1. First Things First: What Is "Arc Quenching"?
When a circuit breaker opens, the moving contact and the fixed contact separate. At the instant of separation, the electric field strength across the gap rises sharply. The metallic bridge that was conducting current is pulled apart, the insulating medium between the contacts breaks down, and a sustained discharge forms — this is the electric arc.
The temperature of an arc can reach several thousand degrees Celsius. It continues to conduct current, meaning the breaker "looks open but the circuit is still live." If the arc is not extinguished in time, the consequences include:
1Contact Erosion
Prolonged arcing burns the contact surface, causing material evaporation and wear that directly shortens electrical life.
2Insulation Damage
High-temperature arcs decompose the surrounding medium, producing conductive gases and metal vapor that reduce insulation strength.
3Interruption Failure
Arc restriking causes interruption failure — short-circuit current keeps flowing and the fault spreads.
4Internal Arc Fault
Pressure inside the enclosure rises sharply, potentially triggering an internal arc fault that endangers personnel and equipment.
That is why arc-quenching capability is the number-one hard metric for a vacuum circuit breaker. And the choice of arc-quenching medium sets the ceiling for how efficiently that arc can be extinguished.
2. The Core Question: Why Can Vacuum Quench an Arc?
The answer can be condensed into one sentence: in a vacuum there is "nothing" to burn, so the arc has no fuel to sustain itself.
Breaking this logic down reveals three key mechanisms:
1. Vacuum Has Extremely High Dielectric Strength
At standard atmospheric pressure, the breakdown field strength of air is roughly 3 kV/mm. In a high-vacuum environment (typically below 10⁻⁴ Pa), there are almost no gas molecules available for impact ionization, so the dielectric strength across the contact gap can be several times higher — or more. This means: at the same voltage level, a vacuum circuit breaker needs only a very small contact gap to withstand the recovery voltage without restriking.
2. There Is No Ionizable Medium in a Vacuum
Air, SF₆ and oil all extinguish arcs by being decomposed, cooled and recombined by the arc — essentially "consuming themselves" to carry away heat. A vacuum breaker is different: with almost no gas molecules present, the arc can only be sustained by metal vapor evaporating from the contact material.
When the AC current crosses zero, the supply of metal vapor is cut off instantly. Meanwhile, the contact structure inside the vacuum interrupter (typically spiral-groove or cup-shaped with a longitudinal magnetic field) forces the arc to rotate at high speed under the magnetic field and spread evenly across the contact surface, preventing localized overheating. After current zero, the plasma rapidly diffuses and recombines, and the contact gap regains its dielectric strength within an extremely short time — the arc is extinguished and does not restrike.
3. Fast Quenching, Low Energy, Long Life
Because arc voltage in a vacuum is low and arcing time is short (typically about half a cycle), the total arc energy is greatly reduced. This is the fundamental reason why vacuum circuit breakers achieve long electrical life, minimal contact wear and low maintenance requirements.
3. Vacuum vs. Air vs. SF₆ vs. Oil: Arc-Quenching Media Compared
Once you understand the principle, it becomes easy to see why vacuum arc quenching has become the mainstream in medium voltage. The table below compares four common arc-quenching media:
| Parameter | Vacuum | Compressed Air | SF₆ Gas | Insulating Oil |
|---|---|---|---|---|
| Quenching Principle | No medium to ionize; natural extinction after current zero | High-pressure airflow blows and cools the arc | Electronegative gas absorbs electrons; powerful quenching | Oil/gas decomposition cooling; bubble quenching |
| Voltage Range | Mainly MV (12–40.5kV) | MV and HV | MV, HV and EHV | LV and legacy MV |
| Electrical Life | Long; many interrupting operations | Moderate | Relatively long | Short; requires oil changes |
| Maintenance | Very low; can be maintenance-free | Higher; requires air supply system | Pressure monitoring and gas recovery required | High; periodic oil replacement |
| Environmental Impact | No greenhouse gas emissions | Clean | High GWP; increasingly regulated | Oil leakage and contamination risk |
| Footprint | Small; enables compact switchgear | Large | Medium; requires gas compartments | Large |
These advantages have made vacuum arc quenching the de facto standard in medium-voltage distribution — and have made the Voltage Fixed Indoor Vacuum Circuit Breaker a workhorse in industrial and mining facilities, commercial buildings and renewable energy stations.
