Available fault current explained

Available fault current is the largest current that would flow at a given point in an electrical system if a bolted short circuit happened there. It is set by the utility supply, the transformers and the conductors between the source and that point, and it is usually far larger than any normal load current. Every breaker, fuse and piece of equipment must be able to withstand it and, where it is meant to, interrupt it safely.

Why fault current is so large

Under normal conditions, the load limits the current. A heater, a motor or a lighting circuit has enough impedance to keep the current at its design value. In a short circuit, that load is bypassed. The only things limiting the current are the impedances of the utility system, the transformers and the cables leading to the fault. Those are designed to be low so the system delivers power efficiently, which means the current in a fault can be many times the normal current.

Fault current is usually expressed in kiloamperes (kA). Figures in the tens of kA are common at the main switchboard of a commercial building, and higher figures occur close to large utility transformers.

Where it is highest

Fault current is highest close to the source and drops as you move further away, because each length of cable and each transformer adds impedance. The main switchboard next to the utility transformer sees the most. A panel at the end of a long feeder sees much less. A small transformer that steps 600 V down to 120/208 V creates a new, lower level of fault current on its secondary side, governed largely by its own size and impedance.

Interrupting ratings and withstand ratings

Two ratings matter when comparing equipment to fault current:

• Interrupting rating. The highest current a breaker or fuse can safely interrupt. If the fault current is higher, the device may fail violently instead of clearing the fault, with a risk of fire, explosion and injury.
• Short-circuit current rating or withstand rating. The fault current an assembly such as a panelboard, switchboard, motor control centre or control panel can withstand until the protective device upstream clears the fault.

The Canadian Electrical Code requires equipment to be suitable for the fault current available where it is installed. In practice, the engineer calculates the available fault current at each piece of equipment and checks that each rating exceeds it. Some systems use series ratings, where a tested combination of an upstream and downstream device is rated together. These only apply to the exact combinations the manufacturer has tested and certified, and they must be documented and maintained.

How fault current is calculated

A short-circuit study builds a model of the system and calculates fault current at each bus. The inputs are:

1. Utility contribution. The utility provides the available fault current or the system impedance at the point of supply. Ask for it in writing, and ask whether it allows for future changes to their system.
2. Transformers. Size, voltage and percentage impedance from the nameplate.
3. Conductors. Size, material, length and how they are installed, for every feeder in the model.
4. Motor contribution. Running motors briefly feed current back into a fault, which adds to the total.
5. Other sources. Generators, solar PV inverters and battery systems can also contribute, usually less than the utility, depending on how they operate.

The calculation is normally done in specialized software. A simple hand estimate, assuming the utility can supply unlimited current, gives a conservative upper bound for the transformer secondary and is sometimes used for quick checks.

What changes fault current

This is why equipment that was rated correctly when installed can become underrated later. The building may not have changed at all. The utility may simply have upgraded its system.

What a short-circuit study report should include

• The utility fault current or impedance used, with its source and date
• A single-line diagram of the modelled system, with bus names that match the equipment labels
• The assumptions made where data could not be collected, such as estimated cable lengths
• Calculated fault current at each bus, for the operating scenarios that matter, such as utility only and utility plus generator
• A table comparing each device's interrupting rating and each assembly's withstand rating with the available fault current
• A clear list of underrated equipment and recommended fixes
• Any series ratings relied on, with the tested combinations identified

Keep the model files as well as the report. Updating an existing model when something changes is much faster than starting again.

A typical case

A building owner replaces an old panel in a mechanical room with a new one bought on price. The new panel's breakers have an interrupting rating suitable for a small residential service. The mechanical room is fed by a short feeder from the main switchboard, close to the utility transformer, so the available fault current there is much higher than the new breakers can interrupt. Under normal load nothing happens. In a fault, the breaker could fail to clear it. A short-circuit study, or even a check of the original drawings, would have caught this before the panel was ordered.

Why it matters for arc flash

Available fault current is one of the main inputs to an arc flash study. Higher fault current often means more incident energy, but not always: higher current can make a breaker or fuse trip faster, which reduces the duration of the arc. This is why arc flash labels must come from a study and not be guessed from fault current alone. See what happens in an arc flash study.

How ehZee can help

Our short-circuit and coordination studies model the system from the utility supply down to the panels, compare available fault current with each equipment rating, and identify anything underrated. The same model supports selective coordination and arc flash work.

Common questions