Selection of Uc,Up,Iimp and In for Surge Protective Devices (SPDs)-jinlispd.com

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Selection of Uc, Up,Iimp and In for Surge Protective Devices (SPDs)

August 20 , 2025

Selection of Maximum Continuous Operating Voltage Uc and Voltage Protection Level Up for Surge Protective Devices (SPDs)

According to the requirements of national standards GB 50343-2012 and GB/T 18802.12-2024, the principle of rated impulse withstand voltage Uw * 0.8>SPD voltage protection level Up should be before the wire enters the system.

If Uc is chosen too low, it will affect the lifespan and stability of theSPD, while if Uc is chosen too high, it will inevitably result in a high Up of the SPD, which will affect the protection effect of the protected equipment. If an appropriate Uc is chosen, we can balance the protection effect and safe operation of the SPD. We should follow the principle that the maximum continuous operating voltage Uc of the power protector ≥ the maximum continuous operating voltage Ucs of the power system.

SPD connected at (if applicable)	Distribution network system configuration
SPD connected at (if applicable)	TN systyem	TT system	IT system
Between phase line and neutral line	1.1U√3 or 0.64U	1.1U√3 or 0.64U	1.1U√3 or 0.64U
Between phase line and PE line	1.1U√3 or 0.64U	1.1U√3 or 0.64U	1.1U
Between phase line and PEN line	1.1U√3 or 0.64U	Not applicable	Not applicable
Between neutral line and PE line	1.1U√3	1.1U√3	1.1U√3 or 0.64U
Between phase line and phase line	1.1U	1.1U	1.1U
Note: U is the line to line voltage of the low-voltage system
This is the value under the most severe fault condition, so 10% tolerance was not considered.

——Refer to GB/T 18802.12-2024 Table 6 Minimum recommended values of Uc for SPD in various power supply systems

Selection of surge protective device (SPD) impulse current and nominal discharge current

Lightning protection level	Main distribution box		Sub distribution box	Equipment room distribution box and electronic information equipment ports that require special protection
	LPZ0 and LZP1 boundary		LPZ1 andLZP2 boundary	Boundaries of subsequent protection zones
	10/350μs type Ⅰ test	8/20μs type Ⅱ test	8/20μs type Ⅱ test	8/20μs type Ⅱ test	Compound wave type Ⅲ test
	Iimp（kA)）	In（kA)）	In（kA)）	In（kA)）	Uoc(kV)/Isc(kA)
A	≥20	≥80	≥40	≥5	≥10/ ≥5
B	≥15	≥60	≥30	≥5	≥10/ ≥5
C	≥12.5	≥50	≥20	≥3	≥6/ ≥3
D	≥12.5	≥50	≥10	≥3	≥6/ ≥5

——Refer to GB 50343-2012 Table 5.4.3-3 Recommended values of impulse current and nominal discharge current parameters of power lineSPDs.

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