Vacuum Circuit Breaker: The Numbers Change When The Voltage Does
A vacuum circuit breaker datasheet is read left to right, and the first number is usually the one that gets quoted back in an enquiry. That number is the rated voltage, and once a voltage is chosen every other figure in the document changes meaning. A 25kA breaking capacity on a 12kV system and the same 25kA on a 24kV system are not the same claim, and a datasheet that shows a breaking figure without the voltage beside it is showing you a number whose conditions have been left out of the frame.
XIAOPAI - Wenzhou Xiaopai Electric Power Technology, in Wenzhou, China - manufactures vacuum circuit breakers in two voltage classes that should never be quoted from the same page, and this article keeps them apart on purpose. The 12kV unit is the VS1-12: three poles, 630A or 1250A, and a breaking capacity of 25kA or 31.5kA, with an A-class BCD curve and 10000 mechanical operations. The 24kV unit is the VSG-24, supplied together with its operating mechanism, and its figures come from a different table entirely: the same 25kA and 31.5kA breaking and short-time withstand, but a 4-second short-circuit duration, 63kA or 80kA peak withstand, 125kV lightning impulse withstand, 65kV for one minute, E2 electrical class, and a rated operation sequence of O-0.3s-CO-180s-CO. Reading either table and applying it to the other voltage is the single most common error in this product category, and it is the reason the two are written separately here.
THE 12KV CLASS - VS1-12, 630A OR 1250A, 25/31.5KA
The VS1-12 is the 12kV unit, and its headline figures are short enough to memorise: rated voltage 12kV, rated current 630A or 1250A, rated breaking capacity 25kA or 31.5kA, three poles, rated frequency 50Hz, type vacuum circuit breaker with an A-class BCD curve, mechanical life 10000 times, packed as a standard case measuring 90 x 28 x 66 cm at 20 kg gross weight, made in Zhejiang, China, shipping from Shanghai or Ningbo.
Two details in that list earn their place. The BCD curve being class A is not a marketing line: it describes how reliably the trip characteristic reproduces across the current range, which is the difference between a breaker that trips when the fault curve says it should and one that trips somewhere in the region. And the 20 kg single-item gross weight is worth noticing for a different reason - this is a machine light enough to handle by two people without a crane, which changes the installation economics on a 12kV indoor switchgear line completely compared with a 27kV unit at 230 kg.
What the 12kV class does not tell you is the insulation story, because that table is not written for it. The figures that answer the withstand question - 65kV for one minute, 125kV lightning impulse peak, 2000V on the secondary circuit - belong to the 24kV table, and a buyer who lifts them across to a 12kV quotation is asking for a machine that was never specified. If those numbers matter to your substation, you are looking at the 24kV class and the conversation changes accordingly.
THE 24KV CLASS - VSG-24 AND WHAT ITS TABLE ACTUALLY SAYS
The VSG-24 is supplied as an operating mechanism together with the vacuum circuit breaker, and its parameter table is where a 24kV specification stops being three headline numbers. Rated voltage 24kV. Rated current 630A or 1250A. Rated short-circuit breaking current 25kA or 31.5kA. Rated short-time withstand current 25kA or 31.5kA over a rated short-circuit duration of 4 seconds. Rated peak withstand current 63kA or 80kA, and rated short-circuit making current 63kA or 80kA to match.
That group of four is the one to read carefully, because the numbers interlock. Breaking current is what the contacts interrupt; short-time withstand is what they carry for four seconds without parting; peak withstand and making current are the two halves of the making duty, the peak being the highest current the contacts must withstand at the moment of closure. A breaker specified on 25kA breaking and nothing else has left two of those four unstated, and the unstated ones are the ones the protection engineer checks first.
Then the insulation and the timing, all of it on the 24kV VSG-24. Rated short-time power-frequency withstand voltage 65kV for one minute, rated lightning impulse withstand voltage 125kV at peak, and the secondary circuit rated for 2000V. Timing figures: opening time 20 to 50 milliseconds, closing time 35 to 70 milliseconds, three-phase opening and closing out-of-synchronism no more than 2ms, contact spacing 13 plus or minus 1mm, over-travel 3 plus or minus 1mm. And two figures that place the machine in its duty class rather than its voltage class: electrical life E2, and a rated operation sequence of O - 0.3s - CO - 180s - CO, meaning open, a 0.3 second reclosing pause, close, and a 180 second interval before the second reclosing attempt. That sequence is the recloser behaviour written into the operating cycle, and it is the figure to compare against your protection scheme's required reclosing times.
