Công cụ tính khả năng chịu ngắn mạch của cáp

How this is calculated#

The tool checks that a conductor survives a fault current for the time the protective device takes to clear it. It uses the adiabatic equation, S = √(I²t) / k. It assumes that no heat leaves the conductor during the fault.

1. Temperatures. It reads the start temperature and the short-circuit temperature limit for the insulation. The insulation class select can name a 110 °C cross-linked type. A joint in the run can set a lower limit than the insulation.
2. k factor. It reads k from Table 52 for the conductor material and the two temperatures. A larger temperature rise gives a larger k.
3. Minimum size. S = √(I²t) / k gives the smallest conductor, in mm², that stays within the limit. I is the fault current in amperes and t is the clearing time in seconds.
4. Withstand. It turns the equation round for the size you pick: I = k × S / √t. That is the largest fault current the conductor survives for that time.
5. Result. The size you pick passes when it is at or above the minimum.

The Standard select decides which part it reads. It reads AS/NZS 3008.1.2 for New Zealand or AS/NZS 3008.1.1 for Australia. Take the clearing time from the device curve at the fault current. A longer time needs a larger conductor.

Tables without a standard named are in the part of AS/NZS 3008.1 you pick.

Common faults, worked with the tool#

Each line states its conductor. Every line uses copper and the New Zealand tables, with no joint unless the line names one.

• Submain, 6 kA for 0.1 s on 25 mm² PVC. The minimum size is 17.09 mm² with a k of 111. The 25 mm² cable withstands 8.78 kA.
• The same fault on 25 mm² XLPE. The minimum size is 13.27 mm² with a k of 143. The cable withstands 11.31 kA.
• 25 mm² XLPE with a soldered joint. The joint sets the limit at 160 °C. The minimum size is 18.99 mm² with a k of 99.9. The cable withstands 7.9 kA.
• 10 kA for 0.2 s on 16 mm² PVC. The minimum size is 40.29 mm² with a k of 111, so it fails. The 16 mm² cable withstands 3.97 kA.

Keep the result#

Every input sits in the page address, so the link in your browser reproduces the calculation. The PDF button prints the result with its steps and table references. Saving a calculation to a project needs a free account.

See also: Prospective fault current at a main switchboard, Arc flash incident energy at a 400 V switchboard and Short-circuit withstand and the k factor.