Type 1 SPD Technology: Carbon Spark Gap vs MOV — How They Work & When to Use Each
Quick Answer: Carbon Spark Gap vs MOV in Type 1 SPDs
For Type 1 SPDs at the service entrance, carbon spark gap is the industry standard. It switches and diverts lightning energy to earth — it does not absorb it. MOV clamps and absorbs. At 10/350 µs energy levels (~200× greater than Type 2), MOVs degrade progressively. Carbon spark gaps don't — and they actively extinguish follow-on current from 230/400 V mains.
Key Takeaways
- Carbon spark gap = diverts: Energy goes to earth via arc discharge — device does not heat up with each lightning event.
- MOV = absorbs: Energy converts to heat in ZnO grain structure — degrades with every 10/350 µs event.
- Follow-on current: Carbon spark gap extinguishes it actively (up to 100 kArms). MOV self-extinguishes — but risks thermal runaway under TOV as it ages.
- Type 2 still uses MOV: At distribution panel level, MOV is correct — surge energy is lower and tight clamping (Up ≤ 1.5 kV) is needed.
Most engineers specifying Type 1 surge protective devices focus on IEC 61643-11 parameters — Iimp, Uc, Up — without examining the internal technology. But the choice between carbon spark gap and MOV-based Type 1 SPDs affects performance in ways those headline numbers don't capture: residual voltage behaviour, follow-on current management, leakage current, and long-term degradation under repeated 10/350 µs events.
Why Type 1 SPDs Face a Different Problem than Type 2
At the service entrance (LPZ 0→1), a direct or nearby lightning strike couples into the installation as a 10/350 µs impulse current. This waveform lasts 350 µs — 17× longer than the 8/20 µs Type 2 waveform. The specific energy (W/R, in J/Ω) is approximately 200× greater than an equivalent-peak 8/20 µs surge.
A Type 2 MOV-based SPD at the service entrance would absorb this energy directly — and fail. The Type 1 device must handle this energy without being destroyed. This is the core design challenge, and why carbon spark gap and MOV perform so differently at the Type 1 position.
Carbon/Graphite Spark Gap: How It Works in Type 1 SPDs
A carbon or graphite spark gap uses carbon-based electrodes separated by a precisely controlled gap, housed inside an arc-chopping chamber. When a lightning transient exceeds the spark-over voltage, an arc forms — then the chamber geometry takes over.
The Arc-Chopping Chamber: How Follow-On Current Is Extinguished
- Arc elongation: The chamber geometry forces the arc to move along the electrode surfaces, physically lengthening it and increasing arc voltage.
- Arc splitting: In multi-carbon designs (Phoenix Contact triggered multi-carbon), the arc splits into multiple parallel short arcs — combined arc voltage exceeds a single two-electrode gap.
- Counter-voltage build-up: Total arc voltage rises above mains supply voltage — creating a counter-voltage that extinguishes follow-on current within microseconds.
DEHN's RADAX Flow technology and Phoenix Contact's triggered multi-carbon spark gap both achieve follow-on current extinction up to 100 kArms — even in high prospective short-circuit current installations, the spark gap extinguishes before the upstream fuse blows.
Why this matters for system availability: If a Type 1 SPD cannot extinguish follow-on current quickly, the upstream fuse or MCB trips after every lightning event — taking the installation offline. Carbon spark gap technology with active extinction keeps the system running after a lightning event, with only the SPD status indicator flagging inspection needed.
