By Kieu nguyen/ 16 September, 2019


According to the US Institure of Lightning Safety: “Surge is one of major cause damage to electrical equipments such as overheated transformers, broken power lines, getting fire or human damage in the worst case.” Each surge event can destruct up to trillions dong.


What is surge?


Surges, or transients, are brief over voltage spikes or disturbances on a power waveform that can damage, degrade, or destroy electronic equipment within any home, commercial building, industrial, or manufacturing facility.


And surge can originate from many sources:


  • House or manufacturing place nearby lightning area.
  • The switching (on and off) and operation of certain electrical loads such as lamp, heat system, motor, laser printer, photocopy machine… – whether due to intentional or unintentional operations – can be a source of surges in the electrical system.
  • The outside wires are broken.

How does lightning strike cause surge?


As reported by the Earth Physics Institute, Vietnam get 2 million lightning strikes per year. Vietnam is located in the center of Asia thunderstorms, one of three strongest one in the world.

Vietnam thunderstorm season is relatively long starting form April and ending in October.

Lightning is a naturally occurring electrostatic discharge between two electrically charged regions in the atmosphere or ground temporarily equalize themselves. The velocity of lightning is up to 36.000km/s because of ions rapid movement. We usually catch the brilliant flashes before the thundering because the sound speed moves at 1.230 km/h and the light speed travels at 299.792 km/h in normal air condition.

Lightning can reach the temperature of 30,000°C, 20 times the temperature needed to turn silica into glass (it only takes 1330°C to melt SiO2).

The Air discharge of lightning will be producing a wide range of magnetic field which the radius up to thousands of meters. High amperage of lightning generate a rapid but extremely strong transient magnetic field, and the closer the center of lightning is the greater the damage.


Although the lightning does not hit electrical equipment directly, the power surge still causes damage to devices. Therefore, surge protection should be taken to protect those from direct lightning strikes as well as transient.

How to prevent surge?

Beside preventing lightning directly by devices such as lightning rods and grounding systems, we must also prevent surge by surge protection devices.

Surge protective device (SPD) is divided into 3 types (Type) based on IEC 61643-11 standard and EN 61643-11 standard according to 3 different levels of testing (Class):


Type 1 SPD
It protects electrical installations against direct lightning strokes. It can discharge the back-current from lightning spreading from the earth conductor to the network conductors.

Type 1 SPD is characterized by a 10/350 µs current wave.

This type is also called primary protection: installed at building having lightning rod.

Type 2 SPD
The Type 2 SPD is the main protection system for all low voltage electrical installations. Installed in each electrical switchboard, it prevents the spread of over voltages in the electrical installations and protects the loads.

Type 2 SPD is characterized by an 8/20 µs current wave.

This type is also called secondary protection: installed at places absent lightning rod.

Type 3 SPD
These SPDs have a low discharge capacity. They must therefore mandatory be installed as a supplement to Type 2 SPD and in the vicinity of sensitive loads.

Type 3 SPD is characterized by a combination of voltage waves (1.2/50 μs) and current waves (8/20 μs).

These type 3 devices are terminal protection: installed for camera, television, phone…

SPD normative definition:


Direct lightning stroke

Indirect lightning stroke

IEC 61643-1

Class I Test

Class II Test

Class III Test

IEC 61643-11/2011

Type 1: T1

Type 2: T2

Type 3: T3

EN/IEC 61643-11

Type 1

Type 2

Type 3

Former VDE 0675v




Type of test wave



1.2/50 + 8/20

Note 1: There exist T1 + T2 SPD (or Type 1 + 2 SPD) combining protection of loads against direct and indirect lightning strokes.
Note 2: some T2 SPD can also be declared as T3.

Common characteristics:

  • Uc: Maximum continuous operating voltage
This is the A.C. or D.C. voltage above which the SPD becomes active. This value is chosen according to the rated voltage and the system earthing arrangement.
  • Up: Voltage protection level (at In)
This is the maximum voltage across the terminals of the SPD when it is active. This voltage is reached when the current flowing in the SPD is equal to In. The voltage protection level chosen must be below the over voltage withstand capability of the loads. In the event of lightning strokes, the voltage across the terminals of the SPD generally remains less than Up.
  • In: Nominal discharge current
This is the peak value of a current of 8/20 µs waveform that the SPD is capable of discharging minimum 19 times.

Special check for each Type of SPD:

    • Type 1 SPD: Iimp – Impulse current

      This is the peak value of a current of 10/350 µs waveform that the SPD is capable of discharging of discharging at least one time
    • Type 2 SPD: Imax – Maximum discharge current
      This is the peak value of a current of 8/20 µs waveform that the SPD is capable of discharging once.
    • Type 3 SPD: Uoc – Open-circuit voltage applied during class III (Type 3) tests.

Surgefree SPD Class I, II, III following IEC 61643-11 and EN 61643-11 standard on below:

Reference: Wikipedia, electrical-installation