Selectivity means that in the event of a malfunction, only the nearest higher-level device will power on. This serves as a preliminary check. upper/lower rated current ratio ≈ 1.6 and above It shows the possibility of selectivity in the overload region; Short circuit selectivity is only possible with manufacturer coordination tables. It is verified.

Overload pre-check: In(upper) / In(lower) ≥ ~1.6
Example calculation: Upper 100 A, lower 40 A → ratio 2.5 ≥ 1.6 → selectivity possible under overload; short-circuit selectivity should be verified with the manufacturer's table.

Selectivity / Coordination Pre-Check

Preliminarily assess the possibility of selectivity based on the rated current ratio of the upper and lower protective devices.

Rated current ratio
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Notes: This is a preliminary check. The general practical ratio for selectivity in the overload region is ≈ 1.6; Short-circuit selectivity can only be verified using manufacturer selectivity (coordination) tables and tripping curves. Full selectivity can be achieved with current limiting, time-phasing, or zone-selective interlocking (ZSI).
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How is it calculated?

  1. Enter the rated current (A) of the upper (feeding) device.
  2. Enter the rated current (A) of the lower (powered) device.
  3. Press Check; the ratio and selectivity assessment will be displayed.

Frequently Asked Questions

What is selectivity?
In the event of a malfunction, only the protective device closest to the fault will trip, leaving the higher levels energized; this is crucial for continuity.
How is selectivity achieved?
Rated current tiering, time delay, current limiting, or zone selective interlocking (ZSI) are used; precise verification is performed with manufacturer selectivity tables.
Does this tool guarantee complete selectivity?
No, it's only a preliminary indication for the overload region; short-circuit selectivity must be verified with tripping curves and manufacturer's tables.

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This tool and its content are for general informational purposes only; they do not constitute a binding inspection/declaration of conformity. Definitive results will be determined through on-site inspection in accordance with relevant legislation and standards. Get a quote →