• Core Technical Advantages of CRESIN CSCB
• Comparison of Three Backup Protection Solutions
• Critical Defects & Hidden Risks
• CRESIN CSCB Series Dedicated SPD Backup Protector (SCB, Recommended Standard Matching Solution)
• Core Technical Advantages of CRESIN CSCB
• SPD Short-Circuit Protection & Selective Tripping Principle of CSCB
• Four Common SPD Backup Wiring Errors & Corresponding Safety Hazards
• Global Market Application & CRESIN Supporting Technical Services
• Application Scenarios of CSCB Series
• Frequently Asked Questions (FAQ) About SPD Backup Protection
• Conclusion

TECHNICAL GUIDE | ELECTRICAL SAFETY

Lightning surges, DC switching transients and abnormal grid overvoltage can gradually damage Surge Protective Devices (SPD). When an SPD fails due to thermal breakdown or short circuit, continuous follow current may cause cabinet fires, busbar damage and system outages, creating serious safety risks in PV combiner boxes, low-voltage distribution panels and industrial control cabinets.

According to IEC 61643-12 and GB/T 18802 standards, an independent short-circuit backup protection device (SCPD) is required upstream of each SPD. This article explains the key role of SPD backup protection, common installation mistakes, and introduces CRESIN CSCB series dedicated backup protector as a reliable solution for Class I and Class II SPD applications.

Core Purpose of SPD Backup Protection

SPD and backup protection undertake completely differentiated protection missions and cannot replace each other:

Core function of SPD: Divert microsecond-level lightning surge current to PE conductor, clamp transient overvoltage, and protect inverters, PLCs, DC equipment and distribution equipment. SPD has no capacity to cut off sustained power-frequency short-circuit current.

Core function of backup protection: Once the SPD suffers thermal runaway or short-circuit fault, quickly isolate the faulty SPD, block continuous follow-current arc, eliminate cabinet fire risks, and avoid power failure of the entire circuit caused by SPD damage.

Therefore, a complete surge protection system requires both SPD and dedicated backup protection working together. SPD is responsible for absorbing and limiting transient surge energy, while backup protection ensures safe disconnection when the SPD reaches the end of its service life or experiences internal failure.

CRESIN CSCB series is specially developed as an SPD dedicated backup protection device, achieving precise coordination between surge discharge and fault isolation. During normal lightning surge events, CSCB remains stable without unnecessary tripping, ensuring uninterrupted surge protection. When the SPD fails and generates dangerous follow current, CSCB rapidly disconnects the faulty branch, suppresses AC/DC fault arcs and prevents secondary damage to electrical systems.

By combining SPD with CSCB, the protection system provides a complete safety solution: protecting downstream equipment from transient overvoltage while preventing fire hazards and system outages caused by SPD failure. This coordinated protection approach is widely applicable to photovoltaic DC systems, low-voltage AC distribution, industrial control cabinets, energy storage systems and other critical electrical applications.

Without qualified backup protection, three severe on-site accidents will occur：

The failed SPD keeps conducting power-frequency current, generating high-temperature arcs and igniting flame-retardant cabinet housing;

Short-circuit current burns main busbars, resulting in full power shutdown of industrial facilities or electrical systems and huge economic losses;

Overcurrent spreads upstream to transformers, triggering main switch tripping and wider power outages.

Comparison of Three Backup Protection Solutions

Ordinary gG/gL Fuse (SPD backup fuse)

Working Principle：Cut off the circuit by melting the fusible metal under continuous overcurrent; low inductance with no extra residual voltage superposition during surge discharge.

Advantages：Low procurement cost, good current selectivity, small incremental residual voltage during surges, high short-circuit breaking capacity.

Disadvantages：Disposable component that must be replaced after blowing; remote fault alarm is unavailable; easy mismatching with SPD rated current.

Application Limitation：Only suitable for small distribution scenarios with low short-circuit current; cannot fully distinguish lightning impulse current and power-frequency fault current, prone to accidental blowing under large lightning surges.

Ordinary Miniature Circuit Breaker (MCB, SPD circuit breaker)

Working Principle：Bimetallic strip for overload protection and electromagnetic coil for instantaneous short-circuit tripping; reusable by manual reset after fault clearance.

Advantages：Resettable without replacement, visible mechanical tripping indication for convenient daily maintenance.

Disadvantages: Internal electromagnetic components introduce additional inductance, which may increase residual voltage during surge discharge and reduce SPD protection performance. Ordinary MCBs are not specifically designed for SPD coordination and may have insufficient arc suppression capability during SPD short-circuit faults.

Application Limitation: Not recommended as a dedicated SPD backup protection device in high-risk applications such as PV systems and industrial power distribution, where accurate coordination between surge current and fault current is required.

CRESIN CSCB Series Dedicated SPD Backup Protector (SCB)

Working Principle: Uses selective thermal-magnetic tripping technology to distinguish between normal lightning surge currents and dangerous power-frequency follow currents. CSCB remains conductive during surge discharge and disconnects the faulty SPD branch when short-circuit failure occurs.