Carbon Spark Gap Performance Characteristics
- Zero leakage current in standby — no metering issues for pre-meter installation, no RCD nuisance tripping in IT systems
- No cumulative degradation from 10/350 µs events — carbon gaps divert energy to earth rather than absorbing it; repeated lightning surges do not progressively degrade spark-over voltage
- Lower residual voltage than MOV-based Type 1 — ~20 V arc voltage during conduction reduces energy stress on downstream Type 2 devices
- TOV immunity — spark-over voltage well above mains TOV levels; no thermal runaway risk from loss-of-neutral or ground fault events
MOV-Based Type 1 SPDs: How They Work and Their Limitations
MOV-based Type 1 SPDs use large-diameter zinc oxide varistor discs — often stacked or paralleled — to provide the required Iimp rating. The MOV clamps and absorbs surge energy simultaneously. When the surge passes, it returns to high impedance — no follow-on current, because the voltage-dependent resistance re-establishes before mains voltage can sustain conduction.
This self-extinguishing behaviour is the MOV's key advantage at Type 2. At the Type 1 position, however, three problems arise:
1. Cumulative thermal degradation. The 10/350 µs waveform deposits ~200× more energy per event than 8/20 µs at the same peak current. Every event degrades the MOV's ZnO grain structure. Over multiple lightning seasons, clamping voltage drifts and surge handling capacity decreases. A carbon spark gap does not absorb this energy.
2. Leakage current and TOV thermal runaway risk. MOVs have measurable leakage current at normal operating voltage — microamperes at room temperature, increasing with temperature and aging. Under TOV events (loss of neutral raising phase voltage), an aging MOV can enter thermal runaway — a failure mode that can cause fire. Carbon spark gaps have zero leakage current and are immune.
3. Higher residual voltage at lightning current levels. The MOV's clamping voltage increases steeply with surge current. At 25 kA (10/350 µs), residual voltage is significantly higher than the test-condition Up value. A carbon spark gap maintains ~20 V arc voltage regardless of current magnitude.
MOV thermal runaway at Type 1 position: Under sustained TOV — such as loss-of-neutral raising phase voltage to 400 V on a 230 V system — aging MOVs with increased leakage current are the primary cause of Type 1 SPD thermal runaway fires. Carbon spark gaps are not susceptible to this failure mode.
Head-to-Head: Carbon Spark Gap vs MOV in Type 1 SPDs
| Parameter | Carbon/Graphite Spark Gap | MOV Array (Type 1) |
|---|---|---|
| Operating mechanism | Switching — arc discharge to earth via carbon electrode chamber | Clamping — voltage-dependent resistance absorption |
| Energy handling | Diverts to earth — device does not absorb energy | Absorbs as heat in ZnO grain structure |
| Follow-on current | Actively extinguished — up to 100 kArms | Self-extinguishing — no active extinction mechanism |
| Leakage current | Zero — purely resistive gap when non-conducting | Small but present (µA range); increases with aging |
| TOV withstand | Excellent — no thermal runaway risk | Risk of thermal runaway under sustained TOV as device ages |
| Degradation under 10/350 µs events | Minimal — electrode erosion only; spark-over voltage stable | Progressive ZnO grain degradation; clamping voltage drifts |
| Residual voltage at high current | ~20 V arc voltage — stable regardless of surge current | Up increases steeply with current — higher residual at 10/350 µs levels |
| Pre-meter installation | Permitted — zero leakage, no metering or RCD issues | Problematic — leakage current can affect metering or trip RCDs |
| Typical Iimp ratings | 12.5–100 kA (10/350 µs) per pole | 12.5–25 kA (10/350 µs) per pole |
| Key manufacturers | Phoenix Contact (FLASHTRAB), DEHN (DEHNventil), TrilPeak | Various — common in lower-cost Type 1+2 combined devices |
Need IEC-Certified Type 1 SPDs with Carbon Spark Gap Technology?
TrilPeak Type 1 and Type 1+2 combined SPDs — IEC 61643-11 Class I certified, Iimp 12.5–50 kA, DIN rail, factory-direct.
