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Explanation of SPD Technical Term

Surge protective device (SPD)

The appliance used to limit transient over voltage and discharge the surging current, it includes one of non-linear element at least.

Voltage switching type SPD

SPD that it shows high impedance when there has no surging, and becomes low impedance while surging voltage occurs . Voltage switching type SPD generally composed of discharging gap, gas discharging tube, thyratron (silicone controllable rectifier) and three-ending bidirectional silicone switching element. Sometimes it also call “short circuit type SPD”

Voltage limiting type SPD

SPD that it shows high impedance when there has no surging, but the impedance will decrease gradually while surging current and voltage rise up. Common non-linear element: piezoresistance, and clamping diode, Sometimes it also call “Clamping SPD”

Combination SPD

SPD constructed of voltage switching and voltage limiting elements. The characteristics curve can take on the characteristics of voltage switching or limiting type, or both.

Modes of protection

Protective element of SPD can be connected between phase line to phase line (L-L), phase line to earth (L-PE), phase line to neutral line (L-N), neutral line to earthing line (N-PE) or their combination. These connections call protection modes.

SPD disconnector

The device that isolates SPD from power supply network (interior and / or outer)

ln nominal discharge current

The 8/20 wave current peak that passing by SPD, used for grading SPD under class II test, or prospective treatment of SPD class I II test

Limp impulse current

It determined by current peak value and charge Q, it shall be performed according to the program for test of acting load. It is used for SPD classification Class I test

Maximum discharge current forclass II test

Passing through SPD, the peak value of current with 8/20 wave, determined by the program for Class II acting load. lmax larger than ln

Maximum continuous operating voltage

Maximum AC virtual voltage or DC voltage allowed to be applied on SPD continuously, the value equal to rated voltage.

Up voltage protection level

It indicates the performance of SPD limiting the voltage between terminals being connected. The value can be selected from the sheet of prior values. This value shall be larger than highest limited voltage.

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