Transformer inrush at ang breaker nito
Tingnan ang switch-on inrush ng transformer laban sa breaker nito.
Reads AS/NZS 3000:2018. Tables checked 2 Oct 2026.
Subukan ang isang job 4
Nilalagyan ng halimbawang value ang bawat job. Palitan ang mga ito ng sarili mong value.
- Stop a transformer tripping its breakerWhy an isolation transformer trips a Type B breaker at switch-on, and what holds.
- Pick a breaker for a site transformerThe breaker and cable for a portable site transformer.
- Check a control transformer breakerWhether a small control transformer on an extra-low voltage secondary holds its breaker.
- Size the breaker for a step-down transformerThe smallest breaker of each type for a single‑phase step-down transformer.
Mga input
Worked example gamit ang mga default na input.
- Rating
- 15 kVA
- Mga phase
- Three phase
- Primary voltage
- 400 V
- Secondary voltage
- 400 V
- Mga circuit sa secondary
- Isa, walang sariling proteksyon
- Inrush, × full load current
- 12
- Tumatagal ang inrush
- 0.1 s
- Uri ng breaker
- Curve C
- Rating
- 32 A
Ang breaker sa switch-on
Maaaring mag-trip
- Inrush current
- 259.8A
- Primary full load current
- 21.65A
- Pinakamaliit na Type B na kakasya
- 100 A
- Pinakamaliit na Type C na kakasya
- 63 A
- Pinakamaliit na Type D na kakasya
- 32 A
- Secondary conductor, hindi bababa sa
- 21.7A
Design aid lang ang resultang ito. I-verify ito laban sa kasalukuyang standard.
Mga hakbang ng kalkulasyon
- Primary full load current21.65A
- Inrush, 12 × ang full load current sa loob ng 0.1 s259.8A
- Uri C 32 A, karaniwang instant band mula160A
- Uri C 32 A, karaniwang instant band hanggang320A
- Ang breaker sa switch-onResultaMaaaring mag-trip
- Pinakamaliit na Type B na kakasya100 A
- Pinakamaliit na Type C na kakasya63 A
- Pinakamaliit na Type D na kakasya32 A
- Secondary conductor, hindi bababa sa21.7A
AS/NZS 3000:2018 Cl 4.14.2.2
Tungkol sa calculator na ito
How this is calculated
When a transformer is switched on, its core can draw a large current for a few cycles while the magnetic flux settles. This inrush can be ten or more times the full load current. It lasts about a tenth of a second, and it is often enough to trip the breaker on the primary. This tool checks whether it does, and finds a breaker that rides through it.
The tool takes the primary full load current from the transformer tool, then multiplies it by the inrush multiple you enter, for the time you enter. No standard on hand prints these two figures, so they are your assumptions. Take them from the maker's data sheet. The multiple is a peak, and the tool compares it with the breaker's band, which is given in RMS multiples of the rating, as if it were RMS: that errs towards a trip.
It then places that current on the trip band of the breaker, the same band the MCB trip curve tool draws. The breaker holds when the band's earliest trip is no sooner than the inrush lasts. It trips when the latest trip is sooner, and it may trip in between. For each type, B, C and D, the tool finds the smallest rating that carries the full load current and holds.
With one circuit on the secondary and no protection there, AS/NZS 3000 lets the secondary cable go without its own breaker if it carries the primary current scaled by the voltage ratio. The tool gives that current.
| Step | Table or clause | What it decides |
|---|---|---|
| Full load current | The transformer tool | The current the inrush multiplies |
| Inrush | The multiple and time you enter | The current and time the breaker sees |
| The breaker's band | AS/NZS 3000 Cl 2.5.3.1 and Table 8.1, and a typical band | Holds, may trip or trips |
| The secondary cable | AS/NZS 3000 Cl 4.14.2.2 Exception | The least capacity of an unprotected secondary cable |
| An extra-low voltage transformer | AS/NZS 3000 Cl 4.14.1 Exception 2 | Whether Cl 4.14 applies at all |
Worked examples
- A 15 kVA three‑phase isolation transformer on a C32 draws 259.8 A of inrush, inside the C32's typical instant band, so the breaker may trip at switch-on. A 32 A Type D holds.
