Tap-changer tank: the oil compartment that keeps diverter-switch oil apart from transformer oil, and how its tightness is type-tested
On-load tap-changer diverter or selector switch compartment · separate oil, sealed head and bushings · pressure, vacuum and leak-rate tests under IEC 60214-1 · static pressure and transformer drying that the compartment must survive
An on-load tap-changer switches under load, and its diverter or selector switch arcs every time it does. That arcing breaks down the surrounding oil, so the switch sits in its own oil compartment, the tap-changer tank, sealed from the transformer's main oil. The compartment has to stay pressure-tight and gas-tight through the transformer's drying process, years of operations and the static head of its conservator; how that tightness is proven is the main thing to ask a supplier.
Oil compartments (tap-changer tanks, diverter or selector switch vessels) of on-load tap-changers in oil-filled power transformers: separation of tap-changer and transformer oil, tightness, pressure and vacuum tests, static pressure and drying-process limits. Complete tap-changers and motor drives are covered on the Tap changers page, the tap-changer conservator on the Conservator tank for an on-load tap-changer page, the main transformer conservator on the Conservator tank (expansion tank) page, and tap position sensing on the Tap position indicator page.
CNFX editorial research · Sources checked October 19, 2026 · 3 sources
Transformer Magazine's tap-changer column explains that on-load tap-changers have their own oil compartment for the diverter or selector switch because arc breaking severely deteriorates that oil and produces particles, soot and gases that must not reach the transformer oil; the active IEC and IEEE tap-changer standards therefore require a tightness or container-integrity test, passed only if the compartment is pressure-tight and gas-tight. [2]
The same column notes that "there is a huge number of older units installed in the field which may show some leakage". Dissolved tap-changer gases can then reach the transformer oil through a compartment that is not gas-tight (as seen with some old pressboard cylinders), through leaking gaskets, or through an air space shared with the transformer in one oil expansion tank, which can distort the transformer's dissolved-gas analysis; IEC 60599 gives higher acetylene values for such "communicating OLTCs". [2]
IEC 60214-1:2014 covers on-load tap-changers of resistor and reactor type, de-energized tap-changers and motor drives; it "applies mainly to tap-changers immersed in mineral insulating oil according to IEC 60296"; its second edition added requirements for vacuum-type and gas-insulated tap-changers and changed the type tests to fit service conditions. [3]
Ask whether the compartment shares any air space with the transformer conservator, how the head and bushings are sealed, and for the type test report covering the compartment's pressure and vacuum test.
02
Helium pressure test, vacuum test and the leak rates they must meet
Maschinenfabrik Reinhausen's type test report VR 5E 001e for its VACUTAP VR diverter switches, tested to IEC 60214-1:2014 sub-clause 5.2.6, tested the compartment from inside to outside at a 0.6 bar differential with a helium-air mixture of 10 % helium and vacuum outside, measuring the helium with a leak detector; and from outside to inside with 1.0 bar outside and vacuum inside, measuring the pressure rise over 30 s with a Pirani sensor. [1]
Reinhausen offers its diverter switch oil compartment in three sizes and tested only the large one for pressure and vacuum, stating that the bushings, the head and the number of bushings are identical for all sizes, so the result applies to each. [1]
Editorial calculation: the measured helium leakage is 1.68 ÷ 4.00 = 42 % of the limit and the vacuum rise 0.58 ÷ 1.00 = 58 % of its limit. Since 1 mbar·l = 0.1 Pa·m³, the 4.00 × 10⁻⁵ mbar·l/s limit equals 4.00 × 10⁻⁶ Pa·m³/s, the unit a leak-rate field in Pa·m³/s would use. [1]
Ask for the leak-rate limit and measured value in one unit, the test gas and differential pressure, and which compartment size was tested.
03
Drying cycle, temperature and operations the compartment goes through
Before the mechanical tests, Reinhausen simulated the transformer's kerosene vapour-phase drying on the samples: heating at 80, 90, 100, 110 and 120 °C for 1 h each and then 140 °C for 7 h, 2 h at ≤ 15 mbar, then 36 or 48 h at ≤ 1.65 mbar, for 50 h or 62 h in total. [1]
The switches then made 500,000 operations in clean insulating liquid, half at not less than 75 °C, plus 100 operations at 115 °C and 100 at −25 °C, with switching times staying within their permissible range. [1] The compartment, head gasket and bushings therefore have to keep their seal through both a vacuum-drying cycle at up to 140 °C and service from −25 °C to 115 °C oil.
Editorial calculation: 500,000 operations at an assumed 20 tap changes a day would take 500,000 ÷ 20 ÷ 365 ≈ 68 years; the daily count is an example, not a figure from the report. [1]
04
Compartment details to agree with the tap-changer supplier
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Tap-changer: [type; through-current; highest voltage]
Compartment: [size; head and bushings; oil volume]
Separation: [own conservator or divided; breather]
Tightness: [pressure and vacuum test; leak-rate limits]
Drying: [process, temperature and vacuum the compartment must withstand]
Static pressure: [maximum allowable]
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Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 19 October 2026
Buyer questions
Why does an on-load tap-changer have its own oil compartment?
Arcing in the diverter or selector switch degrades the oil and produces particles, soot and gases that must not reach the transformer oil. [2]
How is a tap-changer oil compartment tested for tightness?
In Reinhausen's IEC 60214-1 type test: a 0.6 bar helium pressure test with a leak detector and a 1.0 bar vacuum test measuring pressure rise over 30 s. [1]
Can tap-changer gases affect transformer DGA?
Yes, through leaky compartments, gaskets or a shared conservator air space; IEC 60599 sets higher acetylene values for communicating OLTCs. [2]
Sources and what still needs a direct check
Sources checked 2026-10-19. A Maschinenfabrik Reinhausen type test report under IEC 60214-1:2014 (published with a public tender), Transformer Magazine's tap-changer column and the IEC webstore abstract of IEC 60214-1.
