2.4 Overcurrent Protection, Circuit Breakers & Discrimination

2.4 Overcurrent Protection, Circuit Breakers & Discrimination

In electrical installation design, protective devices must satisfy two core requirements: isolating dangerous overcurrents before conductor thermal limits are exceeded, and isolating only the faulted circuit while keeping healthy parallel circuits operational. This section details circuit breaker operational mechanics, short-circuit ratings, and selective discrimination principles enforced by DEWA.

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Overload Current vs. Short-Circuit Current

An overcurrent is any current exceeding the rated nominal current capacity ($I_n$) of a circuit or conductor. Overcurrents are subdivided into two distinct thermal and dynamic phenomena:

1. Overload Current

• Definition: An overcurrent occurring in a mechanically sound, undamaged electrical circuit, caused by connecting excessive load apparatus (e.g. plugging multiple heavy appliances into one socket circuit, or an overloaded electric motor).
• Magnitude: Typically $1.1 \times I_n$ to $5 \times I_n$.
• Characteristics: Slow-building thermal effect. Conductor insulation degrades due to $I^2t$ heat accumulation over minutes or hours.
• Protection Mechanism: Inverse-time bimetallic thermal trip element inside the circuit breaker.

2. Short-Circuit Current

• Definition: An overcurrent resulting from an insulation failure or accidental direct connection between live phase conductors ($L-L$) or between a phase conductor and neutral/earth ($L-N$ or $L-PE$) having negligible impedance.
• Magnitude: Extremely high, ranging from $10 \times I_n$ up to tens of kiloamperes ($50\text{kA}$).
• Characteristics: Instantaneous electrodynamic forces and explosive thermal heating capable of melting copper conductors and vaporizing switchgear components within milliseconds.
• Protection Mechanism: High-speed solenoid magnetic trip element or digital electronic pickup.

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MCB Tripping Characteristics (BS EN 60898 Types B, C, D)

Miniature Circuit Breakers (MCBs) manufactured to BS EN 60898 combine thermal overload protection and magnetic short-circuit protection into a compact DIN-rail module. They are categorized into three standardized instantaneous magnetic trip curves based on their magnetic pickup multiplier range:

Instantaneous Magnetic Trip Multiplier Ranges:
Type B:  |--- 3x In to 5x In ---|
Type C:  |------- 5x In to 10x In -------|
Type D:  |-------------- 10x In to 20x In --------------|

Detailed Breakdown of MCB Curves

Calculation Example

A 32A Type C MCB installed in a DEWA sub-distribution board:

• Thermal overload tripping begins above $1.13 \times 32\text{A} = 36.16\text{A}$, and is guaranteed to trip within 1 hour at $1.45 \times 32\text{A} = 46.4\text{A}$.
• Magnetic instantaneous tripping occurs between $5 \times 32\text{A} = 160\text{A}$ and $10 \times 32\text{A} = 320\text{A}$.
• Therefore, to guarantee instantaneous disconnection (within 0.1s to 0.4s) during an earth fault, the minimum earth fault current $I_f$ must be at least $320\text{A}$.

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Industrial Switchgear: MCCBs and Air Circuit Breakers (ACBs)

For higher current ratings and heavy industrial applications, MCBs are replaced by larger switchgear assemblies compliant with BS EN 60947-2:

Molded Case Circuit Breakers (MCCBs)

• Current Ratings: 63A to 1600A.
• Application: Sub-Main Distribution Boards (SMDB) and Main Distribution Board (MDB) incoming/outgoing feeders.
• Trip Units: Thermal-magnetic or microprocessor-based Electronic Trip Units (ETU) allowing field-adjustable settings:
  • $Ir$ (Long-time pickup / Overload setting): Adjustable from $0.4 \times I_n$ to $1.0 \times I_n$.
  • $Isd$ (Short-time pickup): Adjustable for time-graded discrimination.
  • $Ii$ (Instantaneous pickup): Adjustable magnetic short-circuit ceiling.
  • $Ig$ (Ground-fault pickup): Adjustable earth fault pickup and time delay.

