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34 — Calculators

Check discrimination between two protective devices

Discriminating means that under fault only the device in front disconnects, not half the installation. That has to be checked separately in two ranges, because they behave differently — in the overload range the current ratio decides, in the short-circuit range the combination of devices.

Input

Empirical value for devices of the same kind. For fuses of the same series, 1.6 is usually taken.

The current up to which the combination of the two devices is selective. It comes from the manufacturer coordination table, not from a formula.

From the short-circuit current at cable end calculator.

Result

Discrimination in the short-circuit range
—
Discrimination in the overload range
—
Current ratio present
—
Margin to the selectivity limit currentNegative means the short-circuit current lies above it.
—kA

A guide, not a design to standard. We give no warranty for the correctness of the results, the applicable standards and case-by-case verification govern.

Worked example

The default case the calculator starts with, worked through once. Change the values above and it recalculates immediately.

Inputs

Rated current of the upstream device
160 A
Rated current of the downstream device
63 A
Required current ratio in the overload range
1.6
Selectivity limit current from the manufacturer table
3 kA
Short-circuit current at the downstream device
4.2 kA

Result

Discrimination in the short-circuit range
Not achieved.
Discrimination in the overload range
Achieved.
Current ratio present
2.54
Margin to the selectivity limit current
-1.2 kA

Formula

  • Overload: I_n(upstream) / I_n(downstream) ≥ factor
  • Short circuit: I_k at the downstream device ≤ selectivity limit current

Assumptions and standards

  • The selectivity limit current follows from no formula. It arises from the interaction of both characteristics and their limiting behaviour and is given in the manufacturer coordination table. We do not reproduce those tables — they apply per device pair and series.
  • The required current ratio in the overload range is an empirical value for devices of the same kind. For mixed types — fuse against circuit breaker — it does not simply hold.
  • Discrimination between two devices is checked. In a chain with several levels, every pair has to be checked separately.

Frequently asked

Why two separate ranges?

Because two different releases are at work. In the overload range the thermal release responds slowly, and a sufficient gap between rated currents suffices. In the short-circuit range both devices release magnetically within milliseconds — there no current gap helps, only that the downstream device limits the current before the upstream one responds.

Why is the limit current not built into the calculator?

Because it is not a calculable value but a measured one per device combination. Manufacturers determine it in the test lab and publish it in coordination tables. A reproduced table would at best be redundant and at worst wrong.

What if there is no discrimination in the short-circuit range?

Either choose a different device pair whose limit current lies above the short-circuit current, or reduce the short-circuit current — longer feeder, smaller cross-section, upstream impedance. Or deliberately accept the loss where disconnecting a larger area is tolerable.

Calculating is the easy part.

A formula gives you a number. Designing a plant also demands installation method, grouping, discrimination, the standards in force and a look at the installed base. That is what we take on.

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