Protection Coordination: Same Cascade, Different Discrimination Rules

Scenario: A 63A MCCB upstream of a 32A MCB downstream. A 10kA fault occurs on the load side of the MCB. Both devices see the fault. The question: does the MCB trip first (before the MCCB), isolating only the faulted circuit while leaving the rest of the distribution board energized? IEC 60947 calls this "discrimination." NEC calls it "selective coordination." BS EN 60898 calls it "discrimination." AS/NZS calls it "discrimination." They are not just different words -- they have different pass/fail criteria, different testing methods, and different compliance requirements.

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The Scenario

A typical distribution arrangement in a commercial building:

Supply (400V 3-phase)
    |
    [63A MCCB]  --- Upstream device (Type B, 25kA Icu)
    |
    Distribution Board
    |--- [32A MCB]  --- Downstream device (Type B, 10kA Icn)
    |        |
    |      Load circuit (fault location: 10kA bolted fault)
    |
    |--- [32A MCB]  --- Other circuits (should remain energized)
    |--- [20A MCB]
    |--- [16A MCB]

• Upstream device: 63A MCCB, IEC 60947-2 Type B (instantaneous trip at 3-10x In)
• Downstream device: 32A MCB, IEC 60898 / BS EN 60898, Type B (3-5x In)
• Fault current at downstream device: 10kA symmetrical
• Goal: The 32A MCB must trip and clear the fault BEFORE the 63A MCCB trips. If both trip, the entire distribution board loses power -- all circuits go dark, not just the faulted one.

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Terminology: The First Source of Confusion

The critical difference: NEC requires selective coordination for emergency, legally required standby, and critical operations power systems. The other three standards recommend discrimination but do not mandate it for all installations.

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How Each Standard Defines Success

IEC 60947-2: Discrimination by Time-Current Characteristic

IEC 60947-2 (Circuit breakers for industrial use) defines discrimination in Annex A:

Total discrimination: The downstream device clears the fault alone, at all fault levels up to its rated capacity, without the upstream device tripping.

Partial discrimination: Discrimination is achieved up to a stated fault level (the "discrimination limit"), above which both devices may trip.

The IEC method uses time-current characteristic curves overlaid:

1. Plot the time-current curve of the downstream 32A MCB
2. Plot the time-current curve of the upstream 63A MCCB
3. At every fault current level, verify that the downstream device clears faster than the upstream device begins to operate

IEC 60947-2, Annex A test method:

The discrimination limit (Is) is the highest fault current at which the downstream device interrupts the fault without the upstream device opening. This is determined by testing or by characteristic curve analysis.

For our scenario:

Downstream: 32A Type B MCB
  - Instantaneous trip: 3-5 x 32 = 96-160A (magnetic trip zone)
  - At 10kA: operates in < 5ms (within the instantaneous region)

Upstream: 63A Type B MCCB
  - Instantaneous trip: 3-10 x 63 = 189-630A (adjustable)
  - At 10kA: operates in < 10ms (within the instantaneous region)

IEC discrimination assessment:

At 10kA, both devices are in their instantaneous trip zones. The question is whether the downstream MCB can interrupt 10kA and generate an I2t (let-through energy) low enough that the upstream MCCB does not begin to unlatch.

The IEC method compares:

• Downstream pre-arcing I2t (the energy the MCB lets through before arc extinguishes)
• Upstream total I2t threshold (the energy needed to cause the MCCB to trip)

If downstream pre-arcing I2t < upstream total trip I2t, discrimination is achieved.

Typical values (manufacturer data):

Downstream pre-arcing I2t = 15,000 A2s
Upstream total I2t threshold = 50,000 A2s

15,000 < 50,000  -->  DISCRIMINATION ACHIEVED (IEC method)

IEC result: PASS -- total discrimination at 10kA.

