11.3 Breaking Capacity Coordination with PFC

Breaking Capacity Coordination with PFC

Quick Answer: At each protective device, prospective fault current (PFC) must not exceed the device’s breaking capacity (Icu, and Ics where service breaking capacity matters), unless a compliant backup / cascade arrangement covers the duty. Meeting Zs,max for 0.4 s / 5 s disconnection does not automatically prove breaking-capacity coordination.

Two Different Pass/Fail Questions

Candidates often blend Chapter 11 ideas into one blurry “fault” check. Separate them:

A device can sit below Zs,max (good disconnection) yet have Icu far below local PFC (dangerous interruption). Conversely, a 10 kA-rated breaker can still fail verification if Zs is too high for 0.4 s clearing. Both checks are required competence for the capstone.

Prospective Fault Current at the Installation Point

Prospective fault current (PFC) — also discussed as prospective short-circuit current — is the current that would flow for a defined bolted fault at a stated point, limited by the impedance upstream of that point. Chapter 10 developed PFC conceptually; here we coordinate it with device ratings.

Key properties:

• PFC is not the design load Ib and is not the breaker rating In.
• PFC is usually highest near the origin (main switchboard close to a stiff MEN supply) and reduces downstream as cable impedance is added.
• Three-phase bolted faults, phase-to-neutral faults and earth faults can have different prospective values; switchgear selection commonly uses the relevant worst-case figure required by the design method and manufacturer data.

Distributor advice, calculation from Ze / supply data, or measurement with suitable instruments informs the design PFC used for device selection.

Breaking Capacity — Icu and Ics

Circuit-breakers carry interruption ratings such as:

• Icu — ultimate breaking capacity: the highest fault current the breaker can interrupt under defined test conditions (often after which the device may not be suitable for continued service without assessment/replacement per manufacturer rules).
• Ics — service breaking capacity: the fault level the breaker can interrupt and remain fit for further service, expressed as a percentage of Icu or an absolute value depending on product marking.

Fuses have corresponding breaking capacity / rupturing capacity ratings. Whatever the product family, the installation rule of thumb for teaching is:

Device fault interruption rating ≥ PFC at the point of installation

(or a documented cascade/backup arrangement that makes the combination safe).

Worked coordination thought experiment

• Measured / calculated PFC at a main switchboard = 6 kA.
• Domestic MCBs marked 6 kA Icu → borderline acceptable if the figure truly covers the point of installation and any required factors in your design method.
• Same board with PFC 10 kA and only 6 kA MCBs → non-compliant unless upstream backup devices and manufacturer cascade data allow the combination.

Do not “fix” this by claiming the final-circuit Zs is high so earth-fault If is only a few hundred amperes. Breaking-capacity assessment addresses the prospective fault the device may be asked to interrupt for the fault types considered at that board — including low-impedance short-circuits — not only a distant high-Zs earth fault.

Cascade / Backup — When Upstream Devices Help

Sometimes a downstream breaker with modest Icu is used where PFC exceeds that Icu, provided an upstream device (fuse or MCCB) limits let-through energy and manufacturer cascade / backup tables endorse the combination. That arrangement can satisfy interruption safety but often reduces discrimination: a heavy fault may take out upstream protection as well.

Capstone literacy:

• Backup is a documented coordination method, not a hope that “something upstream will blow.”
• Backup does not cancel Zs verification on finals.
• Prefer devices whose Icu alone covers PFC when selectivity and simplicity matter.

Coordinating the Full Fault Story on One Board

A competent checklist at a Queensland switchboard looks like this:

1. Establish PFC (and related prospective values) at the board.
2. Select main and outgoing devices with Icu / Ics (or fuse breaking capacity) ≥ PFC, or apply approved cascade data.
3. For each final / distribution circuit, verify earth-fault-loop impedance against Zs,max for the correct 0.4 s or 5 s time.
4. Confirm Ib ≤ In ≤ Iz overload coordination (Chapter 7) still holds.
5. Confirm RCD additional protection where required (Chapter 8).
6. Confirm MEN, earthing and bonding integrity (Chapter 9).

Skipping step 2 because step 3 passed is a classic incomplete design. Skipping step 3 because devices are “10 kA rated” is equally incomplete.

Relationship Back to Zs and If

Remember the earth-fault teaching current If ≈ U0 / Zs. That If is what the time–current curve uses for disconnection timing. The PFC used for breaking capacity is typically a bolted-fault prospective value at the device — often much larger than the earth-fault If at the end of a long final.

So:

• Distant outlet: Zs may dominate the 0.4 s pass/fail.
• Main board devices: PFC vs Icu often dominates the interruption pass/fail.

Both ends of the same installation must be right.

Test-Day and Defect Themes

On practical assessment:

• Read the kA marking on breakers and compare with stated or measured fault level for that board.
• Do not assume every imported MCB is 10 kA; many domestic devices are 6 kA.
• After a major fault, devices that interrupted near Icu may need replacement even if they appear resettable — follow manufacturer and safety guidance.
• High measured Zs on a final is still a fail even if the board’s breakers are generously rated for PFC.

Exam Traps Specific to This Section

• Equating In = 32 A with “breaking capacity 32 A.”
• Saying Zs,max compliance proves Icu ≥ PFC.
• Using load current or maximum demand as PFC.
• Ignoring that PFC at a sub-board can be lower — and selecting main-board gear using the sub-board’s lower figure incorrectly, or the reverse.
• Claiming RCDs provide short-circuit breaking capacity for bolted faults.

Capstone Closing Line

For Queensland electrical licence work: Zs proves the MEN loop will drive timely disconnection; Icu ≥ PFC proves the device can survive interrupting the available fault. Learn both sentences cold — markers treat them as separate critical EPCs even when a single scenario mentions “fault current.”

Bridge Across Chapter 11

Section 11.1 gave Zs = Ze + R1 + R2 and If = U0/Zs. Section 11.2 fixed 0.4 s versus 5 s. Section 11.3 closes with breaking capacity ↔ PFC. Together they form the short-circuit and earth-fault-loop competence block listed in exam-meta topic domains — essential for a safe, verifiable MEN installation.