# How to select the Uc value of the surge protector?

- Sep 28, 2020-

When we are working on projects, we often encounter different technical specifications, especially the Uc value of three-phase surge protectors, which vary from 255V to 440V. Why is there such a big difference in project requirements? Is the UC value of the SPD surge protector as large as possible?

my country's low-voltage power supply standard is 50HZ, 380/220V, and three-phase four-wire power supply is generally adopted. The voltage between the three phase lines and the neutral line is 220V, which is called the phase voltage; the voltage between the phase line and the phase line is 380V, which is called the line voltage.

According to GB/T 12325-2008 "Power Quality Supply Voltage Deviation": the sum of the positive and negative deviations of the power supply voltage of 35kV and above does not exceed 10% of the nominal voltage; the deviation of the three-phase power supply voltage of 20kV and below is the nominal ±7% of voltage; 220V single-phase power supply voltage deviation is +7% ~ -10% of the nominal voltage.

Therefore, the common 220V AC system has a voltage fluctuation range of 198V~235.4V. Surge protectors are used as high-impedance components connected between the phase line and the PE line, the neutral line and the PE line, and the neutral line and the PE line. When selecting the parameters of the surge protector Uc, the parameter value that should be referred to is the phase voltage in the line. The phase voltage in the common low-voltage power supply line is 220V, so in the 380/220V power supply line, it is also possible to select single-phase Uc as 255V no problem.

So how to choose the UC value of the surge protector?

We understand from the following three aspects:

01 Relationship between UC value and UP value

According to "Protection Device Performance Requirements and Test Methods for Low-Voltage Distribution System" GB18802.1 3.11 stipulates: Uc is the maximum effective value of AC voltage or DC voltage that can be continuously applied to the protection mode of the surge protector. The value of Uc should be selected according to the phase voltage in the low-voltage system.

The selection of the UC value of the surge protector must take into account the impulse withstand voltage rating of the protected equipment.

All levels of surge protectors should be able to withstand the expected discharge current at the installation point, and their effective protection level Up/f should be less than the Uw of the corresponding category of equipment. That is, UP/F＜UW can ensure the effective lightning protection.

UP/F=ΔU+Up(spd)+Up (backup protector)

ΔU is the induced voltage on the connecting wire, namely ΔU=ΔUL1+ΔUL2+ΔUL3

The SPD's Up/f value is related to the Uc value. The larger the Uc value, the greater the SPD's Up value. Up/f is the sum of the induced voltage drop of the connecting wire and the surge protector's Up, so the SPD's Uc If the value is too high, the UP is also high, which is not conducive to protecting the equipment, so the UC is not as high as possible, just as appropriate.

02 The service life of the surge protector

SPD is a high-impedance non-linear device. Under normal conditions, it is in a high-impedance state. Once a transient overvoltage occurs, it will instantly turn on to ground and discharge the transient overcurrent; however, SPD also has a life span, each time it is turned on After that, the SPD will age until it deteriorates, so that the maximum continuous operating voltage will decrease the Uc value. When the Uc value of the SPD is close to the grid voltage, as long as there is a transient overvoltage (including starting voltage or operating overvoltage), it will generate ground The phenomenon of short circuit.

Therefore, in the actual project implementation process, under the premise that the UP value of SPD meets the withstand voltage value of the protected equipment, we habitually choose the Uc value of 320V-385V, which far exceeds the phase voltage in the line. The advantage is: even if SPD Because the conduction is gradually aging, the Uc value can still be greater than the grid voltage for a long period of time, without direct short-circuit to ground, and the service life in the line is also longer.

03 UC selection under different wiring modes on the low-voltage side of the transformer

Low-voltage surge protectors are usually installed on the secondary side of the transformer. When the low-voltage side of the transformer adopts the delta connection mode, there is no neutral point in the three-phase circuit at this time, and the phase voltage = line voltage is 380V. Use a surge protector with a Uc value of 440V or above to protect the equipment circuit.

If the low-voltage side adopts star connection and has a neutral line, the phase voltage is still 220V, and the Uc value of SPD can be selected from 255V-385V according to the equipment.