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BS 7671:2018+A2 / BS EN 60898

BS 7671, Regulation 536.4 states:

"Where discrimination between protective devices in series is required, the characteristics of the devices shall be selected such that, in the event of a fault, only the protective device on the supply side of the fault operates."

BS 7671 references the IEC method (time-current curves and I2t comparison) but adds practical guidance through the IET Guidance Note 6 (Protection Against Overcurrent):

BS method for MCB-MCB or MCB-MCCB coordination:

1. Overload range (up to 10x In): Check time-current curves do not overlap
2. Short-circuit range (above 10x In): Compare I2t values from manufacturer data
3. Verify at the actual prospective fault current at the downstream device location

BS EN 60898 (the MCB product standard) requires manufacturers to provide I2t characteristics for discrimination assessment. This is published as "energy limiting class" data.

BS 7671 additional requirement -- Regulation 536.4.1:

When discrimination is required for safety (e.g., fire alarm circuits, life safety), the designer must verify discrimination across the full range of prospective fault currents, not just at the maximum.

For our scenario:

At overload (100-200A):
  32A MCB trips in 10-60 seconds
  63A MCCB does not trip (100A is within its rated current)
  --> Discrimination achieved

At moderate fault (500A):
  32A MCB trips in 0.1-0.5 seconds (thermal-magnetic transition)
  63A MCCB trips in 5-30 seconds (thermal region only)
  --> Discrimination achieved

At high fault (10kA):
  Both devices in instantaneous zone
  I2t comparison: 15,000 < 50,000
  --> Discrimination achieved

BS 7671 result: PASS -- discrimination achieved across full range.

However: BS 7671 explicitly recognises that total discrimination may not always be achievable and permits "back-up protection" (Regulation 536.4.3) where the upstream device provides fault-breaking capacity that the downstream device lacks. In a back-up arrangement, both devices may trip simultaneously, and this is acceptable if the upstream device is rated to break the fault current.

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NEC / NFPA 70:2023 -- Selective Coordination

NEC defines selective coordination in Article 100:

"Selective Coordination (Selective). Localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the selection of overcurrent protective devices and their ratings or settings."

NEC requires selective coordination for specific systems:

The NEC requirement is absolute for these systems. It is not "recommended" or "where required by the designer" -- it is mandatory. This is stricter than IEC, BS, or AS/NZS for these specific system types.

NEC assessment method:

NEC does not prescribe a specific testing method (unlike IEC Annex A). Instead, it requires that the designer demonstrate selective coordination through:

1. Manufacturer coordination tables (published selectivity matrices)
2. Time-current curve analysis (manual or software-based)
3. Testing to UL standards (UL 489 for MCBs, UL 1008 for transfer switches)

For our scenario:

If this is a general distribution circuit (Article 240):

Selective coordination is not required. The 63A MCCB and 32A MCB can be a series-rated combination per Article 240.86, where both may trip during a fault. NEC permits this with appropriate labelling.

If this is an emergency system (Article 700):

Selective coordination IS required. The designer must prove that at every fault level from overload up to the available fault current, the 32A MCB trips before the 63A MCCB.

NEC 700.32: "Emergency system(s) overcurrent devices shall be
selectively coordinated with all supply-side overcurrent
protective devices."

This means the I2t analysis is mandatory, and if the devices do not discriminate at any fault level, the design must be revised (different devices, time-delay settings, or fuses instead of MCBs).

NEC result for emergency system: Assessment required per 700.32. With our specific devices (I2t: 15,000 < 50,000), selective coordination is achieved at 10kA. But the designer must verify this for the FULL range up to the available fault current at the 63A MCCB location.

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AS/NZS 3000:2018

AS/NZS 3000, Clause 2.5.5 addresses discrimination:

"Where required, discrimination between protective devices in series should be provided such that, in the event of an overcurrent, the protective device immediately on the supply side of the fault operates before the upstream protective device."

Note the word "should" -- this is a recommendation, not a mandatory requirement in the Australian standard for general installations.

