During fault, the equipment—particularly the circuit breakers, busbars, and enclosures—must interrupt or tolerate that surge without breaking. In case the fault current available surpasses the rating of a breaker, it would explode rather than trip. If your internal busbars of the switchboard are not capable of tolerating the thermal or mechanical stress, they might bend, rupture, or trigger an arc flash.

Each item of electrical equipment has a short-circuit rating. Here in Australia, that is usually stated as kA for a specified time (e.g., 25 kA for 1 second). These ratings are then to be compared to the calculated actual fault current at the site of installation. And that’s not purely for safety reasons—it’s a condition of AS/NZS 3000 and other Australian Standards.

Step 1: Calculating Fault Current

Before choosing equipment, you must determine what fault current it will be subjected to. The most straightforward method is to estimate the prospective short-circuit current (PSCC) at the main switchboard and key points downstream.

Suppose you’re supplying a main switchboard from a 500 kVA transformer with 5% impedance. You can approximate the fault current at the transformer secondary as follows:

Ifault = Transformer kVA / (Vsecondary × %Z)
  

For a 500 kVA transformer at 415 V:

Ifault = 500,000 / (√3 × 415 × 0.05) ≈ 13.9 kA
  

That means the equipment at that point must be rated for at least 13.9 kA—and realistically, you’d round that up and select devices rated at 18 kA or 25 kA for safety and future-proofing.

For points further downstream, fault current reduces due to the impedance of the cable runs. You’ll need to take into account the type, length, and size of the cables to calculate voltage drop and resulting impedance.

Step 2: Selecting Devices with Sufficient Breaking Capacity

Once you have the fault current at a given point, your task is to select protective equipment—such as circuit breakers or fuses—with rated breaking ability (in kA) greater than the calculated fault current.

This rating is also called the breaking capacity, Icu, or short-circuit current rating. You don’t want to use a breaker rated for 10 kA if your fault current is 14 kA. It may not trip at all, or worse, blow explosively under fault conditions.

Good practice in Australia is to choose breakers with a buffer over the calculated one. For instance, if you’ve calculated it as 13.9 kA, opt for an 18 kA or even a 25 kA breaker, subject to cost and availability.

There are current-limiting devices provided by some manufacturers, which can limit the peak fault current available to downstream equipment. These are particularly valuable when upgrading existing old systems that weren’t planned to operate at high fault levels.

Step 3: Busbar and Switchboard Withstand Ratings

Fault current doesn’t only impact breakers—it impacts the entire assembly. Your switchboard, both the internal busbars and mechanical structures, also have to be rated to tolerate the fault.

The two parameters to consider are:

Thermal withstand rating (in kA for 1 second or 3 seconds)
Mechanical (peak) withstand rating (in peak kA)

For instance, if the fault level calculated is 20 kA for 1 second, your switchboard has to be type-tested to withstand at least that current without being destroyed. That means the enclosure, supports, terminations, and cabling.

Under AS/NZS 61439, switchboards must undergo verification for these withstand levels through type testing or design rules. If you’re custom-building a board or using a modular system, make sure it has been verified for your fault current scenario.

Step 4: Coordination and Selectivity

It’s not enough for equipment to simply survive the fault—you want the right device to trip. That means coordination.

Suppose you have a master breaker and a number of sub-circuit breakers downstream. When one of the sub-circuits experiences a fault, you would want only the sub-circuit breaker to trip, not the master one. This maintains continuity of supply to the rest of the installation.

This is where selectivity comes in. When selecting protection devices, examine their tripping curves and verify if they provide selective coordination during fault conditions. In certain instances, you might need to use breakers of the same make and model to ensure selectivity.

Step 5: Considering Future Upgrades

The biggest mistake is designing for today’s load without providing for tomorrow’s.

What if you install more equipment? What if your solar inverter array is increased? What if the utility replaces their transformer?

All these can boost the fault current available. When you pick equipment, it’s a good idea to think ahead. Selecting breakers and busbars slightly larger than today’s calculated current may not be adequate five years from now.

The aim is to choose equipment with ratings that won’t merely pass the test—but will have room to spare to deal with future expansion, system revisions, or supply upgrades.

Step 6: Documentation and Labeling

Australian Standards, such as AS/NZS 3000, call for good documentation of your fault current calculations. This involves:

The method employed (manual or software)
The calculated figures at each point
The equipment ratings chosen based upon these calculations

It’s also common practice to mark switchboards with the fault current at that point. This ensures that future electricians, inspectors, and maintenance crews can easily check the adequacy of the equipment.