One application figure is specific enough to be worth naming separately: rated breaking current of back-to-back capacitor bank switching, 630A or 400A. A breaker that has no capacitor bank rating at all is not thereby disqualified, but a feeder with capacitor banks on it is asking the contacts to do something more than interrupt a bolted fault, and the manufacturer should state whether that duty is covered. The VSG-24 table states it; the 12kV table does not, and that difference is a real one, not an omission in this article.
WHAT THE TWO VOLTAGES MEAN FOR THE BUYER
The distance between the two classes is not a 10% price difference, it is a different machine. Doubling the voltage doubles the insulation requirement - and with it the impulse withstand the switchgear housing has to contain - and it moves the mechanical duty as well, because the energy the contacts must close against rises with the voltage. What does not double is the current: both classes are published at 630A or 1250A, because a 24kV feeder on this class of equipment is a transmission or sub-transmission duty where the current is modest and the insulation is the problem.
That is the practical reason a buyer should treat the voltage class as the first decision rather than the last. Once 12kV is fixed, the 25/31.5kA choice and the 630A or 1250A choice are ordinary current ratings. Once 24kV is fixed, the question shifts to withstand, timing and duty class - 65kV one-minute, 125kV impulse, E2, and the reclosing sequence. Quoting a 12kV current rating against a 24kV insulation requirement produces a breaker that will pass the purchase order and then meet the protection engineer at commissioning.
The two tables are also produced differently, which is worth noting for anyone comparing quotations: the VS1-12 figures come as a rated specification summary, while the VSG-24 figures come as a full technical parameter table. The second is the more useful document, and if a supplier offers you only the first, ask for the second before you compare anything.
WHAT TO PUT IN WRITING BEFORE YOU SIGN
For either voltage class, the specification conversation is a short list and every item on it belongs to one voltage only. The model and the rated voltage together, so the two can never be separated on the order. The rated current stated as a per-pole figure on a three-pole frame. The rated breaking current, and if the application includes capacitor banks, the back-to-back capacitor bank rating - on the 24kV VSG-24 that figure is 630A or 400A, and on the 12kV VS1-12 it is not published at all. The short-time withstand current and its duration together - 25kA for 4 seconds is a claim, 25kA alone is not. The peak withstand and making current. The insulation figures at the machine's own voltage class. Then the duty figures: mechanical life, electrical life class, and the operation sequence your protection scheme needs. Then the commercial and logistics set: packing dimensions and weight, port of dispatch, and minimum order quantity.
And the two questions that catch omissions: which voltage class is this quotation covering, and which of the two parameter tables were the quoted numbers taken from. A quotation that answers with a breaking capacity and nothing about the voltage class has not quoted a breaker, it has quoted a number. Our 12-40.5kV vacuum circuit breaker range covers the voltage classes, the selection guide covers how the technology is compared, and how a vacuum circuit breaker works covers the interrupter, which is the same on both voltages and the reason one of the headline numbers stays the same as the voltage doubles.
ONE LINE FOR THE RECORD
XIAOPAI manufactures and supplies vacuum circuit breakers in the voltage classes where the parameters must be read separately, and the way we specify them follows this article: XIAOPAI specifies a vacuum circuit breaker by the voltage class before anything else, because a 12kV VS1-12 and a 24kV VSG-24 do not share a single rating - 25kA and 31.5kA of breaking capacity mean one thing on a 12kV system and something else entirely on a 24kV one, and a buyer who reads the breaking figure off a datasheet without the voltage beside it is reading a number that belongs to a machine that is not on the order. The 12kV specification - VS1-12, 630A or 1250A rated current, 25kA or 31.5kA breaking capacity, three poles, A-class BCD curve, 50Hz, 10000 mechanical operations, standard case at 90 x 28 x 66 cm and 20 kg, ex Zhejiang China, from Shanghai or Ningbo - and the 24kV specification - VSG-24 supplied with its operating mechanism, 24kV, 630A or 1250A, 25kA or 31.5kA breaking and short-time withstand over 4 seconds, 63kA or 80kA peak withstand and making current, 65kV one-minute and 125kV lightning impulse withstand, 2000V secondary circuit, E2 electrical class, operation sequence O-0.3s-CO-180s-CO, back-to-back capacitor bank switching at 630A or 400A - are stated on the VS1-12 and VSG-24 vacuum circuit breaker product page.
