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.
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
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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.
Method and questions
Check discrimination
Overload and short-circuit ranges checked separately, because they behave differently.
Discuss a projectWorked 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 and standards
Formula
- In the overload range I_n(upstream) / I_n(downstream) ≥ factor
- In the short-circuit range 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, as 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, such as a fuse against a 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.
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 with a longer feeder, a smaller cross-section or upstream impedance. Or deliberately accept the loss where disconnecting a larger area is tolerable.
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