I2t and adiabatic withstand in Australian cable protection
How I2t, clearing time, k value and conductor area relate to Australian cable withstand checks.
What I2t Means
I2t describes the energy-related effect of fault current over time. In Australian cable and protective-conductor review, it is used with clearing time, conductor area and a k value source to screen thermal withstand.
The key relationship is:
I2t = I^2 x t.
Adiabatic withstand is compared with:
k2S2 = (k x S)^2.
The check is commonly expressed as I2t <= k2S2. The values must all belong to the same project context: fault-current point, protective device, clearing time, conductor material, insulation basis and k-value source.
Units And Inputs
The units matter. Fault current is often discussed in kA, but the I2t relationship uses amps. One kA is 1000 A. Clearing time is entered in seconds. Conductor area is entered in mm2. The k value must match the conductor material and insulation or thermal basis.
| Input | What it means | Why it matters |
|---|---|---|
| Fault current | Entered prospective fault current in kA, converted to A for the arithmetic. | I2t rises with the square of current. |
| Clearing time | Device clearing time or entered assumption in seconds. | Energy exposure increases with time. |
| Conductor area | Metric conductor size in mm2. | Withstand capacity changes with area. |
| k value | Source value for conductor material and insulation basis. | The constant must match the conductor and thermal assumptions. |
| Device let-through | Optional entered A2s value from product data. | A device curve or let-through value must be product-sourced. |
Worked Unit Check
For a 5.5 kA fault current and 0.1 s clearing time:
I2t = 5500^2 x 0.1 = 3,025,000 A2s.
For a 16 mm2 conductor with k = 115:
k2S2 = (115 x 16)^2 = 3,385,600 A2s.
The entered conductor withstand is above the entered I2t value, with about 360,600 A2s margin. That does not select the conductor or device; it only screens the values entered.
| Field | Example entry | Reading |
|---|---|---|
| Fault-current point | Switchboard or circuit where PSCC was recorded. | The withstand check must use the same fault-current location. |
| Clearing time | 0.1 s protective-device clearing time or entered project assumption. | I2t changes with time as well as current. |
| Conductor data | 16 mm2 conductor, k value 115 from the selected source. | Withstand capacity depends on conductor assumptions. |
| Device data | Fuse, MCB, MCCB or other product context. | Let-through data and clearing assumptions must match the device. |
Let-Through And k Values
Use A2s, kA, seconds and metric conductor sizes consistently. When AS/NZS 3008 context, manufacturer curves, device let-through data or project assumptions control the conductor data, keep those sources close to the calculation.
The k value is not a decorative constant. It depends on conductor material, insulation and temperature basis. The optional device let-through value must come from the actual product data or project source.
When conductor material, insulation context or device data is uncertain, keep the value in review rather than presenting it as a finished withstand decision.
Next checks
- Use I2t cable withstand when fault current, clearing time and conductor data are ready.
- Use the fault-current chart to understand nearby protection arithmetic.
- Use protection terms to keep device wording consistent.
Boundaries
- This page does not provide a complete k-value table or conductor constants.
- It does not select protective devices or cable sizes.
- Product data, current rules context, project conditions and competent engineering review remain controlling inputs.
I2t and adiabatic withstand in Australian cable protection in Australian work
Explain I2t and adiabatic withstand so fault current, clearing time, k value and conductor area stay connected. The useful record is not just the final number or definition; it is the combination of the entered value, the supply arrangement, the equipment or circuit reference and the decision that still needs project review.
Help users understand I2t and adiabatic withstand in Australian cable and protective-conductor review before using fault-current and clearing-time data. Keep the wording in Australian English, use metric units, and treat 230/400 V a.c. at 50 Hz as the ordinary low-voltage context unless the project information states a different value.
Checks that belong beside this page
- Regional scopeAustralia; public cable protection calculator terminology.
- Standards contextAustralian cable protection and Wiring Rules context; current project data, product data and competent review remain controlling sources.
- Review timingReview cadence: Annual I2t wording review or sooner if calculator or guide wording changes.. Update trigger: Update when I2t calculator inputs, fault-current chart rows or cable withstand guide wording changes.
Practical use sequence
For I2t and adiabatic withstand in Australian cable protection, begin with the circuit, board, load, cable route, equipment item or measurement record named by this topic. Record the value basis, such as current, voltage, kW, kVA, route length, test value, product rating, tariff value or worksheet field, then note the review boundary so another person can see what this page explained and what still needs a project decision.
The strongest I2t and adiabatic withstand in Australian cable protection record is specific enough to be reviewed later. It names the Australian context, keeps the entered assumptions visible, separates calculation support from compliance decisions and shows where a calculator, table, chart or term definition should be opened next.
Record detail to capture
Keep the I2t and adiabatic withstand in Australian cable protection note attached to Australia; public cable protection calculator terminology.. When the page is used during design, estimating, testing, review or study work, write the entered assumption, the source that supplied it, the circuit or equipment label and the person or role that must review the following action.
For I2t and adiabatic withstand in Australian cable protection, the review cadence is Annual I2t wording review or sooner if calculator or guide wording changes. and the update trigger is Update when I2t calculator inputs, fault-current chart rows or cable withstand guide wording changes.. If the topic connects to Protection device terms, open that page after the current definition, input basis or boundary has been recorded, so the next calculation inherits the right Australian context.
A complete I2t and adiabatic withstand in Australian cable protection record leaves four facts visible: why the page was opened, which Australian supply or equipment context applies, which value remains user-entered and what later change would alter the result. That makes the page useful as a working note instead of a loose reference copied away from the project evidence.
- InputsWrite down the entered values and their units before comparing results across calculators or reference pages.
- LocationKeep the value tied to the board, circuit, cable run, equipment terminal, supply point or worksheet row it describes.
- Authority checkCompare the record with current Australian requirements, DNSP or local authority conditions and product information.
- Review trailRetain the date, edition context and reason for the calculation so later changes can be traced.
Connected AUWiring pages
Use the related pages for I2t and adiabatic withstand in Australian cable protection when the topic needs a calculator input, lookup table, formula chart or supporting term before the project record is complete. Open Protection device terms first when that page owns the next value or definition.
- Protection device termsDevice terms used beside cable withstand checks.
- Fault-current relationshipFault current relationship used near I2t review.
- I2t cable withstandReview entered fault current, clearing time and conductor data.
- Short-circuit currentEstimate entered fault-current values where appropriate.