SSD K1 increases the short-circuit withstand of the surge-protection system from 25 kA to 100 kA. The SPD backup device fits within four TH35 DIN-rail modules, replacing a panel-mounted switch-disconnector with NH knife-blade fuse-links - the manufacturer states switchboard space savings of up to 50%. Selection is reduced to a single item: one disconnector serves the entire K1 SPD series, eliminating the risk of installing a fuse-link with a current rating outside the range specified in the SPD datasheet. The fuse striker and microswitch transmit the backup-device state to the monitoring system, so supervision can detect both varistor end-of-life in the SPD and operation of the backup protection.
The disconnector carries lightning impulse current Iimp 12.5 kA with a 10/350 µs waveform and nominal discharge current In 20 kA with an 8/20 µs waveform - the surges flowing through the SPD during a discharge event. Short-circuit withstand Isccr is 100 kA, minimum trip current Itrip 500 A, rated operating voltage Ue 500 V AC (50/60 Hz). It works with SPDs having maximum continuous operating voltages Uc of 175, 275, 320 and 440 V, in TT, TN and IT systems, for L–N, L–PE(N) and N–PE protection modes. Limited current per pole reaches 17.08 kA and Joule integral I²t per pole 85.28 kA²s.
The SPD backup device must meet two requirements simultaneously: it must carry the impulse current flowing through the varistor without damage and interrupt short-circuit current after varistor failure. The first requirement increases the necessary gG fuse-link current rating, while the second reduces it - hence the permissible current ranges shown in SPD datasheets. SSD has both parameters dimensioned directly, and its fuse-link is cylindrical, size 14×51 mm, type K1-SFD-M. Versions: SSD1-2PG-M (45001201) in 1+1 configuration, SSD1-3P-M (45001301) in 3+0, and SSD1-4PG-M (45001401) in 3+1, occupying two, three and four modules respectively. The four-pole version is 107 mm wide, 107 mm high (110.8 mm with clip), 70 mm deep and weighs 0.71 kg. Temperature range −40…+80 °C, humidity 5–95%, operation up to 2000 m above sea level, IP20 protection, overvoltage category III, pollution degree 2, PA6 CT1 and PA66 CT1 enclosure with UL94 V-0 flammability class. Terminals accept 1.5–35 mm² solid and 1.5–25 mm² stranded conductors, tightening torque 3.5 N·m; signalling contact 240 V / 2.5 A AC or 24 V / 3 A DC.
Backup protection becomes necessary when the prospective short-circuit current at the installation point exceeds the K1 SPD's own Isccr of 25 kA or when upstream overcurrent protection exceeds 315 A gG. A station main switchboard or a switchboard supplying vehicle charging positions will often meet at least one of these conditions. The manufacturer specifies the Surge-Trap® K1 12.5 kA with SSD K1 combination for vehicle charging infrastructure and power-distribution switchboards. The product standard in the datasheet is given as draft technical specification prIEC TS 61643-06.
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