International standard IEC 61643-11 Edition 1.0 (03/2011) defines the characteristics and tests for surge protection device (SPD) connected to low voltage distribution systems
This is the A.C. or D.C. voltage above which the SPD becomes active. This value is chosen according to the rated voltage and the system earthing arrangement.
Up: Voltage protection level (at In)
This is the maximum voltage across the terminals of the SPD when it is active. This voltage is reached when the current flowing in the SPD is equal to In. The voltage protection level chosen must be below the overvoltage withstand capability of the loads. In the event of lightning strokes, the voltage across the terminals of the SPD generally remains less than Up.
In: Nominal discharge current
This is the peak value of a current of 8/20 µs waveform that the SPD is capable of discharging minimum 19 times.
In corresponds to a nominal discharge current that a SPD can withstand at least 19 times: a higher value of Inmeans a longer life for the SPD, so it is strongly recommended to chose higher values than the minimum imposed value of 5 kA.
Type 1 SPD
Iimp: Impulse current
This is the peak value of a current of 10/350 µs waveform that the SPD is capable of discharging of discharging at least one time。
IEC 62305 standard requires a maximum impulse current value of 25 kA per pole for three-phase system. This means that for a 3P+N network the SPD should be able to withstand a total maximum impulse current of 100kA coming from the earth bonding.
Ifi: Autoextinguish follow current
Applicable only to the spark gap technology. This is the current (50 Hz) that the SPD is capable of interrupting by itself after flashover. This current must always be greater than the prospective short-circuit current at the point of installation.
Type 2 SPD
Imax: Maximum discharge current
This is the peak value of a current of 8/20 µs waveform that the SPD is capable of discharging once.
If you compare 2 SPDs with the same In, but with different Imax: the SPD with higher Imax value has a higher "safety margin" and can withstand higher surge current without being damaged.
Type 3 SPD
Uoc: Open-circuit voltage applied during class III (Type 3) tests.