When to Specify Each Technology
Specify Carbon Spark Gap When:
- Building has an external LPS — Type 1 SPD is mandatory per IEC 60364-5-53; lightning coupling events are expected
- High lightning exposure — exposed structures, telecom towers, industrial facilities, wind turbines, high-Ng regions
- System availability is critical — active follow-on current extinction prevents upstream fuse trips after lightning events
- Pre-meter installation or IT earthing system — zero leakage current is required
- Long service life required — installations where SPD replacement is difficult or costly
MOV-Based Type 1 May Be Appropriate When:
- Low lightning exposure — buildings in low-Ng areas where Type 1 activation is a rare event
- Space-saving Type 1+2 solution — compact combined device where space is limited and lightning risk is moderate
- Cost-sensitive applications — where the incremental cost of carbon spark gap is not justified by the risk profile
Most common specification error: Installing an MOV-based "Type 1+2 combined SPD" at the service entrance of a building with an external LPS, without verifying actual energy handling under the 10/350 µs waveform. Many products labelled "Type 1+2" only meet the minimum Iimp ≥ 12.5 kA at test conditions — not across repeated real-world events. Carbon spark gap Type 1 SPDs typically offer Iimp 25–100 kA per pole with stable long-term performance.
TrilPeak Type 1 SPDs: Carbon Spark Gap Technology
TrilPeak's Type 1 and Type 1+2 combined SPD range uses carbon spark gap technology in the Type 1 stage — consistent with the approach of DEHN and Phoenix Contact. The carbon spark gap module handles the 10/350 µs lightning current impulse by diverting it to earth, while the coordinated downstream MOV stage provides tight clamping voltage (Up ≤ 1.5 kV) for equipment protection.
Key specifications of TrilPeak's Type 1 SPD range:
- Iimp: 12.5–50 kA (10/350 µs) per pole — IEC 61643-11 Class I certified
- DIN rail mounting with pluggable cartridge for fast module replacement
- Status indicator: green (operational) / red (requires replacement)
- Single-phase and three-phase for TN-S, TT, TN-C, and IT earthing systems
For applications requiring Type 1 + Type 2 in a single device, TrilPeak's Type 1+2 combined SPD integrates the carbon spark gap Type 1 module with an MOV Type 2 module — eliminating the 10 m cable coordination distance required when the two types are installed separately per IEC 61643-12.
Frequently Asked Questions — Type 1 SPD Technology
A sealed GDT — used in signal line SPDs for RS485, Ethernet, and telecom — cannot serve as Type 1 element for two reasons. First, the 10/350 µs lightning current waveform carries ~200× more energy than the 8/20 µs waveform the GDT is rated for — this would destroy the sealed ceramic housing. Second, once a GDT fires on a 230/400 V AC power line, the mains supply drives large follow-on current through the GDT's ~20 V arc state — and the sealed GDT has no arc-chopping mechanism to extinguish this current. The carbon spark gap solves both problems: it handles high 10/350 µs energies by diverting rather than absorbing, and its arcing chamber geometry actively extinguishes follow-on current.
Follow-on current is the mains power-system current that continues flowing through an SPD after the surge has passed, driven by AC mains voltage rather than the surge itself. When a Type 1 SPD fires, it enters a low-impedance arc state. If it cannot extinguish the arc quickly, the 230/400 V mains drives current through this path — potentially exceeding several kiloamperes, sufficient to blow the upstream fuse or cause fire. Carbon spark gap SPDs with arc-chopping technology (DEHN RADAX Flow, Phoenix Contact multi-carbon) extinguish follow-on current within microseconds, keeping upstream fuses intact and restoring normal operation without intervention.
No. A Type 1 carbon spark gap SPD alone is not sufficient for equipment protection. The carbon spark gap's ~20 V arc voltage means its protection level (Up) is primarily determined by lead inductance — typically 2–4 kV at the SPD terminals. Most equipment requires Up ≤ 1.5 kV (IEC 60664 Category II). A downstream Type 2 MOV-based SPD — with ≥ 10 m cable separation, or using a combined Type 1+2 device — brings residual voltage down to equipment-safe levels. Type 1 handles the high-energy lightning impulse; Type 2 refines the clamping voltage.
