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PD IEC TR 61912 ā Low-voltage Switchgear and Controlgear Overcurrent Protective Devices Technical Report Series (Short-Circuit Ratings and Selectivity)
Part 1: Application of short-circuit ratings; Part 2: Selectivity under over-current conditions
š PD IEC/TR 61912 is the UK adoption of technical reports from the IEC 61912 series, developed under the IEC committees for low-voltage switchgear and controlgear (TC 121A, and formerly TC 17), and adopted in the UK through Committee PEL/17 (Switchgear, controlgear and HV-LV co-ordination) and its subcommittee PEL/17/2 (Low voltage switchgear and controlgear). The series addresses overcurrent protective devices in low-voltage switchgear and controlgear: Part 1 (2007) covers the application of short-circuit ratings, and Part 2 (2009) covers selectivity under over-current conditions. The technical reports provide important engineering guidance for protection coordination in distribution systems and motor circuits.
š” The central challenge of low-voltage distribution protection is “reliably interrupting a short circuit while coordinating selectively under normal overload”. Part 1 answers “is the breaking capacity sufficient”, while Part 2 answers “which level should trip on a fault” ā together they form a complete overcurrent protection coordination scheme.
1. Scope and Series Structure š
This series contains two technical reports, addressing short-circuit rating application and selectivity respectively:
| Document | Subject | Content | Year |
|---|---|---|---|
| PD IEC/TR 61912-1:2007 | Short-circuit ratings | Low-voltage switchgear and controlgear ā Overcurrent protective devices ā Application of short-circuit ratings | 2007 |
| PD IEC/TR 61912-2:2009 | Selectivity | Low-voltage switchgear and controlgear ā Overcurrent protective devices ā Selectivity under over-current conditions | 2009 |
ā ļø Technical reports (TR) are informative, guiding documents that do not specify mandatory clauses. They should be used together with the corresponding normative low-voltage equipment standards (e.g. IEC 60947-2 circuit-breakers, IEC 60947-6-2 control and protective switching devices, IEC 60269 fuses, IEC 60898-1 household MCBs, IEC 61009-1 residual current operated circuit-breakers with integral overcurrent protection).
2. Part 1 ā Application of Short-Circuit Ratings ā”
Part 1 (2007) addresses protection coordination of low-voltage assemblies and switchgear under short-circuit conditions. Core content includes:
- Principle of application ā installation: the design principles for short-circuit protection of low-voltage assemblies (switchboards, distribution boards, etc.) and switchgear;
- Characteristics ā low-voltage assemblies: the short-circuit withstand and limiting characteristics of switchboards, distribution boards, etc.;
- Characteristics ā switching devices (self-protection): the self-protection capability of switching devices against short-circuit current;
- Switching devices applied as SCPD: breaking capacity and coordination requirements when a switching device is used as a short-circuit protective device (SCPD);
- Various SCPDs: fuse-combination units and fuses, circuit-breakers to IEC 60947-2, control and protective switching devices (CPS) to IEC 60947-6-2, RCBOs to IEC 61009-1, etc., as SCPDs;
- Interpolation for an alternative SCPD: an annex gives the interpolation method for the suitability of an alternative SCPD, with IĀ²t examples.
šØ The first principle of short-circuit protection is “breaking capacity ā„ prospective short-circuit current”: if the SCPD’s breaking capacity is insufficient, it may fail to interrupt or even explode and ignite on a fault. Selection must verify the prospective short-circuit current at the installation point and consider upstream/downstream coordination and self-protection.
3. Part 2 ā Selectivity under Over-Current Conditions š
Part 2 (2009) defines selectivity between upstream (UD) and downstream (DD) protective devices under over-current conditions. Core content includes:
- Scope of selectivity requirements: definition of the selectivity requirements between upstream and downstream protective devices;
- Selectivity determination: methods for determining the selectivity curve (e.g. time-current characteristic) and the selectivity limit, plus the IĀ²t energy criterion;
- Various coordination combinations: selectivity between circuit-breaker and circuit-breaker, circuit-breaker and fuse, circuit-breaker and CPS, circuit-breaker and motor protection overload relay, fuse and fuse, fuse and other devices, etc.;
- Selectivity types: definition and application of total selectivity and partial selectivity.
š” Selectivity ensures that “the protective device closest to the fault operates first, while upstream devices do not”, limiting the outage to the minimum. In practice this is achieved by time-current curve separation, energy (IĀ²t) matching, or zone interlocking; verify against the manufacturer’s selectivity tables or curves during selection.
4. Engineering and Application Practice š ļø
- š Breaking capacity verification: per Part 1, calculate the prospective short-circuit current at the installation point and ensure the breaking capacity of the selected circuit-breaker, fuse or CPS is sufficient, considering motor starting and transformer inrush conditions.
- š Selectivity coordination: per Part 2, verify the time-current characteristics or selectivity tables of upstream and downstream devices to ensure full selectivity (or an assessed partial selectivity) under overload and short circuit.
- š§® Energy (IĀ²t) matching: in the short-circuit region, verify the upstream/downstream IĀ²t energy so that the upstream device does not operate before the downstream device has interrupted; adjust ratings or use current-limiting devices where necessary.
- š Documentation: prepare a protection coordination table and selectivity analysis report as the basis for distribution system design, acceptance and operation.
5. Frequently Asked Questions ā
ā Q: How do Parts 1 and 2 differ?
A: Part 1 covers the application of short-circuit ratings (breaking capacity) and self-protection, answering “is the breaking capacity sufficient”; Part 2 covers selectivity under over-current conditions, answering “which level trips on a fault”. Together they form a complete protection scheme.
A: Part 1 covers the application of short-circuit ratings (breaking capacity) and self-protection, answering “is the breaking capacity sufficient”; Part 2 covers selectivity under over-current conditions, answering “which level trips on a fault”. Together they form a complete protection scheme.
ā Q: What is an SCPD?
A: An SCPD (Short-Circuit Protective Device) is a device such as a fuse, circuit-breaker, control and protective switching device (CPS), or RCBO used to interrupt fault current quickly on a short circuit. Its breaking capacity should be no less than the prospective short-circuit current at the installation point.
A: An SCPD (Short-Circuit Protective Device) is a device such as a fuse, circuit-breaker, control and protective switching device (CPS), or RCBO used to interrupt fault current quickly on a short circuit. Its breaking capacity should be no less than the prospective short-circuit current at the installation point.
ā Q: What are total and partial selectivity?
A: Total selectivity means that at any overcurrent value only the downstream device operates and the upstream does not; partial selectivity means selectivity is satisfied only within a specified current range, beyond which both may operate. In practice the choice balances cost and risk.
A: Total selectivity means that at any overcurrent value only the downstream device operates and the upstream does not; partial selectivity means selectivity is satisfied only within a specified current range, beyond which both may operate. In practice the choice balances cost and risk.
š Q: How is upstream/downstream selectivity achieved?
A: Through time-current characteristic separation (upstream delay greater than downstream), energy (IĀ²t) matching, or zone interlocking. Verify against the manufacturer’s selectivity tables or curves during selection rather than relying on experience values.
A: Through time-current characteristic separation (upstream delay greater than downstream), energy (IĀ²t) matching, or zone interlocking. Verify against the manufacturer’s selectivity tables or curves during selection rather than relying on experience values.