According to the standard, the declared Up value of a multi-mode SPD is the maximum residual voltage value measured across all its protection modes. For example, for a three-phase SPD, if the residual voltage is 1.5 kV in L-PE mode, 1.8 kV in L-N mode, and 2.0 kV in N-PE mode, then the Up value of this SPD should be specified as 2.0 kV. This means that the protection level provided by this SPD to the system is determined by its weakest protection mode.

This regulation provides crucial guidance for engineers in selecting surge protective devices (SPDs). It requires engineers to consider not only the nominal Up value of the SPD but also to verify that it provides the necessary test data for all required protection modes, and that the Up values for all relevant modes are below the withstand voltage of the protected equipment. Different grounding systems, such as TN-S, TT, or IT, may require different combinations of protection modes. For example, in a TT system, protection between N and PE is critical. If the Up value of the selected SPD in the N-PE mode is too high, even if its Up value in the L-PE mode is low, it will not provide effective protection for equipment on the N-PE circuit, thus creating a vulnerability during common-mode lightning current surges. Therefore, verifying insulation coordination based on the worst-case Up value is crucial to ensuring a robust surge protection system without weaknesses.