Overview - IEC TR 61912-2:2009 (Selectivity under over-current conditions)
IEC TR 61912-2:2009 is an IEC Technical Report and application guide addressing selectivity (coordination) between over‑current protective devices in low‑voltage switchgear and controlgear. The report summarizes terminology, presents methods to determine selectivity between devices in series, and provides worked examples to help designers optimize supply availability while meeting protection objectives. It complements device standards by focusing on practical coordination of circuit‑breakers, fuses, motor overload relays, RCDs and related equipment.
Key topics and technical content
- Definitions and terms: consolidated glossary and abbreviated terms for consistent use of “selectivity”, “upstream/downstream” devices, fault zones, etc.
- Selectivity determination methods: guidance for assessing selectivity for different device pairings, including:
- circuit‑breaker / circuit‑breaker
- circuit‑breaker / fuse and fuse / circuit‑breaker
- fuse / fuse
- circuit‑breaker / motor overload relay (MOR)
- circuit‑breaker / CPS (connector protective system) and fuse / CPS
- RCD selectivity: principles for selectivity between residual current devices (earth‑leakage and earth‑fault conditions) including time‑delay (Type S) considerations.
- Zone Selective Interlocking (ZSI): operating principles and examples for using ZSI to improve selectivity in multi‑supply systems.
- Over‑current protection relays (OCRs): single‑input energizing quantity relays with dependent/independent timing and their coordination considerations.
- Worked examples and annexes: practical time‑current curve examples, grades of selectivity, and discussion of standing loads affecting overload-zone selectivity.
Practical applications
- Design and verification of LV distribution and motor protection schemes to ensure selective disconnection of faults and maximum continuity of supply.
- Choosing and coordinating protective devices to avoid unnecessary upstream trips and reduce outage scope for critical loads.
- Applying ZSI and OCR strategies where conventional time/current discrimination is limited by fault level or protection device characteristics.
- Economic optimization: using limited or graded selectivity where full selectivity across the entire prospective fault current is unnecessary or impractical.
Who should use this standard
- Electrical design engineers and system integrators working on low‑voltage power distribution
- Protection and relay engineers specifying coordination studies
- Installers, testing and commissioning teams validating selective protection
- Specification writers and procurement specialists evaluating device combinations
Related standards (referenced)
Keywords: IEC TR 61912-2:2009, selectivity, over‑current protective devices, low‑voltage switchgear, coordination, circuit‑breaker, fuse, RCD, ZSI.