Earth fault loop impedance equations
The complete loop is compared with the maximum that produces the required protective-device operating current.
Zs = Ze + Ft × (R1 + R2)Zs,max = U0 / IaIf = U0 / Zswhere:
- Zs
- Earth fault loop impedance[Ω]
- Ze
- External loop impedance[Ω]
- Ia
- Current causing operation in the required time[A]
- U0
- Nominal voltage to earth[V]
Use the current local table or exact device curve for the required disconnection time.
How to use the Earth Fault Loop Impedance Calculator (Zs)
Calculate actual earth fault loop impedance and compare it with a device-based maximum Zs input. Start by choosing the operating mode, then enter values from nameplates, measurements, or project documents in the units shown. The result updates immediately and keeps intermediate quantities visible for review.
- Confirm the system and calculation mode.
- Enter measured or manufacturer-rated values.
- Review the primary result and intermediate metrics.
- Continue with the related protection or equipment-selection checks.
Calculation method and validation
The equations and symbol definitions above show the complete calculation basis. Inputs are checked before calculation so zero, negative, incompatible, or physically impossible combinations are flagged instead of silently producing a misleading result.
How to interpret the result
Use the main result as a starting value and read every supporting metric. Entered resistances represent the complete circuit path Operating-current multiple is selected for the required disconnection time A passing or recommended result means the entered mathematical conditions are satisfied; it does not certify the complete installation or a particular product combination.
- Entered resistances represent the complete circuit path
- Operating-current multiple is selected for the required disconnection time
Selection limits and next checks
Preliminary engineering estimate only. Verify the final design against the current applicable standard, local rules, manufacturer data, and project conditions. Compare the output with the current applicable standard, local rules, exact VIOX product datasheet, environmental conditions, protection coordination, and the engineer's project requirements before procurement or installation.
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Assumptions
- Entered resistances represent the complete circuit path
- Operating-current multiple is selected for the required disconnection time
Important Warnings
- Preliminary engineering estimate only. Verify the final design against the current applicable standard, local rules, manufacturer data, and project conditions.