Selection and Installation of Surge Protectors for Low Voltage Power Distribution System

Selection and Installation of Surge Protectors for Low Voltage Power Distribution System

In recent years, with the increasing modernization, there are more and more electronic
information devices and computers in the civil buildings. The voltage of electronic
information equipment is generally low, and their voltage resistance is relatively low,
so they are highly at risk of lightning electromagnetic pulse. Therefore, the civil
buildings equipped with information system devices should not only be protected from the
direct lightning attack, but also be protected from lightning electromagnetic pulse impulse.
Building an improved over surge protection system is an important part of the electric
engineering design. This article aims to propose practical ways of how to choose the surge
protectors according to characteristics of the buildings and how to install the surge protectors
according to the different low voltage power distribution system. This is a reference for
the personnel in practical engineering application.

1. Classification for Surge Protective Device(abbreviate for SPD,hereinafter referred to as SPD )

(1) Switch type SPD, also known as lightning current arrester. This SPD is in high impedance
   when there is no over surge, but once the over-surge occurs, the impedance will decrease
   to a low value immediately. Normaly, discharge gap, gas discharge tubes, thyristors (SCR) and
   the triac switch are used in these non-linear devices. This kind of SPD is also called crow
   bar SPD.

(2) Voltage limiting SPD. This SPD is in high impedance when there is no over surge. When the
   current and voltage increase, the impedance will decrease continually. MOV and TVS are the
   commonly seen components.

(3) Combined SPD, which are combined by the switch type SPD and pressure limiting SPD. This type
   of SPD possess the characteristic of both switch type SPD and pressure limiting SPD on the basis
   of the their respective parameters and applied voltage characteristics.

2. Key Parameters and Features of SPD

(1) Maximum continuous operating voltage Uc: Maximum system operating voltage which may be applied
   to the SPD continually, which is equal to the rated voltage of surge protector.

(2) Impulse current Iimp: the SPD is used in the first class lightning protection, and Iimp reflects
   the SPD resistance on direct lightning strike(10/350μs wave). It includes Ipeak and Q, and its
   value can be used for the calculation of lightning protection class and power distribution of
   facilities(conductive objects, power lines, communication lines, etc.) in the building.

(3) Nominal discharge current In: the peak current that flow through the 8/20μs wave, which is used
   for the SPD classification of class I test and class II test. For class I test, In should be not
   less than 15kA, and the class II test, In should be not less than 5kA.

(4)Voltage protection level Up: the residual voltage of the nominal discharge current (In), which is
  also called the maximum clamp pressure of SPD. In terms of power SPDs, they can be classified into
  four protection class. The protection class determines the installation position of SPD. In the
  information system, the protection class need to be matched with the voltage resistance of protected
  system and equipment.