With the rapid development of electronic technology, more attention has been paid to lightning strikes in construction. Thus, more overvoltage protection products into the market, the Surge Protective Device (SPD) is also becoming familiar to people.
1 Prerequisites for installing the surge protector
Reasonable division of the protection area, according to the possibility of objects hit by lightning and the degree of attenuation of electromagnetic field intensity, the building is divided into LPZ0A zone, LPZ0B zone, LPZ1 zone…LPZn+1 zone.
2 Characteristics of the surge protector
When SPD is selected in a 220/380V three-phase system, its maximum continuous operating voltage Uc should be selected according to the different grounding system forms.
The In of Class I classification test should not be less than 15kA, and the In of Class II classification test should not be less than 5kA.
To protect the protected device from overvoltage, the voltage protection level Up of SPD should always be less than the impulse withstand voltage Uchoc of the protected device, and greater than the maximum power grid operating voltage Usmax (Usmax < Up < Uchoc).
3 Classification of surge protectors
Voltage switching SPD: High impedance when there is no surge, sudden change to low impedance under surge transient overvoltage, such as discharge gap, gas discharge tube, etc., generally used in LPZ0, LPZ1 zone.
Voltage limiting SPD: High impedance when there is no surge, and as the surge increases, the impedance continues to become smaller, such as varistor, suppression diode, etc., generally used in LPZ1, LPZ2 zone, etc.
Combined SPD: It is composed of voltage switching type and voltage limiting type.
4 Selection of surge protector
The cable of the SPD and the protected device should be as short as possible, within 0.5 m.
When multiple SPDS are installed on the lines, the length of the cable between the voltage switching SPD and the voltage limiting SPD should not be less than 10m, and the length of the cable between the voltage limiting SPD should not be less than 5m.
When the distance between the SPD and the protected device is greater than 30m, install another SPD as close as possible to the protected device.
When choosing SPD, attention should be paid to ensure that the SPD will not burn due to power frequency overvoltage, because SPD is anti-transient overvoltage (μs level), power frequency overvoltage is ms level, the energy of power frequency overvoltage is hundreds of times transient overvoltage, therefore, attention should be paid to the selection of SPD with higher power frequency operating voltage.
SPD protection: Each level of SPD should be protected, and can be protected by circuit breakers or fuses, and the breaking capacity of the protector is greater than the maximum short-circuit current.
In the anti-lightning, the shielding, grounding and equipotential measures of the building should be done first, and a reasonable evaluation of the project should be made to determine the expected maximum surge current that the system should withstand, and then the SPD is installed in common mode, differential mode or full-mode wiring according to different grounding forms, and the parameters of SPD are fully considered to match the protected equipment. At the same time, the rationality of its cost performance is investigated to achieve safety, applicability and economy.