 2 %.

The maximum values of lightning current parameters relevant to LPL I are reduced to 75 % for LPL II and to 50 % for LPL III and IV (linear for I, Q and di/dt, but quadratic for W/R). The time parameters are unchanged.

An assumption is made that 50% of this current is conducted via the building’s earthing system, and 50 % returns via the equipotential bonding SPDs connected to a three wire plus neutral power distribution system. It is also assumed that no additional conductive service exists. This implies that the portion of the initial 200 kA discharge experienced by each SPD is 25 kA. (IEC 62305-2)