Standard requirements for selectivity

Overview

The source is the page 'Standard requirements for selectivity' on ABB's low-voltage solutions site, and it is a manufacturer explanation. Its subject is coordination standards and selectivity evidence for device pairs. Selectivity means that, under a short circuit or overcurrent, only the protective device closest to the fault opens, while the devices upstream stay closed. The record rests on metadata only (ABB copyright); the page content is not reproduced here.

The main point recorded is the separation of two different questions. Time-current tolerance-band comparisons and manufacturer-tested instantaneous selectivity limits answer different coordination questions. The page treats this distinction in the context of IEC/TR 61912-2 and the IEC 60947-2 Annex A discussion. In other words, non-overlapping curves for a pair of devices do not give the same assurance as the tested selectivity limit in the manufacturer's tables for that pair. Local current-ratio and tolerance-band comparisons are therefore advisory only; a tested pair-specific limit remains distinct from those screens.

For a designer this means that, in a selectivity study, the results of quick screening tools must be confirmed with manufacturer-tested device-pair data before the final decision. The limit of the source is that this record rests on a single fact and the summary of the page; specific current limits, curve data or calculation methods are not given here. For detail, refer to the relevant standards and manufacturer documents.

Key points

1. Time-current tolerance-band comparisons and manufacturer-tested instantaneous selectivity limits answer different coordination questions.
2. The distinction is discussed in the context of IEC/TR 61912-2 and IEC 60947-2 Annex A.
3. Local current-ratio and tolerance-band comparisons are advisory only.
4. A tested pair-specific selectivity limit remains distinct from those screening checks.
5. The source is a manufacturer explanation; the record rests on metadata only.
6. The record contains no specific current limits or curve data.

Using it in design

• In a selectivity study, present curve overlap and current-ratio checks only as screening tools.
• Keep the manufacturer-tested selectivity limit for each device pair as a separate data field and base the final decision on it.
• Label advisory results and tested results separately in the report.
• Warn for device pairs with no tested data and ask for confirmation from manufacturer documents.
• Cite IEC/TR 61912-2 and IEC 60947-2 Annex A in standard references.

Who should read it

Electrical designers carrying out protection coordination and selectivity studies in data center power distribution.

Publication details

Authors

ABB

Publisher

ABB

Year

Year not stated

Document language

English

Source type

Manufacturer

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation