3.5 Industrial Switchgear, Circuit Breakers (MCB, MCCB), and Fuses

3.5 Industrial Switchgear, Circuit Breakers (MCB, MCCB), and Fuses

Protective switchgear isolates electrical faults—overloads and short circuits—before conductor insulation melts, fires ignite, or equipment suffers irreparable damage. In Singapore industrial installations governed by SS 638 and IEC standards, protective devices must be meticulously selected based on load current ($I_n$), instantaneous tripping characteristic, breaking capacity ($I_{cu} / I_{cs}$), and selectivity (discrimination).

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1. Miniature Circuit Breakers (MCB - SS 245 / IEC 60898 / IEC 60947-2)

Miniature Circuit Breakers combine dual tripping mechanisms within a compact molded housing:

1. Thermal Element (Inverse-Time Overload Protection): Bimetallic strip that bends under sustained low-level overcurrents.
2. Magnetic Element (Instantaneous Short-Circuit Protection): Solenoid coil that fires an armature to trip the breaker instantly (<10 ms) when current exceeds a predetermined threshold.

Instantaneous Tripping Characteristics (Type B, C, D)

IEC 60898 classifies MCBs into standard tripping curves based on the multiple of rated current ($I_n$) required to trigger instantaneous magnetic tripping:

Example: A 16A Type C MCB trips magnetically between $80\text{ A}$ and $160\text{ A}$. A 16A Type D MCB trips magnetically between $160\text{ A}$ and $320\text{ A}$.

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2. Moulded Case Circuit Breakers (MCCB - IEC 60947-2)

Moulded Case Circuit Breakers are used for higher current ratings (typically 16A up to 1600A; up to 100A for LEW Electrician licence scope) and higher fault interrupting duties in sub-distribution boards.

Adjustable Trip Settings

Unlike fixed-rating MCBs, industrial MCCBs feature adjustable trip units:

• Thermal Overload Current ($I_r$): Adjustable between $0.7 \text{ to } 1.0 \times I_n$ to match exact cable current-carrying capacity.
• Short-Time / Magnetic Pickup ($I_m$): Adjustable between $5 \text{ to } 10 \times I_r$ to coordinate with downstream devices.
• Electronic / Microprocessor Trip Units (LSI): Provide precise independent adjustments for Long-time delay (overload), Short-time delay (selective short circuit), and Instantaneous trip (catastrophic fault).

Short-Circuit Breaking Capacity Parameters

• Rated Ultimate Short-Circuit Breaking Capacity ($I_{cu}$): The maximum short-circuit fault current (in kA) that the MCCB can successfully clear at rated voltage. Following an $I_{cu}$ fault test sequence (Open - 3 min - Close/Open), the breaker is considered unsafe for continued service.
• Rated Service Short-Circuit Breaking Capacity ($I_{cs}$): The short-circuit current that the MCCB can clear repeatedly without suffering performance degradation or loss of capability. $I_{cs}$ is expressed as a percentage of $I_{cu}$ (typically 50%, 75%, or 100%). In critical industrial plants, MCCBs should be specified with $I_{cs} = 100% I_{cu}$.

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3. High Rupturing Capacity (HRC) Fuses (BS 88 / IEC 60269)

HRC fuses remain widely specified in industrial switchgear due to their extremely high prospective breaking capacity and ultra-fast current-limiting performance.

Construction & Arc Quenching

An HRC fuse consists of a high-grade ceramic barrel enclosing a pure silver fuse element with calibrated neck reductions. The barrel is tightly packed with high-purity silica quartz sand. Under short-circuit conditions, the silver element vaporizes in less than 2 to 5 milliseconds. The surrounding quartz sand fuses with the silver vapor to form a non-conducting glass fulgurite, rapidly quenching the arc.

Fuse Classifications

• Class gG: Full-range breaking capacity fuse for general application (protects both overloads and short circuits in cables and distribution feeders).
• Class gM: Dual-rated motor circuit protection fuse (e.g., 63M100). The first digit (63A) denotes continuous current rating for thermal sizing; the second digit (100A) denotes the motor starting withstand characteristic.

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4. Discrimination and Selectivity (Series Device Co-ordination)

Selectivity (discrimination) is the coordination of series-connected protective devices such that a fault occurring on a final sub-circuit causes only the protective device immediately upstream of the fault to operate, while main upstream switchboard breakers remain closed.

Rules for Achieving Selectivity under SS 638

1. Fuse-to-Fuse Discrimination: Upstream HRC fuse rating must be at least $1.6 \times$ the downstream fuse rating (e.g., 100A upstream to 63A downstream).
2. Fuse-to-MCB Discrimination: Upstream HRC fuse rating should be at least $2.0 \times$ the downstream MCB rating to ensure the MCB operates magnetically before the fuse begins to melt.
3. MCCB-to-MCCB Discrimination: Achieved by setting a short-time delay ($t_{sd} = 100 - 300\text{ ms}$) on the upstream Category B MCCB, allowing the downstream Category A MCCB to clear the fault instantaneously without tripping the main feeder.

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5. Prospective Fault Current (PFC) & Safety Verification

Prospective Fault Current ($I_{pf}$) is the maximum prospective short-circuit current that can flow at a specific point in an electrical installation during a solid zero-impedance line-to-line or line-to-earth fault.

Mandatory SS 638 Safety Rule

The Rated Short-Circuit Breaking Capacity ($I_{cu}$ or $I_{cn}$) of EVERY protective device installed in a switchboard MUST BE EQUAL TO OR GREATER THAN the Prospective Fault Current ($I_{pf}$) at its point of installation.

$I_{cu} \ge I_{pf}$

If a sub-distribution board installed near a transformer room experiences a prospective fault current of $15\text{ kA}$, installing standard 6 kA MCBs is a catastrophic safety hazard. Under fault conditions, an undersized breaker will fail violently, creating an explosive arc flash, destroying switchgear, and risking fatal injury to personnel.