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Protection coordination refers to the systematic arrangement and interaction of protective devices within an electrical distribution network to ensure that faults are isolated in a controlled and orderly manner. The objective is to minimise the impact of electrical faults by ensuring that only the smallest possible section of the system is de-energised, thus maintaining continuity of service elsewhere. This involves setting the protective devices, such as circuit breakers, fuses, and relays, so upstream devices (closer to the power source) and downstream devices (closer to the load) operate in a hierarchical sequence.
Critical Aspects of Protection Coordination:
| Designation | Title | Sections |
|---|---|---|
| AS/NZS 3000:2018 | Electrical Installations “Wiring Rules” | Clause 2.5.7.2.2 Clause 2.5.7.2.3 |
AS/NZS 3000:2018, also known as the Wiring Rules, is a joint Australian/New Zealand standard that outlines the requirements for electrical installations.
Clause 2.5.7.2.2 states: Discrimination is achieved using a discrimination study, the ratios shown below or the manufacturer’s data and tables. Discrimination need not apply above the arcing fault current Iarc, which is deemed to be in the range of 30% to 60% of the prospective short-circuit current. Discrimination need not be applied where protective devices are in series on the same circuit, such as in UPS-connected supplies.
Clause 2.5.7.2.3 states: C2 is the downstream circuit breaker, and C1 is the upstream circuit breaker. F2 is the downstream fuse, and F1 is the upstream fuse.
| Scenario | Requirements | How to achieve |
|---|---|---|
| Rating of C2 ≥ 800 A | Discrimination is required between overload curves and instantaneous settings, but need not apply above the arcing fault current (Iarc). | By the manufacturer’s data |
| Rating of 250 A ≤ C2 < 800 A | For ratings of C2 greater than or equal to 250 A, and less than 800 A, discrimination shall be provided between overload curves. | Discrimination is deemed to be achieved if the overload setting of C1 ≥ 1.5 × C2, e.g. C1 1000 A with C2 630 A |
| Rating of C2 < 250 A | Discrimination is required between overload curves and is recommended up to the instantaneous setting (Ii) or short-time pickup (ISD) of C1 but need not apply above the arcing fault current (Iarc). Exception: For ratings of C2 ≤ 80, discrimination is not required. | Discrimination is deemed to be achieved if C1 ≥ 1.5 × C2, e.g. C1 MCB marked C63 with MCB C2 marked C40 (i.e. both C curves). |
| Fuses | Provide discrimination (selectivity) on overload. | Discrimination (selectivity) between HRC fuses is deemed to be achieved— For overload when F1 ≥ 1.6 × F2, e.g. 16 A with 10 A; and For short-circuit when F1 ≥ 2 × F2, e.g. 20 A with 10 A1. |
| C2 and F1 | Provide discrimination (selectivity) between the overload curve and the instantaneous or short delay setting (ISD) and the time-current curve of F1. | - |
Notes:
Protection coordination is a crucial aspect of electrical system design, and several terms are used interchangeably or in specific contexts to describe this concept. Here are the key terms:
| Terms Used | Meanings |
|---|---|
| Selective Coordination | Ensures that only the protective device closest to the fault operates, isolating the fault with minimal disruption to the rest of the system. |
| Discrimination | Refers to the ability of protective devices to distinguish between normal operating currents and fault conditions, ensuring that only the appropriate device responds to a fault. |
| Coordination of Protective Devices | General term for setting protective devices so they operate in a predetermined sequence to isolate faults effectively. |
| Overcurrent Coordination | This is specific to coordinating devices that protect against overcurrent conditions, such as circuit breakers and fuses, to ensure they operate correctly together. |
| Time-Current Coordination | It involves setting the time and current characteristics of protective devices so they respond appropriately to fault conditions and maintain a coordinated response. |
| Cascade Coordination | Refers to a hierarchical approach where protective devices are coordinated to trip in succession, starting from the downstream device closest to the fault and moving upstream if necessary. |
| Series Coordination | Focuses on coordinating devices in series within a circuit to ensure the appropriate device trips based on fault conditions. |
| Protection Grading | The process of assigning different protection settings to devices in various zones or levels of the electrical system ensures optimal fault isolation and system protection. |
A protective device coordination study, also known as a coordination analysis or protection coordination study, is a detailed analysis designed to ensure that protective devices within an electrical power system operate in a coordinated manner. The primary goal is to optimise the performance of these devices to isolate faults efficiently while minimising disruption to the rest of the system.
1. Data Collection:
2. System Modelling:
3. Time-Current Characteristic (TCC) Curves:
4. Coordination Analysis:
5. Simulation of Fault Conditions:
6. Verification and Adjustment:
7. Documentation: