Three vacuum circuit breakers, three different jobs. The VS1 is an indoor switchgear breaker, the ZW32 is a pole-mounted outdoor unit for 12–24 kV overhead lines, and the ZW7 is a 40.5 kV outdoor breaker for substation feeders. They overlap in one place only — all three interrupt fault current in a vacuum bottle — and choosing between them is decided long before the electrical calculation, by where the breaker has to sit and what it has to protect.
This guide sets out the decision in the order it actually happens: indoor or outdoor, then voltage class, then breaking capacity, then mechanism and mounting detail. It closes with a type-code decoder and the specification fields that prevent a wrong order. If you need the general principles first, the vacuum circuit breaker overview covers the interrupting principle and the switching duty.
The first decision is environmental, and it eliminates two thirds of the options immediately:
The second decision is voltage class. ZW32 tops out around 24 kV; 35 kV systems need ZW7 or its equivalents. Between them they cover the medium-voltage distribution range that most export projects actually buy.
An indoor breaker lives in a controlled environment and can rely on the cubicle for insulation clearance, cable termination and personnel protection. An outdoor breaker must carry all of that itself: its own insulation housing, its own terminal arrangements, its own operating mechanism enclosure, and a design that tolerates rain, dust, UV and condensation.
That difference explains the physical shape of the two families. The VS1 is a rectangular chassis designed to slide into a cubicle, with the primary contacts presented at the rear. The ZW32 is a sealed epoxy pole assembly on a frame, with bushings that accept overhead conductors, because nothing else is going to protect it.
Practical consequences:
| Consideration | Indoor (VS1) | Outdoor (ZW32 / ZW7) |
|---|---|---|
| Insulation coordination | Provided by the cubicle and the breaker together | Entirely the breaker’s responsibility |
| Maintenance access | Through cubicle doors, with the truck withdrawn | Requires a ladder, a platform or a lift |
| Environmental rating | Assumes a dry, ventilated switchroom | Rated for pollution level III and 2,000 m altitude as standard |
| Typical buyer | Building, plant and substation switchgear packages | Utility distribution, rural electrification, overhead line projects |
The VS1 is the most widely copied indoor vacuum breaker pattern in the 12 kV class, and for good reason: it is simple, well understood and supported by a large switchgear ecosystem.
The fixed-versus-withdrawable choice is its own decision and worth separating from the model choice: a withdrawable truck lets you rack the breaker out for maintenance and replace it with a spare in minutes, at the cost of a more complex cubicle and more mechanical parts to maintain.
The current range available in this family is listed on the VS1 series page; the withdrawable variant is described separately under the handcart version.
The ZW32 is the breaker you see on overhead distribution lines: a compact outdoor unit mounted on a pole bracket or a frame, usually with a current transformer set and a controller for automatic reclosing.
The permanent-magnet actuator is the detail worth understanding before ordering. It replaces the spring-charging mechanism with a magnetic holding force and a capacitor-driven trip, which reduces the part count in the mechanism and makes the unit quieter and more consistent in operation. The trade-off is that the actuator needs a reliable DC supply and a controller that manages the capacitor charging — a failure mode that does not exist on a spring mechanism.
Note also the “-12G” suffix: it adds a visible isolation gap in series with the breaker, so the line can be isolated for work without a separate isolator. That single feature often decides between two otherwise identical offers. Both arrangements are shown on the ZW32 outdoor breaker page and the ZW8-12G page for the earlier pattern.
At 35 kV system voltage the breaker is a different animal. The ZW7 is a 40.5 kV outdoor unit mounted on a frame or a structure, with three separate pole assemblies and a common mechanism, and it is typically used as the incoming or outgoing breaker in a 35 kV outdoor substation.
| Parameter | VS1 (12 kV) | ZW32 (12–24 kV) | ZW7 (40.5 kV) |
|---|---|---|---|
| Application | Indoor metal-clad switchgear | Pole-mounted overhead line | Outdoor substation feeder |
| Rated current | 630–3,150 A | 400–630 A | 1,250–2,000 A |
| Breaking capacity | 20–40 kA | 12.5–20 kA | 25–31.5 kA |
| Mechanical life | ~30,000 operations | ~10,000 operations | ~10,000 operations |
| Mechanism | Spring, fixed or handcart | Spring or permanent magnet | Spring, frame mounted |
| Typical add-on | CT/PT, protection relay in the cubicle | Recloser controller, sectionaliser function | External CTs, protection panel |
The pattern in that table is worth reading twice: the pole-mounted ZW32 has the lowest current and breaking ratings of the three, and that is not a limitation of design effort — it is what an overhead distribution line needs. A 630 A, 20 kA breaker is ample for a 12 kV feeder at the end of a rural network. Specifying more there buys nothing.
