How to Choose a Surge Protective Device (SPD): Key Points and Common Misconceptions

The selection of a Surge Protective Device (SPD) requires comprehensive

evaluation based on the specific application scenario, power system

characteristics, and equipment protection requirements. Below are the

core factors to consider during the selection process:

Define the Protection Level
SPDs are generally classified into three types based on installation

location and protection strength:

Type I (Class B): Used in the main distribution board of a building to

withstand direct lightning strikes or high-intensity surges. The nominal discharge current typically ranges from

50 kA to 100 kA.

Type II (Class C): Installed in floor-level or equipment distribution cabinets to protect against induced

lightning surges. The nominal discharge current usually ranges from 20 kA to 40 kA.

Type III (Class D): Mounted near sensitive equipment to handle low-intensity surges. The nominal discharge current is generally between 5 kA and 10 kA.

Calculate the Nominal Discharge Current Appropriately
The nominal discharge current indicates the maximum surge current that

the SPD can safely divert. Selection should consider regional lightning

risk and equipment criticality:

High lightning activity areas (e.g., coastal or mountainous regions):

SPDs with a capacity of 50 kA or above are recommended.

General areas: SPDs with 20 kA–40 kA capacity are suitable.

Protection of sensitive equipment: A capacity of around 10 kA is usually sufficient.

Voltage Protection Level (Up) Must Be Lower Than Equipment Withstand

Voltage
The Up value must be lower than the rated withstand voltage (Uw) of the

protected equipment. For example, if the equipment’s withstand voltage

is 1.5 kV, select an SPD with Up ≤ 1.2 kV. A lower Up value offers

better protection but may reduce the service life of the SPD.

Match the System Voltage
The SPD’s rated voltage must match the power supply system:

Single-phase 220 V system: Choose an SPD with Un = 230 V.

Three-phase 380 V system: Select an SPD with Un = 400 V.

DC systems: such as 48 V, 500 V, or 1500 V, require corresponding DC SPD products.

Response Time Should Be Short
The response time of an SPD is measured in nanoseconds—the faster, the better. For precision electronic equipment or high-speed communication systems, select an SPD with a response time below

10 ns.

Choose the Correct Installation Location
The SPD should be installed as close as possible to the equipment it

protects, and the grounding conductor should be kept short

(recommended length under 1 meter). A multi-stage protection approach can be adopted: Type I in the main distribution board, Type II in sub-distribution boards, and Type III near terminal equipment.

Look for Standards and Certifications
Products should comply with international or national standards such as

IEC 61643 or GB/T 18802, and carry certifications like CE, UL, or ISO

to ensure reliability and safety.

Environmental Adaptability

Humid or corrosive environments: Choose models with a high protection

rating (e.g., IP65).

Areas with large temperature fluctuations: Ensure the operating

temperature range meets onsite conditions.

Maintenance and Supporting Measures
SPDs degrade over time due to surge impacts and require regular

inspection and timely replacement. Additionally, ensure the grounding

system is reliable, with a recommended ground resistance of less than

10 Ω.

Common Misconceptions to Avoid During Selection:

Blindly pursuing high specifications: Over-specification increases costs without necessarily enhancing protection.

Ignoring equipment withstand voltage: If the SPD’s Up value is higher

than the equipment’s withstand voltage, it will fail to provide

effective protection.

Neglecting signal port protection: Signal lines are also paths for

surge intrusion. Data ports of sensitive equipment should be equipped

with appropriate SPDs.

In summary, SPD selection should comprehensively consider parameters

such as nominal discharge current, voltage protection level, and response time, while also emphasizing product certification, proper installation, and ongoing maintenance. For professional support, contact manufacturers like Kaitai Electric for technical consultation or custom solutions.