Switchroom Short Circuit Rating Checker
Check whether switchgear in a switchroom can safely withstand the prospective short-circuit current available at the board. Enter the fault level and the equipment's rated short-circuit withstand to see the margin, how hard the rating is being worked, and a clear adequate / inadequate verdict.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the prospective (available) fault current at the board in kA, taken from your fault study or utility figure.
- Enter the switchgear's rated short-circuit withstand in kA from the nameplate or datasheet.
- Read the adequacy status, the withstand margin, and the utilisation of the rating.
How it works
Margin is the rated withstand minus the prospective fault current. Utilisation expresses the fault current as a percentage of the rated withstand. If the rated withstand is greater than or equal to the fault current the equipment is adequate; otherwise it is inadequate and must be upgraded or the fault level reduced. Ratings are RMS symmetrical short-time (typically 1 s or 3 s) values.
Worked example
Worked example. A board with a 25 kA prospective fault and switchgear rated 31.5 kA gives a margin of 6.5 kA and 79.37 % utilisation - Adequate. Raise the fault level to 40 kA and the margin becomes -8.5 kA at 126.98 % utilisation - INADEQUATE.
Common mistakes
- Comparing only against the short-time withstand and ignoring the peak (make) rating, which is stressed by asymmetric DC offset.
- Using a fault duration longer than the equipment's rated time, which invalidates the I2t withstand.
- Forgetting that fault levels rise over time as the network grows, eroding the margin.
Frequently asked questions
What margin should I aim for?
Any non-negative margin is technically compliant, but designers usually keep headroom for future network growth that increases the available fault current.
Does this cover the peak make rating?
No. This tool checks the RMS short-time withstand only. Always separately confirm the peak (make) rating and that the fault clearing time is within the rated duration.
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