The selection of Level 1 (Class B) and Level 2 (Class C) surge protectors (SPDs) is a key link in the protection of lightning protection in electrical systems, with the aim of effectively discharging lightning current and limiting overvoltages in layers, thereby protecting the safety of equipment and personnel. Here are the key factors manufacturers should consider when choosing between primary and secondary surge protectors:
1. Selection of Class 1 Surge Protector (Class B SPD).
Primary SPDs are typically installed at the building’s power inlet (such as a main distribution cabinet) to withstand large energy surges from direct or nearby lightning strikes.
1. Impulse Current (IMIMP)
– Definition: A parameter that simulates the impact capacity of lightning currents in kA.
– Selection recommendations: Choose according to the building’s lightning protection level (such as GB 50057 or IEC 62305 standards).
– General Industrial/Commercial Buildings: Iimp ≥ 12.5 kA (10/350μs waveform)
– High-risk areas (e.g., high-rise, thunderstorm-prone areas): Iimp ≥ 20 kA or higher
2. Large Continuous Operating Voltage (Uc)
– It should be higher than the higher operating voltage of the system, leaving a safety margin.
– For example: 380V three-phase system, Uc ≥ 440V AC; Single-phase system Uc ≥ 275V AC
3. Voltage protection level (Up)
– The lower the better, but it needs to be cooperated with the lower SPD.
– Typically Class B SPDs have an Up of 1.8–2.5kV.
4. Installation location and wiring method
– Must be installed on the inlet side of the main distribution cabinet, near the meter or main switch.
– The wiring should be short and thick (0.5m ≤ recommended) to reduce the inductor voltage drop.
5. Arc extinguisher and detachment
– Class B SPDs are mostly gap type or combination type, which requires reliable arc extinguishing capabilities.
– Built-in thermal trip device, automatic disconnection in case of failure, to prevent fire.
2. Selection of secondary surge protectors (Class C SPDs).
Secondary SPDs are installed in distribution cabinets or at the front of critical equipment to further limit residual voltage and protect sensitive equipment.
1. Nominal Discharge Current (In)
– Typically uses an 8/20μs current waveform.
– 20 kA (per pole) is recommended for ≥, 40 kA is optional for high demands.
2. Voltage Protection Level (Up)
– Lower requirements, typically up ≤ 1.5 kV, preferably ≤ 1.0 kV.
– Ensure the security of back-end devices (e.g., PLCs, servers).
3. Large Continuous Operating Voltage (Uc)
– Same as Class B requirements, the system voltage needs to be matched.
4. Response time
– Grade C is mostly zinc oxide varistor type with fast response (<25ns).
5. Cooperation with the upper SPD (energy coordination)
– Distance fit: The cable length between Class B and Class C is recommended ≥ 10m, and the line inductance is used to achieve natural decoupling.
– Decoupling: If the distance is insufficient, an decoupling inductor or an integrated B+C SPD is required.
– Up Coordination: Class C Up should be significantly lower than Class B, creating an effective voltage gradient.
3. Comprehensive selection principle
| Project | Class 1 SPD (Class B) | Secondary SPD (Class C) |
|——|—————-|—————-|
| Key features | Dissipation amplification energy lightning current | Limit residual pressure, fine protection |
| Key Parameters | Iimp（10/350μs） | In（8/20μs） |
| Typical value | ≥12.5kA | ≥20kA |
| Up (Voltage Protection Level) | ≤2.5kV | ≤1.5kV (preferably 1.0kV≤) |
| Mounting Location | The main distribution cabinet incoming end | Distribution cabinet or equipment front end |
| Types of Technology | Clearance type, combination type | Varistor Type (MOV) |
4. Other precautions
1. Standard Compliance:
– Certification standard: GB/T 18802.1 / IEC 61643-11
– Lightning protection level: according to GB 50057, GB 50343, etc
2. Environmental Adaptability:
– Temperature, humidity, contamination level (e.g. IP protection)
3. Monitoring and Maintenance:
– Choose a product with status indication (window) or remote alarm
4. Brand and Quality:
– Give preference to well-known brands that have passed third-party certifications (such as CQC, TUV, UL).
5. System Type Matching:
– Different grounding systems such as TN, TT, and IT need to use SPDs with corresponding wiring methods
summary
When choosing primary and secondary surge protectors, manufacturers should follow the principles of “front-end drainage, back-end voltage limiting, step-by-step derating, and coordination”. The primary SPD focuses on the ability to resist high current, and the secondary SPD focuses on low residual voltage and fast response, and the two can be used together to build an effective multi-level protection system to ensure the safe and reliable operation of the electrical system.