Cascading Protection Devices
Cascading in electrical systems (for kA rating) refers to the coordination between circuit breakers such that a downstream (lower-rated) circuit breaker is protected by an upstream (higher-rated) circuit breaker during short-circuit (fault) conditions. This allows the downstream breaker to have a lower short-circuit withstand rating (kA rating) than what the actual fault level at that point would require.
🔧 What Is the kA Rating?
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kA (kiloampere) rating is the short-circuit current withstand capacity of a circuit breaker.
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It indicates the maximum fault current the breaker can safely interrupt without being destroyed.
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For example, a breaker with a 6 kA rating can safely interrupt a fault current of up to 6,000 amps.
⚡ The Problem:
In a distribution system, the fault current close to the supply source (like a transformer or main DB) can be very high, e.g., 25 kA, while downstream devices may be rated for only 6 kA or 10 kA.
Replacing all downstream breakers with high kA-rated breakers is expensive and unnecessary. This is where cascading (also known as back-up protection) comes in.
🔁 Cascading / Back-Up Protection:
This is a method where:
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The main upstream breaker (high kA rating) is designed to break the high fault current.
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The downstream breakers (lower kA rating) are protected because the upstream breaker clears the fault before the downstream one is damaged.
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This allows the use of cheaper, lower-kA-rated breakers in sub-distribution boards.
✅ Conditions for Cascading to Work:
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Manufacturer-tested combinations: Cascading must be tested and approved by the manufacturer. You cannot assume protection without documentation.
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Distance and wiring between upstream and downstream breakers must match the tested setup.
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Same brand / product line is often required to maintain valid coordination.
🧮 Example:
| Position | Breaker Rating | Fault Level | Comment |
|---|---|---|---|
| Main DB | 25 kA MCCB | 25 kA | Can clear full fault |
| Sub DB | 6 kA MCB | 16 kA | Protected by upstream MCCB |
| Final DB | 4.5 kA MCB | 6 kA | Protected by Sub DB breaker |
Without cascading, the 6 kA breaker would fail at 16 kA fault level. With cascading tested and applied, the upstream breaker clears the fault and protects the downstream device.
📘 Standards Reference:
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IEC 60947-2 and IEC 60898 (for circuit breaker coordination)
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SANS 10142-1:2024 – Refer to section on protection coordination (usually in Chapter 6 or annexes).
⚠️ Important:
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Cascading ≠ Selectivity. Cascading is about protection, while selectivity is about ensuring only the faulty circuit is disconnected.
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Use the manufacturer’s cascading tables or software (e.g., Schneider's Ecodial, ABB's DOC, etc.) for proper coordination.
🔚 Summary:
Cascading allows lower-rated breakers to be used in high-fault areas by relying on an upstream breaker to provide back-up protection, provided the arrangement is tested and approved by the manufacturer.
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