The 40.5 kV unit is available as a frame-mounted outdoor breaker; details are on the ZW7-40.5 page. The full product range is indexed under the vacuum circuit breaker category.
Chinese-manufactured breakers follow a type designation that encodes the rating in the model number. Once you can read it, comparing two offers takes seconds.
| Segment | Meaning | Example: VS1-12/630-20 |
|---|---|---|
| VS1 / ZW32 / ZW7 | Product family code | VS1 = indoor vacuum breaker family |
| First number | Rated voltage in kV | 12 = 12 kV |
| Second number | Rated current in A | 630 = 630 A |
| Third number | Rated short-circuit breaking current in kA | 20 = 20 kA |
Suffix letters carry the mechanical detail: T or M for spring versus permanent-magnet mechanism, W for withdrawable and F for fixed, and a pole-distance figure for the phase spacing. A model number missing the breaking capacity tells you the offer is incomplete — that value cannot be assumed from the family.
The breaking capacity is not a quality indicator, it is a number that must exceed the prospective fault current at the installation point — with margin, but not indefinitely more. Two inputs set it:
Selecting a 31.5 kA breaker where 20 kA would do is money spent on a number that never gets used. Selecting 20 kA where the fault level is 24 kA is a safety failure. The calculation method, including the tolerance assumptions, is set out in the impedance and fault level guide.
Projects with frequent switching duty — capacitor banks, motor starters, arc furnaces — sometimes reach for a circuit breaker when a vacuum contactor is the correct device. The two interrupt current in the same way but are built for different operating lives: a breaker is designed for occasional fault interruption, a contactor for hundreds of thousands of load operations. The distinction, and when the overlap causes trouble, is covered in the contactor versus breaker comparison.
| Item | Value to state |
|---|---|
| Family and mounting | Indoor cubicle (VS1, fixed or withdrawable) or outdoor (ZW32 / ZW7) |
| Rated voltage | 12 kV, 24 kV or 40.5 kV — and the system’s highest voltage, not just the nominal |
| Rated current | From the load calculation, with the ambient correction applied |
| Breaking capacity | kA, from the fault level study at the installation point |
| Mechanism | Spring or permanent magnet; manual trip or motor charged |
| Control voltage | DC110, DC220 or AC220 for the closing and trip coils |
| Integration | Cubicle type (e.g. KYN28), CT ratio, protection relay, interlocks |
| Accessories | Auxiliary contacts, anti-pumping, mechanical interlock, heater |
The control voltage is the field that most often arrives wrong on a purchase order. It is set by the station battery, not by the breaker, and a mismatch means the breaker cannot be commissioned until the coil is replaced.
Choose the family by environment and voltage first — indoor VS1 for cubicles, ZW32 for pole-mounted overhead lines up to 24 kV, ZW7 for 40.5 kV outdoor substations — then size the breaking capacity from the fault level, then settle the mechanism and accessories. Reading the type code correctly at the quotation stage prevents most of the mismatches that only surface at commissioning.
If you can confirm the system voltage, the fault level at the point of installation and whether the breaker sits indoors or on a pole, the engineering team can match a specific frame and confirm the accessory set against your cubicle or pole arrangement.
The VS1 is an indoor vacuum circuit breaker designed to be mounted inside metal-clad switchgear, available up to about 3,150 A and 40 kA. The ZW32 is an outdoor pole-mounted breaker for 12–24 kV overhead lines, typically 400–630 A and 12.5–20 kA, often supplied with a recloser controller. They serve different installation environments, not different performance tiers.
The ZW7-40.5 family, rated 40.5 kV for a 35 kV system, with typical ratings of 1,250–2,000 A and 25–31.5 kA. It is an outdoor frame-mounted breaker used as incoming or outgoing protection in 35 kV substations. Indoor 35 kV applications use cubicle-mounted breakers of a different family with the same 40.5 kV rating.
Calculate the prospective short-circuit current at the breaker’s installation point — transformer contribution plus motor and network contribution — and select the next standard breaking capacity above it. Standard values in medium voltage are 20, 25, 31.5 and 40 kA. Oversizing is safe but costly; undersizing is a safety failure, so the calculation should be documented rather than estimated.
A spring mechanism charges a spring with a motor and releases it to close the contacts; it is mechanical, well understood and needs no special supply beyond the charging motor. A permanent-magnet actuator holds the closed position magnetically and trips by discharging a capacitor, which reduces moving parts and noise but depends on a reliable DC supply and a controller managing capacitor charge.
It can be installed indoors in a suitable enclosure, but it is designed as an outdoor device and that is where its cost advantage lies. Installing an outdoor breaker indoors usually means paying for weather protection you do not need, while losing the cubicle integration, interlocks and cable termination arrangements that an indoor breaker provides.
