Fault current is the huge rush of electric current through a circuit in the event of a fault. The fault could be short circuit between phases, line-to-earth fault, or phase-to-neutral fault. In a fault, the resistance (or impedance) within the circuit is reduced to very low values, so more enormous current can pass—a thousand amps or more.

To picture it, think of a water pipe at regular pressure. Now think of that pipe bursting all of a sudden—that’s your electrical fault. The pressure explodes, and the pipe (or electric system) can’t handle it. The purpose of fault current ratings is to make sure the system doesn’t burst under that pressure.

Where Does Fault Current Come From?

Most fault current is provided by the electricity supply network and that would be your local electricity distributor’s transformer supplying your site. The lower the impedance of the transformer, the higher current it can deliver on fault.

Fault current may also be contributed by on-site sources. Generators running parallel to the grid, for instance, can supply current back into a fault. Large motors and battery systems are also likely contributors, particularly under fault conditions. These sources need to be accounted for in fault level calculations at the design stage.

What Does the kA Rating Mean?

kA rating is a specification of how much fault current a device is able to interrupt or sustain without danger. For circuit breakers, breaking capacity is the term used that may be referred to as Icu or Ics depending upon the type. It informs you of how much current the breaker can interrupt without harming itself. For switchboards and busbars, the term used is withstand rating, also known as Icw. This rating shows the level of current that the equipment will withstand for a specified amount of time, typically one second, without failing.

Suppose you have a switchboard with a 40kA rating for a second. What this says is the board can be safely loaded with 40,000 amps for a maximum of one second. After that, the components will fail, melt, or even explode.

How Is Fault Current Calculated?

Fault current calculations are regulated by codes like AS/NZS 3000 and AS/NZS 61439. These codes specify how to determine the maximum prospective short-circuit current (PSCC) at various locations on an electrical installation.

To work out fault current, you must be aware of the transformer kVA rating, the transformer impedance percentage, and cable length and size of the supply cables. Contributions from any on-site generators and motors also need to be considered, which can raise the fault level.

Another widely employed formula for fault current is:
Fault Current (A) = (Transformer kVA x 1000) / (V x %Z), where V is the phase-to-phase voltage (normally 400V in low-voltage systems) and %Z is the transformer impedance.
This will provide you with a reasonable estimate of the available fault current at the transformer terminals, which can then be modified for distance and cable impedance.

Typical Fault Current Levels

In various environments, fault current levels are standard. In residential settings, the PSCC is typically between 6kA and 10kA. In commercial and light industrial areas, fault currents may range from 10kA to approximately 25kA. In heavy industry or substations, it’s not unusual to have fault levels above 25kA, and they may even be higher than 50kA at times.

Being close to the supply transformer has a significant influence. The nearer you are to the supply, the less impedance and the greater fault current. The longer cable runs feeding your installation means that the extra impedance could help reduce the available fault current.

Choosing the Right Equipment

Once you’ve calculated the fault level at a given point in your system, the next step is selecting equipment that can handle that current safely. Circuit breakers must have a breaking capacity greater than or equal to the calculated PSCC. Similarly, switchboards must have a withstand rating that meets or exceeds the same value.

It’s also crucial to ensure that busbars and cable terminations are able to withstand the thermal and mechanical stresses that a high fault current places on them. This involves precise coordination throughout all elements to ensure predictable behavior of the entire system in the event of a fault.

If you determine that the PSCC is greater than the capability of your proposed equipment, you can use a few alternatives. You might specify more highly rated breakers and switchboards, or you might lengthen the cable to add impedance, which is a natural means of fault current reduction. Another technique is to utilize current-limiting devices like fuses or current-limiting circuit breakers, which lower the peak fault current seen by downstream equipment.

Series Rating (Cascade Protection)

In some situations, lower-rated downstream devices can be protected by a higher-rated upstream breaker. The theory behind this is called series rating or cascade protection. For example, an 50kA main circuit breaker can protect 10kA-rated downstream breakers as long as the set is tested and certified as a system.

These configurations are required to conform to AS/NZS 61439, which demands type testing evidence for the combination. Without it, the use of mismatched equipment may lead to non-conformity and risks to safety.

Arc Faults and Fault Current

Arc faults are not identical with short circuits, but they share a close affinity. An arc fault happens when current bridges an air gap, normally resulting from a defective insulation or inferior connections. While the mechanism differs, the hazard presented by an arc fault is also intensified by the available fault current.

The greater the fault current, the greater the amount of energy that is available to maintain and energize the arc. This can cause major damage, fires, or injury. That’s why fault current levels should be understood and managed as an important initial step to conduct arc flash risk assessments.

Importance of Labelling and Documentation

In practice, it is necessary to label and record the calculated fault current on every switchboard. This will make it safe and fully understand the abilities of the system to maintain or upgrade it in the future.

Standards like AS/NZS 61439 require this documentation. A fault current label should ideally contain the calculated fault level in kiloamperes, the date of the assessment, and the name of the individual or organisation that made the calculation. Specific requirements may be required by some jurisdictions, so always refer to local regulations.