GuideSwitchgear and motor control10 min read
Vacuum circuit breakers
A vacuum circuit breaker opens its contacts inside a sealed vacuum interrupter, a ceramic bottle with nothing in it to sustain an arc. The arc that forms when the contacts part burns in metal vapor from the contacts themselves, and at the first current zero the vapor condenses and the gap recovers its strength within microseconds. That makes the vacuum breaker the standard medium-voltage breaker from 5 to 38 kV: no oil, no gas, no arc chutes to maintain, tens of thousands of operations, and a bottle that lasts the breaker’s life. Nearly every metal-clad lineup built or retrofitted since the 1990s uses them.
The short version
- One vacuum interrupter per phase; copper-chromium contacts, a gap of 8 to 20 mm, a bellows for the moving stem; sealed for life.
- Ratings to read: maximum voltage (4.76, 8.25, 15, 27, 38 kV), continuous current (1,200 to 4,000 A), interrupting current (25 to 63 kA), close-and-latch, BIL, interrupting time (3 or 5 cycles), control voltage.
- A stored-energy spring mechanism closes and trips it; a motor recharges the spring; the breaker draws out of its cell on a racking mechanism.
- Vacuum breakers can chop small inductive currents and cause switching surges; motors and dry-type transformers on them get surge arresters or snubbers.
- The tests: insulation resistance, a vacuum-integrity hi-pot across the open contacts, contact resistance, timing, trip and close coil checks, and contact-erosion measurement.
- Used breakers must match the cell exactly, by maker, type, frame and ratings; roll-in vacuum replacements exist for most old air-magnetic cells.
How a vacuum interrupter breaks current
The interrupter is a ceramic cylinder, sealed at both ends with metal caps, pumped to a hard vacuum and sealed for life. Inside are two contacts, one fixed and one on a stem that moves through a metal bellows, and a metal shield around them that catches the vapor. The contacts are a copper-chromium alloy chosen to erode slowly and to chop little current.
When the contacts part under load, the current doesn’t stop; the last point of contact melts, metal vapor fills the gap, and the current carries on as an arc in that vapor. In a vacuum the arc is diffuse and cool at the hundreds or thousands of amps of a load current, and even a fault arc of tens of thousands of amps is confined and driven across the contact face by the contact’s own magnetic field, so it doesn’t burn a hole. The arc lasts until the alternating current passes through zero, within half a cycle. At that moment the vapor condenses onto the contacts and the shield in a few microseconds, there is nothing left between the contacts to conduct, and the gap of a centimeter or so withstands the voltage that rises across it. The current is off.
The same action in air needs arc chutes a foot long and a blast of magnetic force to stretch and cool the arc; in oil it needs gallons of oil that carbonize with every operation; in SF6 it needs the gas. The vacuum bottle does it in a sealed package the size of a coffee can, which is why the vacuum breaker took over medium-voltage switchgear.
The ratings on the nameplate
| Rating | Typical values | What it means |
|---|---|---|
| Rated maximum voltage | 4.76, 8.25, 15, 15.5, 27, 38 kV | The highest system voltage it may be used on; 15 kV covers 12.47 and 13.8 kV systems |
| Rated continuous current | 1,200, 2,000, 3,000, 4,000 A | What it carries continuously without overheating |
| Rated short-circuit current | 25, 31.5, 40, 50, 63 kA | The symmetrical fault current it can interrupt |
| Close-and-latch (peak) | 2.6 × short-circuit current, peak | The fault it can close onto and hold without the contacts blowing open |
| Short-time current | Short-circuit current for 2 s (3 s on some) | What it carries while a downstream device clears |
| BIL | 60 kV at 5 kV class; 95 kV at 15 kV; 125 kV at 27 kV; 150 kV at 38 kV | Impulse withstand |
| Interrupting time | 3 cycles (50 ms) or 5 cycles | From trip signal to arc extinction |
| Mechanical endurance | M1: 10,000 operations; M2: 30,000 | Operations before major overhaul |
| Control voltage | 48, 125 or 250 V DC; 120 or 240 V AC | For the trip and close coils and the spring-charging motor; must match the switchgear |
| Type and frame | e.g. 150 VCP-W 500, 1200 A | The maker’s type: 15 kV class, VCP-W family, 500 MVA interrupting class (≈ 20 kA at 15 kV) |
Older nameplates give an interrupting rating in MVA at a rated voltage (250, 350, 500, 750, 1,000 MVA); divide by 1.732 × voltage for the kA. A 500 MVA breaker at 13.8 kV is about 21 kA; at 4.16 kV the same frame may be rated higher in kA. Our impedance guide gives the fault current the breaker must beat.