Advantages: Provides precise SPD coordination without surge misoperation, quickly isolates SPD faults, suppresses AC/DC fault arcs, prevents cabinet fire risks, and improves overall system safety.

Disadvantages: Compared with ordinary fuses and MCBs, dedicated SPD backup protectors require professional selection according to SPD parameters and application conditions.

Application Limitation: Requires correct model matching with SPD protection class, system voltage and discharge current to achieve optimal protection performance.

Critical Defects & Hidden Risks

The internal coil carries large inductance; during lightning surge discharge, inductance will superimpose an extra 1.1~1.25 times residual voltage, reducing SPD protection performance and threatening precision equipment such as inverters and control systems.

Prone to false tripping under large 8/20μs or 10/350μs impulse current, cutting off the complete lightning protection loop during thunderstorms.

Poor coordination with SPD thermal disconnector, slow tripping speed under SPD short-circuit faults, insufficient arc suppression capacity for high-current PV DC cabinets.

CRESIN CSCB Series Dedicated SPD Backup Protector (SCB, Recommended Standard Matching Solution)

Core Design Logic: Lightning surges pass through without tripping, fault currents are cut off within milliseconds

Adopt dual thermal-magnetic selective tripping mechanism to accurately distinguish microsecond-level lightning impulse current and millisecond-level power-frequency fault current:

When the SPD discharges normal lightning surges (8/20μs / 10/350μs waveform): CSCB remains fully conductive without misoperation or extra residual voltage increment.

When the SPD ages and short-circuits to generate continuous follow current ≥3A: CSCB trips within milliseconds to isolate the faulty circuit and thoroughly extinguish AC/DC arcs.

For the official full technical parameter table of CSCB, including detailed electrical specifications, performance data, application information and product features, please visit the CSCB product detail page. All technical documents and related product information are available for reference.

Core Technical Advantages of CRESIN CSCB

Full model coverage: T1 matching models CSCB-SA15S/SA25S for Class I SPDs; T2 matching models CSCB-S20S/S40S/S60S/S80S/S100S for Class II SPDs, supporting full 1P/2P/3P/4P pole specifications.

Strong short-circuit withstand capacity: Max breaking capacity up to 50kA, compatible with high short-circuit current environments of large-scale DC distribution systems.

Optional remote signal contacts: Connectable to plant SCADA monitoring systems to upload SPD fault alarm signals in real time.

Standard 35mm DIN rail mounting; each T2 module is only 18mm wide to save cabinet space; IP20 protection grade, operating temperature -25℃~60℃, adapting to harsh environments such as rooftop PV, ground-mounted power stations and industrial control cabinets.

Compliant with NB/T 42150-2021 standard, perfectly matched with all CRESIN SPD series including CSMS-B40 PV DC surge protective devices.

SPD Short-Circuit Protection & Selective Tripping Principle of CSCB

CRESIN CSCB integrates a double-layer safety tripping structure to realize full-range fault protection for SPDs:

Thermal Overload Tripping (Bimetallic Strip Module)Simulate the thermal capacity curve of the varistor inside the SPD. When long-term leakage current accumulates and causes slow thermal aging of the SPD, the bimetallic strip deforms under heat to cut off the circuit before the varistor reaches the ignition temperature, eliminating hidden fire risks from slow temperature rise.

Instant Short-Circuit Magnetic Tripping (Electromagnetic Coil Module)When the SPD breaks down and short-circuits to generate instantaneous large fault current, the electromagnetic mechanism breaks the circuit rapidly within 0.1s, thoroughly extinguishing AC/DC power-frequency arcs and preventing fault spread.

Unified key parameter for all CSCB models: industrial-grade 3±1A power-frequency tripping current, which accurately identifies dangerous follow current generated by faulty SPDs while ignoring transient lightning impulse current, solving the long-standing industry pain point of misoperation of fuses and ordinary circuit breakers.

Four Common SPD Backup Wiring Errors & Corresponding Safety Hazards

According to field surveys by CRESIN global after-sales teams covering PV and distribution projects in Europe, Southeast Asia and the Middle East, more than 60% of cabinet fire accidents are caused by non-standard backup protection wiring:

Mistake 1: Install backup protection behind the SPD (PE side)

Risk: After the SPD short-circuits, fault current flows directly to the PE busbar without isolation; long wiring will generate induced voltage under lightning surges, drastically raising residual voltage Up and breaking downstream electrical equipment.Standard Rule: CSCB / fuse / MCB must be connected in series at the upstream power incoming terminal of the SPD (positive/negative pole side), between the main busbar and SPD.

Mistake 2: Overlong & Looped SPD connecting cables

Risk: Every 1 meter of cable produces approximately 1μH inductance; under 20kA surge current, an extra induced voltage up to 2kV will be superimposed on equipment terminals, completely offsetting the SPD voltage clamping effect.Standard Rule: The total length of SPD incoming wires and PE cables shall be controlled within 0.5m, no looping and minimize right-angle bends.