4. SanGao's Answer: Fixed Sealed Poles (Embedded Poles)
A good arc-quenching principle still needs a good structure to realize it. Zhejiang SanGao Electric Co., Ltd. has long focused on the research and manufacturing of the Voltage Fixed Indoor Vacuum Circuit Breaker. We cast the vacuum interrupter and the conductive circuit integrally into epoxy resin, forming a fixed sealed pole structure.
What Is a "Fixed Sealed Pole"?
A fixed sealed circuit breaker is short for "fixed sealed pole type circuit breaker." It is still fundamentally a vacuum circuit breaker; what differs from an ordinary vacuum circuit breaker is first the encapsulation material and structure. When you go deeper into the technical differences, the two also differ significantly in insulation method, reliability and maintenance strategy.
Simply put: the original vacuum interrupter is encapsulated in epoxy resin, so that the previously exposed live parts are "hidden" inside solid insulation.
From "Surface Insulation" to "Volume Insulation"
Conventional assembled poles rely on air for external insulation, so insulation performance is heavily affected by ambient humidity, dust and condensation. With an embedded pole, because the vacuum interrupter is embedded in epoxy resin insulation material, the original surface insulation is transformed into volume insulation. No further treatment is required, and high insulation strength is achieved.
This brings two direct benefits: first, it favors the miniaturization design of circuit breakers and switchgear; second, the vacuum interrupter is effectively protected by the resin from impact, collision, dust and other external environmental influences — it can operate reliably even in environments with severe condensation.
Simpler Structure, Higher Reliability
Compared with traditional assembled poles, fixed sealed poles have significantly fewer parts, a simpler structure and easier installation. This simplifies the manufacturing process of the circuit breaker while greatly improving reliability and stability — in electrical equipment, fewer parts usually means fewer failure points.
Maintenance-Free Over the Entire Life Cycle
The vacuum interrupter and related conductive components are simultaneously embedded in epoxy resin solid insulation material, shielding them from external environmental influences. The product is completely maintenance-free over its entire service life, with strong adaptability and wide application in chemical, metallurgical, mining and other fields.
To be objective, the production cost of embedded poles is relatively high. But encapsulating the low-resistance vacuum interrupter and outlet socket in epoxy resin improves the insulation level and anti-pollution capability, and eliminates the influence of condensation — an investment that pays off in long-term operational reliability.
In response to the actual usage environment of switchgear, adopting the fixed sealed pole form better meets the needs of more users, increases switchgear reliability and improves the power supply reliability of the grid.
5. Core Advantages of SanGao Voltage Fixed Indoor Vacuum Circuit Breaker
Combined with the embedded pole process, the SanGao fixed indoor vacuum circuit breaker offers the following main features:
- Good product interchangeability and long-lasting reliability;
- Complete "five prevention" interlocking design, supporting both forward and reverse interlocking as well as five-prevention interlocking requirements;
- Flexible and convenient mechanical interlocking, with a convenient outgoing connection design for fixed circuit breakers;
- Contact temperature measurement function for real-time monitoring of main circuit temperature rise;
- Service life extended to 30,000 operations, suitable for high-frequency operation scenarios;
- Surface strengthening treatment of components to adapt to harsh environments such as coastal areas;
- Normal operating conditions comply with GB/T 11022 and IEC 60694 — "Common Technical Conditions for High Voltage Switchgear and Control Equipment Standards."
