Surge Protection Device

Author: Simon

2023-07-19 17:03

1.What is SPD?

The Surge Protection Device (SPD) is an electrical installation protection system component. This device is connected in parallel to the power supply circuit of the loads that have to protect. It can also be used at all levels of the power supply network, which is the most commonly used and efficient type of overvoltage protection.

Fig 1: Waveform of surge formation & waveform of surge isolation

1.1 The three types of SPD

Type 1 SPD

The Type 1 SPD is recommended in the specific case of service-sector and industrial buildings protected by a lightning protection system or a meshed cage. It protects electrical installations against direct lightning strikes. It can discharge the back-current from lightning spreading from the earth conductor to the network conductors. Type 1 SPD is characterized by a 10/350µs current wave. Generally this class of lightning protector has a maximum surge capacity of 100KA or more.

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 voltage in the electrical installations and protects the loads. Type 2 SPD is characterized by an 8/20 µs current wave. Lightning through-current capacity greater than or equal to 40KA is considered Type II.

Type 3 SPD

These SPDs have a low discharge capacity. They must mandatorily 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). The third type of surge protector is required to reduce the residual surge voltage to 1000V , its lightning through-current capacity should not be less than 10KA.

 

1.2 Surge test waveform (IEC standard)

Surge voltage waveform

Fig 2: Curve for a (1.2/50µs) Voltage Surge Pulse

For voltage waveforms, the induced impulse takes 1.2µs (IEC 60-1) to rise from the start of the rise to 10-90% of the peak induced at that time, and 50µs to reach 50% (IEC 469-1) on the fall. The rise time of a voltage waveform is shorter than that of a current waveform, but it lasts more than twice as long as a current waveform.

Surge current waveform

Fig 3: Curve for a (10/350µs) surge current pulse

Typical lightning current curve for electric strikes through the earth, the current curve for a direct lightning strike on a lightning rod, generally known as the direct lightning waveform (usually Type I Protection). In the 10/350µs waveform, the 10 means that the lightning current takes 10 microseconds to reach 90% of the peak from 0. 350 means that the lightning current decays from 0 through the peak to half the peak (half of the peak) in 350µs.

Fig 4: Curve for a (8/20 µs) Surge current pulse

Typical over-voltage breakdown caused by lightning striking through the earth, generally known as an induced lightning waveform (usually Type II protection). The 8/20µs waveform where 8 means that the lightning current reaches 90% of the peak from 0 in 8µs and 20 means that the lightning current decays from 0 through the peak to half the peak (half of the peak) in 20µs.

The difference between the two waveforms is that the 8/20μs waveform has a fast rise time and decay time, while the 10/350 waveform has a fast rise time and slow decay time, resulting in a difference that the lightning current energy corresponding to the 10/350μs waveform is much greater than that of the 8/20μs waveform, approximately 17 to 21 times greater.

 

2. Why is the SPD needed?

2.1 Damage caused by Surge

As buildings become smarter and various control systems are introduced, the losses caused by Surges are rapidly increasing. In our country, there are no accurate studies on the losses caused by Surge, but in the US, electronic system failures due to high voltage and overcurrent are estimated to cause $26 billion in lost time and expense annually in the industry alone. As shown in the table below, 88.3% of unexplained failures in electronic equipment are caused by Surge.

2.2 Natural effects

Direct Strike

Lightning strikes directly at structures, equipment, power lines, etc. It causes voltages of about 20kV or more and overcurrent of several kA to 300kA or more. 

Indirect Lightning

Surge is transmitted through the track by lightning on power transmission and communication lines, which is the most common occurrence, and it has a very large energy of more than 6,000V, which causes the most damage.

Inducement Lightning

Surge occurs due to the Surge of the ground potential due to the inflow of high voltage currents induced through conductors such as power lines, communication lines, and metal pipes buried in the ground near the lightning point.

Discharge

The charge induced by the discharge from the ground and the cloud, within the cloud, and between the cloud and the cloud flows into the power line, metal body, or surface, damaging the equipment.

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