What is Iimp—Iimp of Class I SPD
Frequently Asked Questions
What is Iimp?
Iimp is the impulse discharge current for Class I SPDs, defined by crest value, charge Q, and specific energy W/R.
What are the preferred values of Iimp for Class I SPD?
Preferred values are 1, 2, 5, 10, 12.5, 20, and 25 kA.
How is specific energy W/R related to Iimp?
W/R = Iimp² × b, where b = 2.5 × 10⁻⁴ s.
What is a lightning protection level (LPL)?
LPL is a number related to a set of lightning current parameters values relevant to the probability that associated maximum and minimum design values will not be exceeded in naturally occurring lightning.
How many lightning protection levels are defined in IEC 62305?
IEC 62305 defines 4 levels: I, II, III, and IV.
What portion of lightning current is assumed to flow through each SPD in a three-wire plus neutral system?
It is assumed that 50% of the current is conducted via the building’s earthing system, and 50% returns via SPDs, so each SPD experiences 25 kA from an initial 200 kA discharge.