6. Technical Features at a Glance
| Item | Description |
|---|---|
| Product Type | Voltage Fixed Indoor Vacuum Circuit Breaker |
| Pole Type | Fixed sealed pole (embedded pole); vacuum interrupter integrally cast in epoxy resin |
| Insulation Method | Volume insulation; high dielectric strength; pollution and condensation resistant |
| Mechanical Life | Extended to 30,000 operations |
| Interlocking | Complete five-prevention interlocking; supports forward and reverse interlocking |
| Additional Function | Contact temperature measurement |
| Environmental Adaptability | Surface strengthening treatment; suitable for harsh coastal environments; operates normally under condensation |
| Maintenance Strategy | Maintenance-free during product service life |
| Standards | GB/T 11022, IEC 60694 |
| Typical Applications | Chemical, metallurgical, mining, commercial buildings, industrial power distribution |
| Customization | Customizable to meet specific standards and operating conditions |
7. Where Are Fixed Embedded-Pole Breakers the Best Fit?
Thanks to their outstanding insulation performance, pollution resistance and maintenance-free operation, SanGao fixed indoor vacuum circuit breakers are especially suitable for the following scenarios:
◆Chemical & Metallurgical
Heavy dust, corrosive gases and humidity fluctuations — air-insulated poles are easily contaminated.
◆Mining & Coastal Areas
High humidity and salt spray demand higher surface anti-corrosion and insulation performance.
◆Commercial Buildings & Hospitals
Compact spaces and limited maintenance windows require compact, maintenance-free solutions.
◆Data Centers
Extremely high requirements for power continuity — equipment reliability directly affects business continuity.
◆Renewable Energy Stations
Solar, wind and energy storage substations — large temperature differences and condensation risk inside enclosures.
◆Industrial Distribution Rooms
High operating frequency demands long mechanical life and reliable five-prevention interlocking.
8. Frequently Asked Questions
How "vacuum" is the vacuum inside a vacuum circuit breaker?
The vacuum level inside a vacuum interrupter is typically on the order of 10⁻⁴ Pa or lower. This pressure is far below atmospheric pressure, with very few gas molecules, so there is almost no medium available for impact ionization — greatly improving both dielectric strength and arc-quenching capability.
If vacuum quenching is so good, why is SF₆ still used in HV and EHV?
At higher voltage levels, the requirements for contact gap, interrupter size and manufacturing process rise sharply, increasing both cost and technical difficulty. That is why vacuum is the mainstream in medium voltage (12–40.5kV), while SF₆ still has its place in EHV. However, as environmental regulations tighten, vacuum and eco-friendly gas alternatives continue to advance.
Why do embedded poles have higher insulation strength than assembled poles?
The key is the change in insulation method. Assembled poles mainly rely on air as external insulation — "surface insulation." Embedded poles embed the vacuum interrupter and conductive parts in epoxy resin, forming "volume insulation." Solid insulation media have far higher breakdown strength than air and are unaffected by humidity, dust or condensation, making insulation more stable and reliable.
Are embedded poles truly completely maintenance-free?
For the main circuit itself, the vacuum interrupter and conductive parts are sealed inside epoxy resin, isolated from the external environment, so no maintenance of the pole body is required during the product's service life. However, the operating mechanism, secondary circuit and interlocks should still be inspected periodically per regulations to ensure long-term reliable operation of the complete unit.
Fixed type vs. draw-out type — how do I choose?
Fixed type offers simple structure, relatively lower cost and reliable contact — suitable for applications with moderate operating frequency and fixed cabinet space. Draw-out type allows easy withdrawal for maintenance and quick replacement — suitable where maintenance convenience is critical. Neither is absolutely better; the choice depends on the distribution scheme, maintenance strategy and budget. SanGao can provide customized solutions — contact us for an evaluation.
Can you customize to our standards?
Yes. SanGao is a high-tech enterprise specializing in the production of Voltage Fixed Indoor Vacuum Circuit Breakers. We can customize products according to your requirements to meet different standards and operating conditions. Feel free to contact us anytime for pricing.
Need Selection Support or a Quote?
Tell us your rated voltage, rated current, short-circuit breaking current and switchgear type. The SanGao technical team will match the most suitable Voltage Fixed Indoor Vacuum Circuit Breaker solution for you, with full customization support.
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