The Different Parameters of a Surge Protective Device (SPD)

Continuous Operating Voltage - Uc

The maximum continuous operating voltage $U_c$ is the maximum root-mean-square (rms) AC voltage that can be continuously applied to the SPD, including a safety margin.

Temporary Overvoltage - Ut

 The temporary overvoltage Ut (TOV) is the maximum rms value acceptable by the SPD for 5 seconds and 120 minutes, either in withstand mode or in a controlled end-of-life mode (disconnection). The Ut parameter is higher than the Ut voltage.

 Discharge current - In and Imax

The maximum discharge current Imax, applicable to Type 2 SPDs, corresponds to the maximum withstand capacity of an SPD without destruction when subjected to a single lightning stroke (8/20 μs wave).

The nominal discharge current In corresponds to the repetitive withstand capacity without destruction (15 strokes of an 8/20 μs wave) of a Type 1 or Type 2 SPD.

Impulse current - Iimp

The impulse current Iimp applicable to Type 1 SPDs, corresponds to the maximum withstand capacity without destruction when subjected to 1 lightning stroke (10/350 μs wave). This test simulates the consequences of a direct lightning strike on the installation.

Total discharge current - Itotal 

The total discharge current flowing through the PE or PEN conductor of a multipole SPD.

Specific energy - W/R

 The energy dissipated during the discharge of the $I_{imp}$ impulse current during the Class I test. It is expressed in kJ/ohm.

 Open circuit voltage - Uoc

This parameter only applies to Type 3 SPDs and corresponds to the maximum acceptable combined wave voltage (maximum value = 20 kV).

 Protectio level - Up

The maximum value of the residual voltage across the SPD terminals during the 8/20 μs current wave test (at the highest value of the declared In or Imp currents) or during the 1.2/50 μs voltage wave test @ 6 kV (if required).

Residual voltage

The value of the residual voltage across the SPD terminals when subjected to an 8/20 μs current wave of a specific value (e.g., 5 kA).

Short-Circuit Current Rating - Isccr

The SPD and its associated disconnecting device (fuse) are tested to safely disconnect up to a maximum short-circuit current value (e.g., 50 kA). This $I_{sccr}$ value must be higher than the prospective short-circuit current of the network at the installation point of the SPD.

 Follow Current Interrupting Rating - Ifi

This criterion applies exclusively to "air spark gap" technology SPDs. After triggering, these SPDs conduct a portion of the network current (follow current) and must be able to interrupt it. This behavior does not apply to low-voltage SPDs based on "varistor" technology

 Maximum DC Voltage- Ucpv

The voltage applicable to the SPD under continuous operating conditions. It must be higher than the maximum PV voltage (Uocstc).

 Short-Circuit Current Rating - Iscpv

The SPD must safely withstand (disconnect) an end-of-life test at a declared short-circuit current value. This $I_{scpv}$ value must be higher than the maximum short-circuit current of the PV line ($I_{scstc}$).