An MOV-based Type 1 or Type 1+2 combined SPD may be adequate for buildings in low lightning-density areas (Ng < 1 strike/km²/year) without an external LPS, where the Type 1 SPD is installed as a precaution and is unlikely to activate more than once or twice in its service life. Carbon spark gap is preferred when: (1) the building has an external LPS; (2) the structure is in an exposed location; (3) the installation is pre-meter or in an IT earthing system; (4) system availability after lightning is critical; or (5) Iimp ratings above 25 kA per pole are required.
A Type 1+2 combined SPD passes both Class I (10/350 µs) and Class II (8/20 µs) tests — the internal design may be a single MOV array or a coordinated spark gap + MOV combination.
A Type 1+2 special combined SPD (Phoenix Contact, DEHN terminology) contains two independently designed and certified elements: a full-performance carbon spark gap module (Type 1) and a separately rated MOV module (Type 2), each operating to its own performance standard. This is the highest-performance Type 1+2 architecture — the spark gap handles high-energy lightning with low residual voltage, and the MOV provides fine voltage clamping at lower surge levels.
Conclusion
At the Type 1 service entrance position, the choice between carbon spark gap and MOV reflects a fundamental difference in how each technology handles the 10/350 µs lightning current waveform.
Carbon spark gap is the industry standard for sound engineering reasons: it diverts rather than absorbs lightning energy, actively extinguishes follow-on current, presents zero leakage current, and maintains stable performance across repeated events. MOV-based Type 1 suits low-lightning-exposure applications and compact Type 1+2 formats where lightning risk is moderate.
The Type 2 MOV stage remains correct for its position — downstream clamping, tight Up, self-extinguishing. The two technologies are complementary. Specifying them correctly, based on actual surge environment rather than headline kA numbers, is the mark of a rigorous IEC installation design.
Related Resources
TrilPeak Type 1 SPD Products
- Type 1 SPD Range — carbon spark gap, IEC 61643-11 Class I certified, Iimp 12.5–50 kA, DIN rail
- Type 1+2 Combined SPD — integrated carbon spark gap (Type 1) + MOV (Type 2)
Technical Guides
- Type 1 SPD Selection Guide — Iimp sizing, Uc, earthing system, IEC 60364-5-53
- MOV Surge Protector: How It Works
- Gas Discharge Tube (GDT) Guide
- Type 1 vs Type 2 vs Type 3 SPD Guide
- Circuit Breaker vs Surge Protector
- Lightning Arrester vs Surge Arrester
Specifying Type 1 SPDs for an IEC Installation?
TrilPeak's engineering team can review your earthing system, Iimp requirements, and coordination distance — factory-direct, OEM/ODM available.
Standards Referenced
- International Electrotechnical Commission. (2025). IEC 61643-11:2025 — Low-voltage surge protective devices — Part 11: Surge protective devices connected to AC low-voltage power systems (2nd ed.). IEC. https://webstore.iec.ch/en/publication/65314
- International Electrotechnical Commission. (2020). IEC 61643-12:2020 — Low-voltage surge protective devices — Part 12: Selection and application principles (3rd ed.). IEC. https://webstore.iec.ch/en/publication/32531
- International Electrotechnical Commission. (2013). IEC 60364-5-53:2013+AMD1:2016 — Low-voltage electrical installations — Part 5-53: Selection and erection of electrical equipment. IEC. https://webstore.iec.ch/en/publication/1586
- International Electrotechnical Commission. (2010). IEC 62305-4:2010 — Protection against lightning — Part 4: Electrical and electronic systems within structures (2nd ed.). IEC. https://webstore.iec.ch/en/publication/6847
- International Electrotechnical Commission. (2020). IEC 60664-1:2020 — Insulation coordination for equipment within low-voltage supply systems. IEC. https://webstore.iec.ch/en/publication/63385