Mechanism, cell and controls
- Stored-energy mechanism
- A closing spring, charged by a motor (or a hand crank), closes the contacts in a few tens of milliseconds and charges the opening spring as it does; the trip coil releases the opening spring. Close and trip are independent of the control supply once charged, which the standards require.
- Coils and auxiliaries
- A close coil, a trip coil (often two, for redundancy), an anti-pump relay so a sustained close signal doesn’t cause repeated closing, auxiliary switches that report the position, and a spring-charged switch.
- Draw-out carriage
- In metal-clad switchgear the breaker rolls into its cell on a carriage with primary stabs at the back and a secondary disconnect for the control wiring, through connected, test and disconnected positions, with interlocks that stop racking while closed.
- Interlocks
- Mechanical: no racking with the breaker closed, no closing between positions. Key interlocks for sequences with other equipment (the Kirk key guide).
- Counter and indicators
- An operations counter, open/closed flags, and a spring-charged indicator; on used breakers the counter is the odometer.
Makers, types and roll-in replacements
| Maker and type | Class | Notes |
|---|---|---|
| Eaton (Cutler-Hammer, Westinghouse) VCP-W | 5 to 15 kV | The most common breaker in North American metal-clad gear since the 1980s; 50 VCP-W, 150 VCP-W; VCP-WR roll-in versions for DHP and other old cells |
| ABB ADVAC, AMVAC, VD4; GE PowerVac (now ABB) | 5 to 38 kV | ADVAC in ABB and GE cells; AMVAC with a magnetic actuator; PowerVac in GE lineups |
| Siemens GMSG (3AH3 interrupters) | 5 to 15 kV | In Siemens GM-SG switchgear; earlier GMI |
| Schneider (Square D) Evolis, HVX, VR | 5 to 38 kV | In Masterclad and MVS gear |
| Powell PowlVac | 5 to 38 kV | In Powell switchgear; also sold as retrofits |
| Retrofit specialists | Any | Roll-in vacuum replacements for GE Magneblast, Westinghouse DHP, Allis-Chalmers, Federal Pacific and ITE air-magnetic cells, with the same interlocks and stabs |
The lineup decides the breaker: a cell accepts only the breaker type, frame and ratings it was built for. That makes a spare breaker for an existing lineup a specific thing to search for, and the breaker listings are written by type for that reason.
Current chopping and switching surges
A vacuum interrupter is so good at interrupting that it can stop a small current before its natural zero, “chopping” it. The energy in an inductive load (a lightly loaded motor, an unloaded transformer) then has nowhere to go but into the stray capacitance, and the voltage spikes. Opening a motor while it still turns can also produce several re-ignitions across the gap, each one a steep-fronted surge that stresses the first turns of the motor’s winding.
The remedies are standard: surge arresters at the motor or transformer terminals, or RC snubbers (resistor-capacitor networks) that slow the voltage rise; and modern contact alloys with chopping currents below about 3 A, which have made the problem small on new breakers. On a used lineup feeding motors or dry-type transformers, check that the arresters or snubbers are there; on a retrofit of an air-magnetic cell to vacuum, add them.
Testing a vacuum circuit breaker
With the breaker withdrawn, cleaned and inspected (cracked interrupters, loose hardware, worn contact fingers on the stabs), the test set is:
Insulation resistance
Phase to phase, phase to ground and across each open interrupter, at 2,500 or 5,000 V DC, against NETA’s minimums for the voltage class.
Vacuum integrity
An AC high-potential test across each open interrupter at the maker’s voltage, typically 27 to 36 kV for a minute on a 15 kV breaker. A bottle that has lost vacuum flashes over at a fraction of that. A magnetron-type vacuum tester does the same job without high voltage on some models.
Contact resistance
With a 100 A micro-ohmmeter across each closed pole: tens of micro-ohms, and within about 50% between phases. A high reading means eroded contacts or a bad joint in the pole.
Timing and travel
A breaker analyzer records contact parting and closing times, pole spread and stem travel; compared with the maker’s figures (opening in about 30 to 45 ms, closing in about 50 to 80 ms, with the three poles within a millisecond or two).
Coils and controls
Minimum pickup voltage of the trip and close coils, the anti-pump function, the auxiliary switches, the charging motor’s current and time, and the counter.
Contact erosion
The erosion indicator on each moving stem (a mark or a gap measured with a gauge) against the limit; the bottle is replaced when it’s reached.
In the cell
Racking through all positions, the shutters, the interlocks, the secondary disconnect, and a primary-injection or secondary-injection test of the relays that trip it.