Mistake 3: Mismatched current rating of backup protection

Hazard 1: Excessively large fuse/breaker: SPD short-circuit fault current cannot blow the fuse or trigger the breaker, and continuous arcs ignite the cabinet.Hazard 2: Undersized backup device: Normal thunderstorm impulse causes accidental tripping, and the whole system loses surge protection during lightning weather.Matching Reference: CRESIN CSMS-B40 PV DC SPD supports a maximum 125A gG/gL backup fuse; the corresponding CSCB model shall be selected based on the SPD maximum discharge current (40kA SPD matches CSCB-S40S, 80kA SPD matches CSCB-S80S).

Mistake 4: One set of backup protection shared by multiple parallel SPD modules

Risk: If a single SPD breaks down, the shared backup cannot isolate the independent faulty branch; short-circuit current spreads to all parallel SPD loops, triggering full cabinet power failure.Standard Rule: Each independent SPD unit or single SPD pole must be equipped with a dedicated CSCB backup protector; sharing one SCPD for multiple SPD groups is prohibited.

Global Market Application & CRESIN Supporting Technical Services

CRESIN supplies SPDs, CSCB backup protectors and complete surge protection solutions for global distributors, solar EPC contractors, panel manufacturers and OEM customers, covering Europe, Southeast Asia, Latin America, Africa and the Middle East markets.

Application Scenarios of CSCB Series

DC Power Systems:For 600V/1000V/1200V/1500V DC distribution systems, matched with CSMS-B40 DC SPDs

Low-voltage AC Distribution:Applicable to main distribution cabinets, sub-panels, and commercial rooftop power systems, matched with CSMS-A15 Class I AC SPDs and CSMS-B40 Class II AC SPDs.

Integrated surge protection solutions for industrial automation, telecom cabinets and energy storage systems.

If you need complete CSCB technical datasheets, installation specifications, product catalogs, and other related documents, please visit our Product Catalog page to access all editable files.

Frequently Asked Questions (FAQ) About SPD Backup Protection

Why does an SPD need backup protection?

An SPD is designed to discharge lightning surge currents and limit transient overvoltage, but it cannot interrupt continuous power-frequency short-circuit current. When an SPD fails due to thermal aging or internal short circuit, a dedicated backup protection device is required to quickly isolate the faulty SPD, prevent arc and fire risks, and maintain system safety.

Can ordinary fuses or circuit breakers replace an SPD backup protection device?

No. Ordinary fuses and circuit breakers are not specifically designed for SPD protection coordination. They may cause false tripping during lightning surges, increase residual voltage, or provide insufficient arc suppression during SPD short-circuit failures. A dedicated SPD backup protection device can accurately distinguish between surge currents and dangerous follow currents.

What is the difference between an SPD backup fuse and an SPD circuit breaker?

An SPD backup fuse cuts off fault current by melting the fuse element. It provides reliable short-circuit protection but requires replacement after operation. An SPD circuit breaker can be reset after tripping, but ordinary circuit breakers may affect SPD protection performance due to internal inductance. A dedicated SCB backup protector provides better coordination with SPD systems.

How does CRESIN CSCB provide SPD backup protection?

CRESIN CSCB adopts a selective thermal-magnetic tripping technology. During normal lightning surge discharge, CSCB remains conductive without unwanted tripping. When an SPD generates continuous follow current due to aging or short circuit, CSCB quickly disconnects the faulty circuit within milliseconds, suppressing arcs and improving system safety.

Where should an SPD backup protection device be installed?

The SPD backup protection device should be installed in series on the upstream power side of the SPD, between the main busbar and the SPD input terminal. Incorrect installation behind the SPD or on the PE side may prevent effective fault isolation and increase equipment damage risks.

How to select the correct CSCB model for an SPD?

The CSCB model should be selected according to the SPD type, protection class, maximum discharge current and system voltage requirements. CRESIN provides various CSCB models compatible with Class I and Class II SPDs, supporting applications such as PV DC systems, low-voltage AC distribution and industrial control systems.

Conclusion

Ordinary fuses and miniature circuit breakers cannot meet the selective coordination requirements of SPD backup protection under complex PV and industrial power environments, with obvious defects such as false tripping, elevated residual voltage and insufficient arc suppression performance.

As shown in the CSCB technical parameter table, CRESIN CSCB is a dedicated SPD backup protector designed for lightning protection. It provides selective isolation between surge currents and power-frequency faults, preventing fire risks caused by SPD failure.

For PV systems, distribution upgrades and panel manufacturing, CSCB provides a safe, standardized and cost-effective SPD protection solution compliant with IEC and domestic standards.

For project matching, sample testing or customized solutions, please visit our Contact Us page. Our engineers provide SPD & CSCB selection guidance, wiring support and technical assistance. We also welcome distributor and OEM/ODM cooperation.