Protection Coordination
Protection coordination per IEC 60898, IEC 60947-2, and IEC 60269. Fault level cascade, TCC curve generation, discrimination analysis, and protection verification.
Configure devices and click Calculate to see protection coordination analysis.
Discrimination, also called selectivity, is the coordination between series-connected protective devices so that only the device nearest the fault operates, leaving upstream circuits unaffected. IEC 60947-2 Annex A defines methods for verifying full and partial discrimination using time-current characteristic curves and manufacturers' selectivity tables for circuit breaker combinations.
How to Select a Circuit Protection Device
- 1Determine the design current — Calculate the design current Ib of the circuit from the connected load. For motor circuits, use the full-load current from the motor nameplate and consider starting current requirements.[IEC 60364-4-43 Clause 433.1]
- 2Select nominal device rating — Choose a protective device with nominal rating In where In is greater than or equal to Ib, and In is less than or equal to the cable's current-carrying capacity Iz after derating.[IEC 60364-4-43 Clause 433.1]
- 3Verify breaking capacity — Ensure the device's rated breaking capacity Icu exceeds the prospective short-circuit current at the installation point. The device must safely interrupt the maximum fault current.[IEC 60947-2 Clause 2]
- 4Check disconnection time — Verify the device disconnects the supply within the required time for the circuit type: 0.4 seconds for 230V final circuits or 5 seconds for distribution circuits, using the earth fault loop impedance.[BS 7671 Regulation 411.3.2]
- 5Verify cable thermal protection — Confirm the device's let-through energy I2t does not exceed the cable's adiabatic withstand capacity k2S2. This ensures the cable survives a short circuit without insulation damage.[IEC 60364-4-43 Clause 434.5]
- 6Check upstream discrimination — Verify discrimination with the upstream device using time-current curves or the manufacturer's selectivity tables. Full discrimination ensures only the device nearest the fault operates.[IEC 60947-2 Annex A]
How Protection Coordination Works
The protection coordination calculator verifies that upstream and downstream protective devices operate in the correct sequence during fault conditions, ensuring selectivity (discrimination) throughout the installation.
The calculator plots time-current characteristic (TCC) curves for circuit breakers and fuses using manufacturer data and standard trip curves (B, C, D per IEC 60898-1, or Type 1/2/3 per NEC). It checks that the downstream device trips before the upstream device across the full range of prospective fault currents. Per IEC 60947-2, selectivity is confirmed when the downstream device clears the fault within its breaking capacity while the upstream device remains closed.
BS 7671 Chapter 43 requires that protective devices disconnect the supply within the time limits of Regulation 411.3.2. Cable I2t withstand (adiabatic limit) is overlaid to verify conductor protection. Results include TCC coordination plots, selectivity assessment, cable damage curves, and recommended device settings.
MCB Instantaneous Trip Ranges (IEC 60898-1)
| Type | Trip Range (× In) | Application | Reference |
|---|---|---|---|
| Type B | 3–5× | Resistive loads, lighting, long cables | IEC 60898-1 Clause 8.6 |
| Type C | 5–10× | Moderate inductive loads, small motors | IEC 60898-1 Clause 8.6 |
| Type D | 10–20× | High inrush loads, transformers, motors | IEC 60898-1 Clause 8.6 |
Source: IEC 60898-1 Clause 8.6.1
Frequently Asked Questions
What is protection coordination and why is it important?
Protection coordination (discrimination or selectivity) ensures that only the protective device nearest to a fault operates, while upstream devices remain closed. This minimizes the extent of supply interruption. BS 7671 Regulation 536.2 recommends discrimination where necessary for safety. IEC 60947-2 Annex A defines total selectivity (discrimination guaranteed up to the maximum prospective fault current) and partial selectivity (discrimination up to a stated current level). Poor coordination can cause nuisance tripping of main breakers, leaving entire installations without power.
How do you achieve time-based discrimination between circuit breakers?
Time-based discrimination relies on the upstream device having a longer tripping time than the downstream device at every fault current level. Per IEC 60947-2, a minimum time grading margin of 0.15-0.4 seconds is required between devices, depending on the breaker technology. For moulded case circuit breakers (MCCBs), the upstream device should have a higher time setting and the curves must not overlap. This method is simple but adds fault clearance time, which increases cable I2t damage and arc flash energy at higher fault levels.
What is current-based discrimination and when is it used?
Current-based (current-grading) discrimination uses the difference in fault current magnitudes at different points in the network. The upstream device instantaneous setting is set above the maximum fault current that the downstream device will see. Per IEC 60947-2, a ratio of at least 1.5:1 between upstream and downstream instantaneous pickup settings is typically needed. This method works well when there is significant impedance (cable length) between devices that attenuates the fault current, providing a natural current difference.
How do I check discrimination between a fuse and a circuit breaker?
Discrimination between fuses and circuit breakers is verified by plotting both time-current characteristics on the same log-log TCC diagram. The fuse total pre-arcing I2t must exceed the circuit breaker total I2t (breaking energy) at every fault current up to the prospective fault level. Fuse manufacturers such as ABB, Schneider, and Eaton publish discrimination tables showing confirmed selectivity pairs. Per BS 88-2 and IEC 60269-1, the upstream fuse minimum pre-arcing time must be at least the downstream breaker total clearing time with a suitable margin at all current levels.
What is the cable damage curve and how does it relate to protection?
