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.