AS/NZS 3000 references AS/NZS 60947.2 (the Australian adoption of IEC 60947-2) for the technical method of assessing discrimination. The approach is identical to IEC:

1. Compare time-current curves in the overload region
2. Compare I2t values in the short-circuit region
3. Determine the discrimination limit

AS/NZS additional consideration -- back-up protection (Clause 2.5.6):

AS/NZS 3000 explicitly permits back-up protection (cascade) arrangements where the upstream device provides fault-breaking capacity for the downstream device. In this arrangement:

• The downstream device has a lower breaking capacity than the prospective fault current
• The upstream device limits the fault current and energy
• Both devices may trip simultaneously
• The combination must be tested or verified by the manufacturer

For our scenario:

32A MCB rated at 10kA Icn
Prospective fault current = 10kA
The MCB can handle the fault alone -- back-up is not needed.

Discrimination check:
  I2t (downstream pre-arcing) = 15,000 A2s
  I2t (upstream total) = 50,000 A2s
  15,000 < 50,000 --> Discrimination achieved

AS/NZS result: Discrimination achieved, but not mandated for general circuits.

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The Critical Difference: Mandatory vs. Recommended

NEC stands alone in requiring selective coordination for specific system types. The other three standards treat discrimination as a design goal, not a compliance requirement.

This has enormous practical implications:

• In a US hospital, the emergency power system MUST have selective coordination at every level -- from the generator breaker down to the final circuit MCB. This often requires fuses (which have more predictable discrimination characteristics than MCBs) or electronic trip units with adjustable time delays.

• In a UK hospital to BS 7671, discrimination is recommended for life safety circuits but the designer has discretion. If discrimination cannot be achieved with standard devices, the designer can accept the risk and document the decision.

• In an Australian hospital to AS/NZS 3000, discrimination is recommended but not enforced. Back-up protection (cascade) is explicitly permitted as an alternative.

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Practical Discrimination Assessment

Method 1: Manufacturer Selectivity Tables

Most major manufacturers (Schneider Electric, ABB, Siemens, Eaton) publish selectivity/discrimination matrices showing which upstream-downstream combinations achieve total discrimination.

Example format (simplified):

The larger the ratio between upstream and downstream ratings, the easier discrimination becomes.

Method 2: Time-Current Curve Overlay

Plot both device curves on the same axes. At every current level, verify the downstream curve is below (faster than) the upstream curve.

Critical zone: The area where both curves enter the instantaneous (magnetic) trip region. For circuit breakers of the same type (both Type B), the instantaneous trip bands may overlap:

32A Type B: Instantaneous at 96-160A
63A Type B: Instantaneous at 189-630A

Between 189A and 630A, BOTH devices are in or approaching their instantaneous zones. In this overlap region, discrimination depends on I2t energy limiting, not time-current curves.

Method 3: I2t Energy Comparison (Short-Circuit Region)

At fault currents above both devices' instantaneous thresholds, the faster device wins. The I2t comparison determines this:

Rule: Downstream pre-arcing I2t < Upstream pre-arcing I2t

If this holds, the downstream device begins to arc and interrupt
BEFORE the upstream device even starts to unlatch.

For our 32A MCB / 63A MCCB combination at 10kA:

32A MCB pre-arcing I2t: ~15,000 A2s (from manufacturer data sheet)
63A MCCB pre-arcing I2t: ~50,000 A2s

15,000 << 50,000  -->  Clear discrimination

The 2:1 ratio between upstream and downstream current ratings (63A:32A) provides comfortable discrimination. As a general rule of thumb:

• Ratio ≥ 2:1: Discrimination usually achievable
• Ratio 1.6:1 to 2:1: Marginal -- verify with manufacturer data
• Ratio < 1.6:1: Discrimination difficult or impossible with standard MCBs

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When Discrimination Fails: Practical Solutions

Solution 1: Increase the Current Rating Ratio

Replace the 63A MCCB with a 100A or 125A MCCB. The larger upstream device has higher I2t thresholds and slower thermal response, improving discrimination.