That is the set our breakers and switchgear get before they ship: contact resistance, insulation resistance, timing and operation, and the vacuum integrity test, written up for the buyer. Our switchgear guide covers the rest of the lineup.
Buying a used vacuum breaker
- Match the cell. Maker, type, frame (continuous current), interrupting rating, voltage class, control voltage, and the exact cell design: a 150 VCP-W 500 1200 A isn’t a 150 VCP-W 750 2000 A, and a breaker for a 1980s cell may not fit a 2000s cell of the same family without conversion. The cell nameplate and the breaker nameplate must agree.
- Read the counter. A few thousand operations is nothing; tens of thousands means the mechanism needs overhaul and the bottles need the erosion check.
- Ask what “reconditioned” means. Cleaned and tested; or cleaned, re-lubricated, coils and auxiliaries replaced, tested; or that with new pole assemblies. The price should follow.
- Get the test report with the vacuum integrity result and timing, dated. A breaker that passes the hi-pot across its open contacts has sound bottles, which is nine-tenths of its value.
- Consider a roll-in for an obsolete cell: a new vacuum breaker built to fit a Magneblast or DHP cell often costs less than a reconditioned original, and the gear keeps its bus.
Questions people ask
How does a vacuum circuit breaker work?
Its contacts sit inside a sealed vacuum interrupter. When they open under load, an arc forms in metal vapor boiled off the contact surfaces, because there is no air to carry it. At the next natural current zero, a few milliseconds later, the vapor condenses onto the contacts and shields, the gap becomes an insulator again almost instantly, and the current stays off. The contact gap of a few millimeters to twenty is enough to withstand the recovery voltage at 5 to 38 kV.
What is the difference between a vacuum circuit breaker and an SF6 breaker?
The medium the arc burns in. A vacuum breaker interrupts in a sealed vacuum bottle and dominates indoor switchgear from 5 to 38 kV. An SF6 breaker interrupts in sulfur hexafluoride gas, which handles higher voltages and outdoor duty (38 kV and up, through transmission voltages), but the gas is a potent greenhouse gas and needs monitoring. Below 38 kV vacuum has displaced SF6, oil and air-magnetic breakers almost entirely.
How long does a vacuum circuit breaker last?
The interrupters are rated for 10,000 to 30,000 mechanical operations (class M1 or M2) and for a number of full-fault interruptions set by contact erosion, typically 30 to 100 at rated short-circuit current. In ordinary service, switching load a few times a day, a breaker and its bottles last 30 to 40 years; the mechanism gets lubrication and the coils and auxiliary switches get replaced along the way. A bottle that has reached its erosion limit or lost vacuum is replaced as a pole assembly.
How do you test a vacuum circuit breaker?
With the breaker withdrawn from its cell: insulation resistance phase to phase, phase to ground and across the open contacts; an AC hi-pot across each open interrupter at the maker’s voltage (on the order of 27 to 36 kV for a minute on a 15 kV breaker), which proves the vacuum is intact; contact resistance with a 100 A micro-ohmmeter, matched between phases; timing of opening and closing with a breaker analyzer; the minimum voltage at which the trip and close coils operate; and the contact-erosion indicator against the maker’s limit. NETA ATS section 7.6 gives the procedure and the acceptance values.
What is a vacuum breaker retrofit or roll-in replacement?
A new vacuum breaker built on a carriage that fits an old switchgear cell designed for an air-magnetic breaker (GE Magneblast, Westinghouse DHP, Allis-Chalmers, Federal Pacific, ITE), with the same stabs, interlocks and secondary connections, so the lineup keeps its bus and cells and gets a modern breaker with no cell modification. Eaton’s VCP-WR, ABB’s AMVAC retrofits and several specialists make them for most vintage cells.
Can a vacuum circuit breaker be used on a motor?
Yes, and it is, in medium-voltage motor starters and switchgear, with one precaution: a vacuum interrupter can chop the small magnetizing current of a motor before its natural zero, producing a voltage spike, and can re-ignite several times while opening a motor that is still spinning. A surge arrester or an RC snubber at the motor terminals, standard practice since the 1980s, absorbs it; modern breakers with low-chop contact alloys make it a smaller problem.
Sources: IEEE C37.04, C37.06 and C37.09 (ratings and testing of AC high-voltage circuit breakers); IEEE C37.20.2 (metal-clad switchgear); ANSI/NETA ATS section 7.6 (medium-voltage breakers); makers’ instruction books (Eaton VCP-W, ABB ADVAC and VD4, Siemens GMSG, Schneider Evolis).