Principle of surge protection device

Introduction

Surge protection Device (SPD) , also known as “Lightning arrester”or “Overvoltage protection Device”, is an indispensable Device in lightning protection of electronic equipment. Its function is to limit the transient over-voltage into the power line and signal transmission line to the range that the equipment or system can bear, or to discharge the strong lightning current into the ground, to protect protected equipment or systems from damage by shock.

The application scope of SPD is mainly the power protection of low-voltage power supply and distribution system and the signal transmission line protection of electronic information system. The former is to suppress the lightning electromagnetic pulse interference (EMP-RRB- coming from the power supply circuit, and the latter is to suppress EMP emp interference coming from the signal circuit. Both the protection object is the electronic information system equipment, devices.

Composition

The type and structure of a Surge protection Device varies for different purposes, but it should contain at least one nonlinear voltage limiting element. The main components of Surge protection Devices are metal oxide varistor (MOV) , silicon avalanche diode or TRANSORB diodes, gas-filled tube, etc. .

1) MOV is an effective surge suppressor, mainly used to prevent catastrophic damage to equipment, but it requires response time, which means that a very short period of peak current may enter the equipment before the threshold circuit is activated. Therefore, the Surge protection Device designed by MOV is not suitable to protect the sensitive equipment with microprocessor. This design is currently used in 90% of the current surge protectors on the market.

2) the avalanche diode or TRANSORB diodes respond faster than MOV and are sensitive to transients, but the maximum capacity of this element is also much smaller than that of a medium sized MOV.

3) the gas-filled tube has a long response time, but it can absorb large amounts of power surges, and is generally designed as a “Last resort”measure to suppress AC surge currents, preventing the entry of power surges not filtered out by the previous stage.

How it works

Since it’s a Surge protection Device, let’s start with the surge. Surge also known as surge, as the name implies is beyond the normal operating voltage of the instantaneous overvoltage. Essentially, a surge is a violent pulse that occurs in a few millionths of a second and can be caused by heavy equipment, short circuits, power switches or large engines. Products with surge arresters effectively absorb the sudden surge of energy to protect the connecting devices from damage.

According to the working principle, the Surge protection Device can be divided into switch type, voltage-limiting type, shunt type or choke type:

1) switch type: it works as a high impedance when there is no transient overvoltage, but as soon as it responds to lightning transient overvoltage, its impedance suddenly changes to a low value, allowing lightning current to pass through. Devices used in such devices are: Discharge Gap, gas-filled tube, thyristor, etc. .

2) voltage limiting type: it works as high impedance when there is no transient overvoltage, but its impedance decreases with the increase of inrush current and voltage, and its current and voltage characteristics are strongly nonlinear. Devices used in such devices include zinc oxide, varistors, transient-voltage-suppression diode, avalanche diode, etc. .

3) split or choke type

Shunt type: in parallel with the protected equipment, it presents low impedance to the lightning impulse and high impedance to the normal working frequency; choke type: in series with the protected equipment, it presents high impedance to the lightning impulse, but to the normal working frequency presents the low impedance. Devices used in such devices include choke coils, high-pass filter, low-pass filter, and quarter-wavelength short circuits.