First-Principles Engineering Breakdown: How an MCB Clears Overcurrents
A Miniature Circuit Breaker (MCB) combines two distinct physical phenomena inside a compact 18mm modular casing: a slow thermodynamic expansion blade for overload protection, and an ultra-fast electrodynamic solenoid plunger for short-circuit clearance.
Thermal Overload Physics: Differential Bimetallic Deflection
Continuous current flows directly through a laminated bimetallic strip composed of two metals with unequal coefficients of linear thermal expansion (α): a high-expansion copper/brass alloy bonded to a low-expansion nickel-iron alloy (Invar, FeNi36). Ohmic Joule heating generates thermal energy:
P_heat = I² ⋅ R_bimetal &implies; ΔT(t) = ΔT_max ⋅ (1 - e^(-t / τ))As the strip heats up, the brass side expands faster than the Invar side, forcing the blade to curve sideways. When the tip deflects beyond the calibrated latch boundary (adjusted via the factory calibration screw), it knocks the trip bar and releases the spring-loaded toggle mechanism.
Electromagnetic Short-Circuit Physics: Solenoid Hammer Strike
When a low-impedance short circuit occurs (e.g. 10× I_n or thousands of amperes), waiting for thermal heating would cause cable melting. In series with the bimetal is a heavy copper wire coil containing a movable soft-iron cylinder (plunger) restrained by a calibrated spring. The magnetic force generated obeys:
F_mag = (N ⋅ I)² ⋅ μ_0 ⋅ A / (2 ⋅ g²) ∝ I²At short-circuit levels, magnetic force overcomes spring tension in less than 3 milliseconds. The iron plunger fires forward like a bullet, striking the moving contact arm directly and knocking it open before the main toggle mechanism has even finished pivoting!
Arc Extinction: Magnetic Lorentz Blowout & De-ion Splitter Chute
Separating contacts carrying thousands of amperes creates a 6,000°C ionized plasma arc. To extinguish this arc before the current peak, the MCB uses electrodynamic magnetic blowout:
F_Lorentz = I × B &implies; V_arc_total = ∑ V_plate ≥ 11 × 35V = 385V > V_supplyLorentz forces drive the arc along copper runner horns into a stack of 11 to 13 insulated steel splitter plates. The arc is chopped into a series of mini-arcs, rapidly cooling the plasma and raising arc voltage above system voltage, which drives the current to zero in 3-8 milliseconds (Current Limitation Class 3).
Time-Current Curves & The Adiabatic Cable Boundary (I²t ≤ k²S²)
BS EN 60898-1 defines three standardized tripping envelopes based on the instantaneous magnetic threshold:
Type B: 3× to 5× I_n (Domestic lighting, socket radials, long cable runs)
Type C: 5× to 10× I_n (Commercial lighting, small motors, pumps)
Type D: 10× to 20× I_n (Transformers, X-ray, heavy industrial inrush)Under BS 7671 Regulation 434.5.2, the circuit breaker must limit let-through energy so that the downstream circuit protective conductor does not exceed its maximum temperature: I²t ≤ k²S².