Type 1 SPD Technology: Carbon Spark Gap vs MOV — How They Work & When to Use Each

Choosing between Type 1 SPD spark gap vs MOV technology is one of the most consequential decisions in IEC 61643-11 SPD specification, yet most engineers focus only on Iimp, Uc, and Up without examining the internal technology. The choice 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.

Quick Answer

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. A Metal Oxide Varistor (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.

• 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 kA_rms). 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.

1. Why Type 1 SPDs Face a Different Problem than Type 2

At the service entrance, Type 1 SPDs must survive roughly 200 times more energy per event than Type 2 devices, which is why the two positions call for different internal technology. At the service entrance (Lightning Protection Zone 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 the Type 1 SPD spark gap vs MOV comparison matters so much more at this position than it would further downstream.

2. Carbon/Graphite Spark Gap: How It Works in Type 1 SPDs

A carbon or graphite spark gap handles the 10/350 µs lightning impulse by switching and diverting it to earth rather than absorbing it. It 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.

2.1 The Arc-Chopping Chamber: How Follow-On Current Is Extinguished

1. Arc elongation: The chamber geometry forces the arc to move along the electrode surfaces, physically lengthening it and increasing arc voltage.
2. 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.
3. 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 kA_rms — even in high prospective short-circuit current installations, the spark gap extinguishes before the upstream fuse blows.

Note: 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.

2.2 Carbon Spark Gap Performance Characteristics

• Zero leakage current in standby — no metering issues for pre-meter installation, no Residual Current Device (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
• Temporary Overvoltage (TOV) immunity — spark-over voltage well above mains TOV levels; no thermal runaway risk from loss-of-neutral or ground fault events

3. MOV-Based Type 1 SPDs: How They Work and Their Limitations

MOV-based Type 1 SPDs clamp and absorb surge energy rather than diverting it, which creates three specific problems at the Type 1 position that don't apply at Type 2. They 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.

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.

4. Type 1 SPD Spark Gap vs MOV: Head-to-Head Comparison

Nine parameters separate carbon spark gap from MOV performance at the Type 1 position, and the pattern is consistent: spark gap wins on longevity and safety margin, MOV wins on cost and compactness.

5. When to Specify Each Technology

The specification decision comes down to lightning exposure, availability requirements, and earthing system — not just budget.

5.1 Specify Carbon Spark Gap When:

• Building has an external Lightning Protection System (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

5.2 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

Note — 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.

6. 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.

7. Conclusion

At the Type 1 service entrance position, the Type 1 SPD spark gap vs MOV choice 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.