In the most adverse of situations, Surge Protection Device need to arrest as much as 100 kA. When distributed into individual strands, the current values amount to 25 kA per strand (using a TN-C) system. That is why we recommend that master distributors of LV unit substations (in buildings qualifying as LPL I protection level) be fitted with the HS 50-50 sealed spark gap with gas filling. Being SPD Type 1, the equipment guarantees the equalisation of potential and disposal of the lightning current as well as the switching surge that is generated in the power supply lines entering the building. It is capable of arresting Iimp currents as large as 50 kA. Unit substations of individual buildings then should be fitted with the SPC25, a combination of SPD Type 1 and 2, that features dual varistors for higher safety and offers 25 kA of arrestable impulse current. Secondary substations and control cabinets should be equipped with SPD Type 2. An example of that class in our product range is the PIII-275, which is offered as a complete, sealed unit or with replaceable modules.