<b><u>Surge Protective Device (SPD) </u></b>
A device designed to limit transitory overvoltage surges and to divert impulse currents
away. Also known as limiters, they include at least one non-linear (non ohmic)
component. The international reference standards are IEC EN 61643-11 e IEC 61643-1.
10/350 µs waveform– Standardized current waveform; it flows through the equipment at the moment it is summited to a direct lightning strike.
8/20 µs waveform – Standardized current waveform; it flows through the equipment at the moment it is subjected to an indirect lightning strike.
1.2/50 μs impulse voltage – Standardized voltage waveform, it is added to the rated mains voltage.
Type 1 surge protective device (SPD) – Surge protective device designed to divert the energy associated to a direct lightning strike or an operation on the electricity grid. The test parameter is the discharge current represented by a 10/350 µs waveform (test class I).
Type 2 surge protective device (SPD) – Surge protective device designed to discharge the energy associated with an indirect lightning strike or an operation on the electricity grid. The test parameter is the discharge current idem 8/20 µs waveform (test class II).
I_imp: impulse current for test class I
This is the discharge current with 10/350 μs waveform that the device is able to
divert towards earth or the network at least one time, without deterioration. It is
used to classify the surge protection devices in test class 1 (the 10/350 µs waveform
corresponds to this definition).
Why is I_imp important? – EC 62305 standard requires a maximum impulse current value per pole of 25 kA. To ensure protection in any installation, the SPD must be correctly sized for the maximum current provided for. Be careful not to confuse the current per pole (25 kA) with the total current (100 kA for a 3P+N network).
I_n: rated discharge current – for test class II This is the discharge current with 8/20 μs waveform that the Class 2 SPD is able to divert (towards earth) at least 20 consecutive times, without deteriorating. It is used to determine the SPD’s level of protection, Up. It is at this In value the level of protection value (Up) is measured.
Why is I_n important? – By law, a SPD with In of at least 5 kA may be installed in any system, even in areas with high frequency of lightning strikes. In any case, it is better not to scrimp on the In: the higher it is, indeed, the longer the life of the SPD will be.
I_max: maximum discharge current for test class II – Peak value of the maximum discharge current with 8/20 μs waveform that a Class 2 SPD is able to withstand at least once. I max is, in general, much greater than In .
Why is Imax important? – The difference between Imax and In indicates when the SPD is working, in nominal conditions, near its limits. The higher the Imax is, for the same In, the safer the SPD is working, far away from its performance limits.
U_n: rated voltage – Rated voltage of the mains network between phase and neutral (RMS AC value).
U_c: maximum continuous voltage (IEC 61643-1) – Maximum voltage to earth that the SPD is able to permanently support without either cutting in or deteriorating.
U_T: resistance to TOV (Temporary Overvoltage) – Maximum RMS or DC voltage which the SPD can be subjected to which exceeds the maximum voltage for continuous operation Uc for a specific and limited time (generally 5 s).
N_g: lightning density Expressed as number of times lightning strikes the ground per km^2 per year.
Protection mode – Common mode (MC)- protection between the active conductors
(phase and neutral) and earth.
Differential mode (MD): protection between the active conductors.
I_n: follow-through current – Current, supplied by the electrical supply grid, which flows through the SPD following an impulse current.
I_n: rated interruption value of the follow-through current Presumed short-circuit current that a SPD is able to interrupt on its own.