Insulation and dielectric
Vacuum interrupter integrity testing applies a withstand voltage across the open contacts of each vacuum bottle. A bottle that holds the voltage still has a good vacuum, while one that breaks down has lost it and cannot safely interrupt current in breakers, reclosers, starters, switches and tap changers.
Vacuum interrupter integrity testing, often called a vacuum bottle test, checks that the sealed vacuum envelope inside a vacuum interrupter still contains a good vacuum. The interrupter is a ceramic or glass bottle with a pair of contacts inside, one fixed and one moving, sealed under a very high vacuum. The vacuum is what allows the small gap between open contacts to withstand the system voltage and to extinguish an arc at current zero. The integrity test applies a specified withstand voltage across the open contacts of each bottle and watches for breakdown. If the bottle withstands the voltage, its vacuum is considered adequate. The measurement is made on medium-voltage vacuum circuit breakers, vacuum reclosers, vacuum contactors in motor starters, vacuum load tap-changer selectors, vacuum switches and similar devices, and the same type of open-contact withstand check is applied to some gas-bottle switches. It is carried out in acceptance and maintenance work, and it is often paired with an insulation measurement and a contact resistance measurement on the same device. It is a dielectric check of the vacuum gap, not a direct measurement of the pressure inside.
A vacuum interrupter has no way for the user to see inside, yet its ability to interrupt current depends entirely on the quality of its vacuum. Seals can leak slowly through flexible metal bellows, brazed joints or ceramic joints over many years, and a bottle that has been struck, dropped or stressed during shipping or installation may be damaged without any outward sign. If the vacuum has degraded, the gap can no longer withstand voltage when open and can no longer reliably interrupt current. An interrupter in that condition may fail to clear a fault, restrike after a switching operation or explode, with severe consequences for the equipment and for anyone near it. A lost vacuum is not repaired in the field, so the only protection is to detect it and replace the bottle. Testing at acceptance verifies that the device arrived and was installed without damage. Testing in maintenance catches slow leakage before the device is called on to interrupt a fault. The test is also the usual answer when an operation count or a contact wear reading raises questions about a bottle's condition.
In a high vacuum there are very few gas molecules to ionize, so the gap between two electrodes can withstand a high voltage across a small spacing. As air leaks in and the pressure rises, the number of molecules in the gap grows, and the gap withstands less voltage. At a certain pressure the gap can break down at the normal system voltage, and at a higher pressure it cannot hold the test voltage at all. A withstand test exploits that behavior. A voltage chosen to be high enough to be a meaningful challenge, but not high enough to damage a sound bottle, is applied across the open contacts, and a bottle with a good vacuum withstands it without breakdown, while a bottle with a degraded vacuum breaks down or shows excessive current. Applying a high voltage across a vacuum gap can generate X-rays as electrons strike the electrodes, which is a safety consideration for the people nearby and one reason the procedure controls the voltage, the exposure time and the distance. Another point is that a gap that withstands voltage gives no information about the contact condition, and the opposite applies as well.
The device is de-energized, isolated from every source, proven dead and grounded under the owner's lockout and permit procedures, and drawn out or racked to the test position where the design allows. Its contacts are opened and held fully open, because the gap must be at the full open spacing for the test to be meaningful, and the position is confirmed visually or by the mechanism. The test set is connected across the open contacts of one bottle at a time, with the other poles and the frame grounded or left as the manufacturer directs. The test voltage and the type of voltage, alternating or direct, are taken from the manufacturer's instructions and the governing procedure, and they are never exceeded. The voltage is raised smoothly to the specified level, held for the required period while the current is watched, and lowered gradually. Because high voltage across a vacuum gap can emit X-rays, the exposure time is limited, personnel keep their distance from the bottle, and the manufacturer's precautions are followed. A breakdown or a current that is unusually high is cause to stop and investigate, not to retest repeatedly. After the test, the device is discharged and grounded, and the contact position and the result for each pole are recorded. If the voltage is direct, a safe discharge of the residual charge is required before touching the device.
The record holds the withstand voltage applied to each bottle and the type of voltage used, the time it was held, whether the bottle withstood the test or broke down, and any leakage current observed. It notes which device, pole and bottle were tested, the confirmation that the contacts were fully open, the contact gap or position indicator where measured, the identity and calibration status of the test set, and any additional observations such as the operation count and the visual condition of the device. Where a bottle failed, the action taken is recorded, for example replacement and retest. These details allow a later reader to see that the same method and voltage were used each time.
No single number decides whether a bottle is acceptable, and this page does not give one. The result is judged by the outcome of the withstand test against the voltage the manufacturer and the governing procedure specify. The first criterion is that the bottle withstands the applied voltage for the full hold period without breakdown, flashover or abnormal leakage current, and a bottle that fails to do so is considered to have lost its vacuum. The second is that the same voltage and the same type of voltage are applied to every bottle of the same design so results can be compared among poles of one device and among similar devices. The third is trending against earlier results on the same device, where a rising leakage current or a lower withstand level is a warning even before a failure. The fourth is cross-checking with other measurements, including insulation resistance, contact resistance, operation count and, where used, a direct vacuum pressure check. The governing text is found in ANSI/NETA ATS-2025 for acceptance testing and ANSI/NETA MTS-2023 for maintenance testing, which describe this check for breakers, switches, starters and tap changers. For the values, refer to your own licensed copies of the standards named above, and to the manufacturer's instructions for each device.
Acceptance and maintenance tests differ in purpose. At acceptance the test confirms that the device was not damaged in shipping or installation and records its starting condition. In maintenance it screens devices that have been in service for years, especially those that have interrupted faults, those that are operated often and those with signs of wear. The equipment class changes the details. On vacuum circuit breakers the test is made pole by pole, across the open contacts. On reclosers with vacuum interrupters the bottles are inside the tank or housing, and access is arranged accordingly. On medium-voltage motor starters the bottles are in the contactor, and the test is made on each pole of the contactor with the contacts open. On load tap-changers that use vacuum interrupters the test is made on the interrupters in the selector or diverter. On gas-insulated switches, an open-contact withstand test checks the gas gap and is less sensitive to vacuum, which is a different question. The test is an indirect check of vacuum, and when more certainty is needed a specialized vacuum measurement is used in addition.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
A high vacuum has very few gas molecules, so the open gap can withstand high voltage. As air leaks in, the gap holds less. A bottle that withstands the specified voltage across fully open contacts therefore has an adequate vacuum, while one that breaks down has lost it.
Yes. High voltage across an open vacuum gap can produce X-rays, so exposure time is limited, people stay away from the bottle, and the manufacturer's precautions are followed. The usual lockout, isolation and grounding procedures apply as for any high-voltage test, and the test set is operated by qualified personnel.
No. Once the vacuum has been lost, the sealed bottle cannot be restored in the field. The interrupter is replaced, following the manufacturer's procedure, and the new bottle is tested before the device returns to service. The old bottle is removed because it can no longer interrupt current.
No. The withstand test only checks the dielectric strength of the vacuum gap. Contact wear, alignment and resistance are checked separately, for example by contact resistance measurements, contact wear indicators and mechanical checks, so a complete assessment uses several tests together.
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