Solution 2: Use Time-Delay Upstream Device

Replace the 63A MCCB with a time-delay (short-time delay) type. The upstream device waits 50-200ms before tripping, allowing the downstream MCB to clear the fault first.

Without delay: Both trip in < 10ms at 10kA
With 100ms delay: MCB trips in 5ms, MCCB waits until 100ms -- discrimination achieved

IEC 60947-2 Category B MCCBs have intentional short-time delay capability (Icw rating). Standard MCBs (IEC 60898) do not.

Solution 3: Use Fuses

Fuses have extremely predictable I2t characteristics and a wide range of time-current curve shapes. A 63A HRC fuse upstream of a 32A MCB provides excellent discrimination because the fuse's pre-arcing I2t at 10kA is typically 100,000+ A2s -- far above the MCB's total I2t.

This is why NEC-compliant emergency systems in the US often use fuses rather than circuit breakers for the upstream protection -- fuses are easier to coordinate.

Solution 4: Zone-Selective Interlocking (ZSI)

ZSI is a communication system between upstream and downstream devices:

1. Both devices detect the fault current
2. The downstream device sends a "restraint" signal to the upstream device
3. The upstream device adds a time delay (typically 50-100ms)
4. The downstream device trips first
5. If the downstream device fails to clear the fault, the upstream device trips after the delay expires

ZSI achieves total discrimination at all fault levels without requiring the upstream device to have inherently higher I2t thresholds. It is increasingly common in modern switchgear, particularly for NEC Article 700 compliance.

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Cost of Compliance: NEC Selective Coordination Premium

Achieving mandatory selective coordination under NEC 700/701/708 adds significant cost:

For a hospital with 50 distribution boards on the emergency system, the selective coordination premium can reach $250,000-$500,000. This is a real cost that NEC imposes and other standards do not.

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Practical Implications for Multi-Jurisdiction Engineers

1. "Discrimination" and "selective coordination" are not interchangeable terms across standards. IEC discrimination permits partial discrimination up to a stated limit. NEC selective coordination (for emergency systems) requires total coordination across all fault levels. Using the wrong term in a specification can lead to under-designed protection.

2. NEC 700.32 is the strictest protection coordination requirement of any major standard. If you are designing emergency systems in the US, budget for a formal coordination study and potentially more expensive protective devices.

3. For general circuits, all four standards are lenient. Discrimination is recommended but not mandated. Back-up protection (cascade) is permitted. In practice, most general distribution boards do not achieve full discrimination, and this is acceptable.

4. The 2:1 current ratio rule of thumb works across all standards. If the upstream device is at least twice the rating of the downstream device, discrimination is usually achievable with standard devices. Below 2:1, engineering analysis is required.

5. Always use manufacturer-specific coordination data. Generic time-current curves are insufficient for formal coordination studies. Contact the device manufacturer for specific I2t data and coordination tables. Most manufacturers provide free coordination software (e.g., Schneider Ecodial, ABB DOC, Eaton XPRT).

6. Fuses are easier to coordinate than circuit breakers. If selective coordination is mandatory and the budget is tight, fuses on the upstream protection are often the most cost-effective solution. The trade-off is manual fuse replacement after a fault versus automatic re-closing with a circuit breaker.

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Summary: Same Cascade, Four Standards

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Related Resources

• Protection Discrimination: MCB-Fuse Coordination -- Why fuses coordinate more easily than MCBs
• Short Circuit Withstand: Distribution Board -- Fault levels that feed into coordination studies
• Earth Fault Loop Impedance Comparison -- Protection speed depends on loop impedance
• Cable Sizing: The 50m Office Feeder -- Cable size affects available fault current
• Protection Coordination Calculator -- Verify discrimination for your specific device combination