Precise Selection, Effective Protection: A Guide to Surge Protective Device (SPD) Selection and Techniques

Surge Protective Devices (SPDs) are critical protection components in

electrical systems, designed to safeguard power and signal line equipment from lightning surges and grid overvoltages. Improper selection can lead to protection failure, equipment damage, and significant economic losses. This article systematically outlines the core techniques and

considerations for SPD selection, helping you choose scientifically and

comprehensively enhance equipment protection levels.

1. The Importance of Correct SPD Selection

Ensure Equipment Safety: Effectively prevent damage caused by

lightning strikes and grid fluctuations.

Enhance System Stability: Suppress voltage fluctuations and reduce the frequency of equipment downtime.

Control Maintenance Costs: Reduce equipment repair and replacement

expenses through reasonable protection.

2. Detailed Explanation of Core SPD Selection Parameters

Current Discharge Capacity (Imax / Iimp)

Meaning: The maximum surge current an SPD can withstand.

Selection Advice:

Main Distribution Cabinet (Type 1): Iimp ≥ 25kA

(10/350μs).

Floor Distribution Box (Type 2): Imax ≥ 40kA (8/20μs).

Terminal Equipment (Type 3): Imax ≤ 20kA.

Voltage Protection Level (Up)

Meaning: The residual voltage limited by the SPD, directly

affecting equipment safety.

Selection Advice:

Primary Protection: Up ≤ 2.5kV.

Secondary Protection: Up ≤ 1.7kV.

Tertiary Protection: Up ≤ 1.2kV (recommended for

sensitive equipment).

Rated Operating Voltage (Uc)

Meaning: The maximum voltage at which the SPD can operate

stably over the long term.

Selection Advice:

Single-Phase System: Uc = 275V–320V.

Three-Phase System: Uc = 385V.

DC System: Match according to the actual voltage

(e.g., 24V/48V/1500V).

Response Time (tA)

Meaning: The reaction time required for the SPD to initiate

protection, in nanoseconds (ns).

Selection Advice:

General Equipment: tA ≤ 25ns.

Precision or Communication Equipment: tA ≤ 10ns.

Transmission Performance (Insertion Loss)

Meaning: The degree of impact the SPD has on signal quality, in decibels (dB).

Selection Advice: Insertion loss should be ≤ 0.5dB to

maintain high signal transmission performance.

3. Selecting SPD Protection Levels by Application Scenario

Type 1 (Primary Protection):
Suitable for main distribution cabinets and building power

entrances, discharging high-energy direct lightning currents. Recommended: Iimp ≥ 25kA,

Up ≤ 1.5kV.

Type 2 (Secondary Protection):
Used for floor distribution boxes or industrial sector cabinets,

suppressing residual surges after primary protection. Recommended: Imax ≥ 40kA, Up ≤ 1.7kV.

Type 3 (Tertiary Protection):
Installed close to sensitive equipment, such as servers and

computers, defending against minor voltage fluctuations.

Recommended: Imax ≤ 20kA, Up ≤ 1.2kV.

4. Common Selection Misconceptions and Countermeasures

Misconception 1: Over-Pursuing High Current Discharge Capacity
Analysis: Over-specification increases costs and may deviate from actual needs.
Strategy: Select based on installation location and lightning risk.

Misconception 2: Neglecting the Voltage Protection Level (Up)
Analysis: A high Up value cannot effectively protect sensitive

equipment.
Strategy: Use SPDs with Up ≤ 1.2kV for precision equipment.

Misconception 3: Failing to Match the Rated Operating Voltage (Uc)
Analysis: Mismatch between Uc and system voltage can easily cause

SPD misoperation or damage.
Strategy: Ensure the SPD's Uc matches the system operating voltage.

Misconception 4: Ignoring Response Time
Analysis: Slow response may fail to protect equipment in time.
Strategy: Prefer models with a response time ≤ 10ns.

Misconception 5: Substandard Grounding System
Analysis: Poor grounding significantly affects the SPD's discharge efficiency.
Strategy: Ensure grounding resistance ≤ 10Ω, with short and

reliable grounding wires.

5. Key Points for Installation and Maintenance

Installation should be performed by professionals, on the output

side of the main switch or branch switch.

The grounding system must be reliable, with grounding resistance ≤ 10Ω and grounding wires as short and straight as possible.

Regularly check status indicators; performance tests should be

conducted after the thunderstorm season.

In high lightning incidence areas, a coordinated multi-level protection strategy (Type 1 + Type 2 + Type 3) is recommended.

6. Conclusion

Scientific selection of Surge Protective Devices is crucial for the safe and stable operation of equipment and systems. By understanding key

parameters in the context of actual scenarios and avoiding common

misconceptions, the maximum protective efficacy of SPDs can be achieved. Choosing professional brand SPD products, such as Kaitai, can

significantly enhance the system's lightning and surge protection

capabilities, extend equipment lifespan, and reduce operational risks,

thanks to their reliable performance and comprehensive solutions.