- The same transformer on a D32 holds the 259.8 A and passes. The 32 A Type D is also the smallest D that carries the 21.65 A load.
- The same transformer on a B32 fails: the inrush is past the top of the typical instant band and the breaker trips. A 32 A Type D would hold.
- A 3 kVA 230/110 V site transformer on a C16 may trip on its 156.5 A inrush. A 16 A Type D holds, and the 110 V cable needs at least 27.3 A.
- A 10 kVA 400/230 V transformer, 10 × inrush, on a D32 holds and passes. A 32 A Type D is the smallest D that holds.
Limits
The inrush depends on where on the wave the transformer is switched and on the flux left in the core, so it differs from one switch-on to the next. The tool uses one figure for the peak and one for the time. A toroidal transformer can draw much more than a laminated one. A soft-start or inrush limiter changes the figures; enter what the maker gives.
The breaker's band is typical of its type, not a maker's curve. The tool does not check a fuse or a moulded-case breaker.
See also: MCB trip curve, Transformer and Cable size.
Mga tanong
Design aid ang resulta. I-verify ito laban sa kasalukuyang standard.
Bakit nagti-trip ang breaker ng transformer kapag ini-switch on?
Kumukuha ang core ng malaking magnetising current sa ilang cycle habang nagse-settle ang flux nito. Ang inrush ay maaaring sampung beses o higit pa sa full load current. Kapag naabot nito ang instant band ng breaker, maaaring mag-trip ang breaker.
Gaano kalaki ang transformer inrush current?
Depende ito sa disenyo at sa punto ng wave kung saan ito sinwitch. Ang 10 hanggang 12 beses ng full load current sa humigit-kumulang 0.1 s ang karaniwang unang hula. Puwedeng humila nang higit ang toroidal transformer. Ang data sheet ng maker ang nagbibigay ng totoong figure.
Aling breaker curve ang angkop sa transformer?
Kadalasan ay Type D, na ang tipikal na instant band ay nagsisimula sa 10 beses ng rating nito. Ang Type C ay nangangailangan ng mas malaking rating para sa parehong inrush, at ang Type B ay mas malaki pa. Hinahanap ng tool ang pinakamaliit sa bawat isa na kakayanin.
Kailangan ba ng secondary cable ng sarili nitong breaker?
Pinapayagan ng AS/NZS 3000 Cl 4.14.2.2 na walang protection ang isang secondary circuit, kung ang cable nito ay dala ang primary rated current na pinarami sa primary-to-secondary voltage ratio. Kapag mas maraming circuit, bigyan ng protection ang bawat isa.
Curve ba ng maker ang breaker band?
Hindi. Ito ay karaniwang band para sa type, ang parehong iginuguhit ng MCB Trip Curve tool. Kapag ang resulta ay May trip, tingnan ang curve ng maker o pumili ng susunod na mas mataas na rating.
Mga clause at table na binabasa ng tool na ito
- AS/NZS 3000:2018 Cl 4.14.1Transformers outside Cl 4.14
- AS/NZS 3000:2018 Cl 4.14.2.2Transformer secondary circuits
- AS/NZS 3000:2018 Cl 2.5.3.1Operating points of circuit-breakers and fuses
- AS/NZS 3000:2018 Cl 4.14Transformer secondary circuits
- AS/NZS 3000:2018 Table 8.1Maximum earth fault loop impedance multipliers by MCB curve
Bawat clause at table, kasama kung paano ito na-verify
Huling na-check ang mga table noong 2 Oct 2026.
Mga kaugnay na tool
- Transformer calculatorCalculate transformer full load current and impedance.Walang standard
- MCB trip curve at cable limitPlot an MCB trip band against the cable it protects.AS/NZS 3000AS/NZS 3008.1.2AS/NZS 3008.1.1
- Transformer size mula sa demandSize a transformer from the demand, with growth and losses.Walang standard
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