The cable damage curve (adiabatic line) represents the maximum time a cable can withstand a given fault current without insulation damage, calculated using I2t = k2S2 from IEC 60364-4-43 and BS 7671 Regulation 434.5.2. On a TCC diagram, the protective device characteristic must lie entirely below and to the left of the cable damage curve, ensuring the device disconnects before cable damage occurs. The constant k depends on conductor and insulation material (k = 115 for PVC/copper, k = 143 for XLPE/copper per Table 43.1 of BS 7671).
What are the maximum disconnection times required by BS 7671?
BS 7671 Regulation 411.3.2 and Table 41.1 specify maximum disconnection times for fault protection. For final circuits not exceeding 63A, the maximum time is 0.4 seconds for TN systems (120V-230V line-to-earth) and 0.2 seconds for TT systems. For distribution circuits, Regulation 411.3.2.3 permits 5 seconds for TN systems and 1 second for TT systems. These times ensure that touch voltages are limited to safe levels during earth faults, protecting persons from electric shock.
Why does a Type B MCB sometimes fail to discriminate with a Type C MCB upstream, even when the upstream device is rated at double the current?
Under IEC 60898-1:2015, the instantaneous trip range for Type B is 3-5 times In and for Type C is 5-10 times In. For a 16 A Type B downstream and 32 A Type C upstream, fault currents above 160 A (5 x 32) put both devices in their instantaneous regions with no guaranteed discrimination. At 500 A, both attempt to trip in under 10 ms and discrimination becomes a race between very fast mechanisms. IEC/TR 61912-2 provides tested combination tables, and many B/C combinations lose discrimination above 3-6 kA. The problem zone is specifically between 5 x In of the upstream device and the maximum prospective fault current. This is why BS 7671 Regulation 536.4 recommends consulting manufacturer discrimination tables rather than assuming ratio-based rules.
What is cascading (back-up protection) under IEC 60947-2, and why does it allow apparently underrated downstream devices?
Cascading per IEC 60947-2:2016 Annex A allows a downstream device rated below the prospective fault current to be used when a specific upstream current-limiting device has been type-tested in combination. The upstream device begins opening during the first half-cycle, inserting arc voltage that reduces peak let-through current before the downstream device completes its opening. The downstream device only ever experiences a fraction of the prospective fault current. This allows 25 kA-rated outgoing MCCBs behind a 50 kA-rated incomer, saving approximately 30-40% on switchgear costs. The trap is that combinations are manufacturer-specific and product-specific. You cannot mix brands or substitute models without revalidation. BS 7671 Regulation 536.3 addresses this.
How does the let-through energy (I2t) of a fuse or MCB interact with the cable's thermal withstand, and why is checking only the trip time insufficient?
BS 7671 Regulation 434.5.2 requires that the protective device's I2t not exceed the cable's k2 x S2. Many engineers check only whether the device trips within the required time for shock protection, but even if trip time is adequate, the let-through energy could damage the cable. This is most acute with fuses at moderate fault currents in the knee region of the TCC. A 63 A gG fuse at 500 A takes approximately 5 seconds with I2t of 1,250,000 A2s. A 6 mm2 XLPE/Cu cable has k2S2 of only 735,564 A2s, so it fails even though it can carry 63 A per Method E. At higher fault currents (3 kA), the fuse becomes current-limiting with much lower I2t. The dangerous zone is the middle ground where faults are not high enough for current-limiting operation.
Why does zone-selective interlocking (ZSI) dramatically improve both protection coordination and arc flash results?
ZSI is a communication scheme per IEC 60947-2:2016 Annex M where a downstream device sends a restraint signal to the upstream device. Without ZSI, achieving discrimination requires the upstream device to have a longer time delay, increasing arc flash energy. With ZSI, when a fault occurs on a final circuit, the downstream MCCB restrains the upstream ACB and trips in approximately 30 ms. If the fault is between devices (no restraint signal), the ACB trips without delay in approximately 50 ms. Without ZSI, the ACB must wait 300 ms on a busbar fault to maintain discrimination, producing approximately 25 cal/cm2. With ZSI, the ACB trips in 50 ms, producing approximately 5 cal/cm2, an 80% reduction. IEEE 1584-2018 Annex B.3 recommends ZSI as one of the most effective arc flash mitigation strategies at minimal cost.
What is the cross-over point problem in fuse-MCB coordination, and why does every protection engineer need to check it manually?
When a fuse is upstream of an MCB, the cross-over point is the fault current where the MCB's instantaneous trip becomes faster than the fuse's minimum pre-arcing time. Below this current, the MCB trips thermally and the fuse curve sits above, maintaining discrimination. Above this current, both devices attempt to operate simultaneously. For a 100 A gG fuse upstream of a 32 A Type C MCB, the MCB trips magnetically at 160-320 A. At 320 A, the fuse pre-arcs in approximately 1 second versus the MCB's 5-15 ms. But at 3000 A, the fuse pre-arcs in approximately 10 ms and the MCB in approximately 5 ms. Manufacturer TCCs show minimum and maximum bands, not single lines, and these bands overlap at high fault currents. IEC 60269-2 and IEC 60898-1 do not provide a standardised verification method, requiring actual I2t data from both manufacturers.
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Standards Reference
- IEC 60947-2 — Circuit breakers
- BS 7671:2018+A2 — Chapter 43
- IEC 60269 — Fuses