Air Circuit Breakers (ACBs)

• Current Ratings: 800A to 6300A.
• Application: Main Low Voltage (LV) Switchboards directly downstream of DEWA transformers.
• Features: High short-time withstand current capability ($I_{cw}$ up to $100\text{kA}$ for 1 second), allowing full time-graded selectivity with downstream MCCBs without opening prematurely.

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Breaking Capacity Metrics: Icu, Ics, and Prospective Short-Circuit Current (Ipf)

When a short circuit occurs, the fault current at the breaker terminals is limited only by the transformer capacity and cable impedance. This maximum potential fault current is called the Prospective Short-Circuit Current ($I_{pf}$ or $I_{pscc}$).

Mandatory Rule for Breaking Capacity

The short-circuit breaking capacity of every circuit breaker installed in a DEWA network must be equal to or greater than the maximum prospective short-circuit current at the point of installation:

$\text{Breaker Breaking Capacity } (I_{cu}) \ge \text{Prospective Short-Circuit Current } (I_{pf})$

Definitions of Breaking Capacity Ratings (BS EN 60947-2)

• $I_{cu}$ (Ultimate Short-Circuit Breaking Capacity): The maximum fault current value that the circuit breaker can interrupt successfully once under standardized test conditions ($O - t - CO$). Following an $I_{cu}$ fault interruption, the breaker is safely cleared but may suffer permanent mechanical degradation and may not be fit for continued service.
• $I_{cs}$ (Service Short-Circuit Breaking Capacity): The fault current value that the breaker can interrupt repeatedly ($O - t - CO - t - CO$) and remain fully operational without loss of performance.
  • Expressed as a percentage of $I_{cu}$ (e.g. $50% I_{cu}$, $75% I_{cu}$, or $100% I_{cs} = I_{cu}$).
  • DEWA specifications strictly require $I_{cs} = 100% I_{cu}$ for main incoming ACBs and MDB switchgear.
• $I_{cn}$ (Nominal Breaking Capacity): Standard breaking capacity metric applied to domestic MCBs under BS EN 60898 (e.g. 6kA, 10kA).

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Principles of Selective Discrimination & Coordination

Selectivity (Discrimination) is the coordination of protective devices such that a fault occurring at any point in the installation is cleared exclusively by the protective device located immediately upstream of the fault, keeping all unaffected circuits fully operational.

Main LV Switchboard (ACB: 2500A, Time-delayed) 
         │
         ├──> SMDB Feeder (MCCB: 250A, Short-time delay)
         │          │
         │          └──> Branch DB (MCB: 32A Type C, Instantaneous)
         │                    │
         │                    └──> [FAULT OCCURS HERE]

Types of Selectivity Coordination

1. Current Selectivity (Ampere Grading): Setting the pickup threshold of the upstream device significantly higher than the downstream device. Operates effectively for low-magnitude overload faults.
2. Time Selectivity (Time Grading): Introducing a deliberate time delay (e.g. 100ms to 300ms grading margin) on the upstream circuit breaker (Category B ACB/MCCB) so that the downstream fast-acting breaker (Category A MCB) clears the fault before the upstream device initiates tripping.
3. Energy Selectivity ($I^2t$ Grading): Coordination based on limiting the total let-through energy ($I^2t$) of the downstream device below the energy threshold required to trip the upstream breaker.

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Fuse Discrimination Ratios & Time-Current Grading Charts

Where high-rupturing capacity (HRC) fuses (BS 88 / BS EN 60269 gG curves) are used in main distribution circuits, discrimination between upstream and downstream fuses in series is achieved by adhering to standardized nominal rating ratios:

Standard Fuse Discrimination Ratio

To guarantee full selectivity between two series-connected gG HRC fuses:

$\frac{\text{Upstream Fuse Rating } (I_{n1})}{\text{Downstream Fuse Rating } (I_{n2})} \ge 1.6 : 1$

• Example: A 100A gG fuse downstream will achieve total discrimination with a 160A gG fuse upstream ($160 / 100 = 1.6$). If a 125A upstream fuse were used ($125 / 100 = 1.25$), both fuses might melt simultaneously during a heavy short circuit, causing an unwanted loss of discrimination.