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Arc Fault Clearing in Switchboards

    What Is an Arc Fault?

    An arc fault is an unintentional electrical discharge between conductors or between a conductor and earth, inside a switchboard. These events release extremely high temperatures, intense light, and pressure waves, which can cause equipment damage and potential injury.


    Common causes of arc faults include:

    • Loose or deteriorated connections
    • Damaged or aged insulation
    • Contamination from dust, moisture, or vermin
    • Accidental contact during installation or maintenance





    What Is Arc Fault Clearing?


    Arc fault clearing is the process of detecting and rapidly disconnecting the electrical supply when an internal arcing fault occurs in a switchboard. The aim is to minimise damage, reduce fire risk, and protect personnel.


    The Two Stages of Arc Fault Clearing

    1. Detection
      The protection system must identify the arc fault extremely quickly—often in milliseconds—to prevent catastrophic damage. This can be achieved through:

      • Over-current protection – detecting a sudden surge in current
      • Arc flash detection relays – using optical sensors to detect arc light
      • Current-limiting fuses – restricting peak fault current
      • Protection relays with short-time pickup – enabling rapid breaker operation


    2. Disconnection

      Once detected, a circuit breaker or fuse isolates the faulty circuit. The clearing time is the total time from the start of the arc to complete isolation. Faster clearing times mean less damage and lower arc flash energy.

    Arc Fault Clearing Time Calculator

    Purpose & Utility

    Estimates the time it takes for a protection device to clear an arc fault. Supports arc flash risk assessment and helps electricians select appropriate PPE and safety measures.

    This calculator estimates the maximum arcing fault clearing time in seconds, based on the fault current and the protective device’s response characteristics.

    Arc Fault Clearing Time (seconds) = (Ke × Ir )÷ (If)1.5

    The actual clearing time is obtained by referencing the manufacturer’s protection curve for the selected breaker type and rating.

    Where:

    • t = Arc fault clearing time (seconds)
    • Ke = constant based on acceptable volume damage (typically derived from arc flash studies or standards, AS/NZS 3000 specifies it to be 250)
    • Ir = current rating of the switchboard (amps)
    • If = 30% of prospective fault current (kilo-amps)

    Input all the values in the AUSINET online calculator. Compare the result with the manufacturer’s curve and validate that the breaker trips within or before the calculated time.



    Manual calculation Steps

    1. Input Ke – Based on an arc flash study or empirical damage threshold. AS/NZS 3000 specifies it to be 250.
    2. Input Ir (Amps) – Rated current of the switchboard
    3. Input If (Amps) – 30% of the Prospective fault current
    4. Apply Formula – (Ke × Ir )÷ (If)1.5
    5. Display Result –  Maximum Arc Fault Clearing Time in seconds
    6. Compare Result with Manufacturer Curve – Validate that the breaker trips within or before the calculated time


    Interpretation of Results

    • Shorter clearing time → Lower arc energy, reduced damage risk
    • Longer clearing time → Higher arc energy, increased hazard; may require PPE upgrades or design changes


    Use the result to:

    • Assess arc flash risk
    • Select appropriate protection devices
    • Determine PPE category and safe working distance
    • Document compliance with WHS and AS/NZS 61439.1 or IEC 61641 (where applicable)