MCB Wiring Diagram: How a Miniature Circuit Breaker Works

A miniature circuit breaker (MCB) does two things a fuse cannot: it resets after a trip, and it protects against both sustained overloads and high-magnitude short-circuit faults using two separate mechanisms. Understanding both mechanisms -- and the trip curves that characterize them -- determines whether you select the right MCB for a given circuit or end up with nuisance trips or, worse, equipment damage.

Inside an MCB: Thermal and Magnetic Trip

Every MCB contains two independent tripping elements stacked in series.

Thermal Element (Bimetallic Strip)

A bimetallic strip carries the load current. When current exceeds the breaker's rated value for a sustained period, the strip heats up and bends, releasing the latch mechanism and tripping the breaker. The tripping time is inversely proportional to the degree of overload -- a 20% overload takes many minutes; a 100% overload trips in seconds.

This element protects against overload -- a condition where the current slightly exceeds the rating over time, as happens with a motor drawing too much current under a heavy mechanical load.

Magnetic Element (Solenoid)

An electromagnetic solenoid carries the same line current. If current spikes above a threshold value instantaneously, the solenoid pulls the trip latch open immediately -- within milliseconds. This protects against short-circuit faults, where an unintentionally low-impedance path drives current to potentially tens of thousands of amps.

The threshold at which the magnetic element trips is what defines the trip curve -- the most important parameter after current rating.

MCB Trip Curves: B, C, and D

IEC 60898 defines the standard trip curves. Each curve specifies the range of current multiples (In) at which the magnetic element operates instantaneously.

Curve B: 3x to 5x In

The magnetic element trips between 3 and 5 times the rated current. This is the most sensitive curve. Use Curve B for:

A Curve B MCB trips easily if the circuit has any significant inrush. A 16A Curve B will trip on 48-80A -- even a small motor starting can cause nuisance trips.

Curve C: 5x to 10x In

The magnetic element trips between 5 and 10 times rated current. This is the most common curve in residential and light commercial installations. Use Curve C for:

A 20A Curve C MCB tolerates momentary inrush up to 200A before the magnetic element fires. Most household appliance motors fall within that window.

Curve D: 10x to 20x In

The magnetic element trips between 10 and 20 times rated current. Use Curve D for:

A 32A Curve D MCB tolerates up to 640A instantaneous before the magnetic trip fires. This gives high-inrush equipment room to start without tripping, while still protecting against genuine short-circuit faults.

Curve K and Z (Less Common)

Curve K (8-12x) is sometimes found in industrial motor circuits. Curve Z (2-3x) is for semiconductor and electronic equipment protection. Both are IEC 60947 devices rather than the household IEC 60898 standard.

Single-Pole, Double-Pole, and Triple-Pole MCBs

Single-Pole MCB

Interrupts one conductor -- the line (hot) conductor. The neutral passes straight through the consumer unit without switching. Used for single-phase branch circuits in a consumer unit (lighting, socket rings). The line conductor connects to the top terminal (line/supply side) and exits at the bottom terminal (load side).

Terminal orientation: supply always connects at the top; load connects at the bottom. This is not just convention -- some MCBs are directional and will not interrupt fault current correctly if installed backward. The label "IN" or a molded arrow on the case confirms the correct line orientation.

Double-Pole MCB

Interrupts both the line and neutral conductors simultaneously. Required for:

Both poles are mechanically linked -- press the trip button and both open together. Terminal labeling: L (line) and N (neutral) on the line side; L and N on the load side.

Triple-Pole MCB

Interrupts all three line conductors of a three-phase circuit simultaneously. Common in three-phase sub-distribution boards and as the main switch feeding a three-phase piece of equipment. Terminal labeling: L1, L2, L3 line side; L1, L2, L3 load side (no neutral switched).

Four-pole MCBs add a switched neutral for three-phase + neutral circuits.

Wiring an MCB: Step-by-Step

Safety Note

Always isolate the supply side before working in a consumer unit or distribution board. In a live consumer unit, the busbars and main incoming terminals remain energized even with all MCBs in the off position. Work on consumer units must be done by a qualified electrician in most jurisdictions. Turn off the utility supply at the meter isolator or service head -- or have the supply isolated by your utility company -- before opening the consumer unit enclosure.

Single-Pole MCB in a Consumer Unit

  1. The line busbar (or phase bar) clips onto or slides into the MCB line-side terminal at the top. Most modern consumer units use a busbar comb that plugs into all MCBs simultaneously.
  2. The load-side bottom terminal accepts the line conductor of the branch circuit cable. Strip 10-12mm of insulation, insert the bare conductor, and tighten the terminal screw to the specified torque (typically 2-3 Nm for residential sizes).
  3. The neutral of the branch circuit cable runs directly to the neutral bar -- it does not pass through the MCB.
  4. The earth of the branch circuit cable runs to the earth bar.
  5. The neutral bar and earth bar connect to each other only at the main incoming earth point. Downstream of the consumer unit they must remain separate (TN-S or TN-C-S systems).

Selecting the Current Rating

Match the MCB rating to the cable current-carrying capacity (CCC) -- not to the load. The MCB protects the cable, not the appliance. If 2.5mm² twin-and-earth cable has a CCC of 27A in free air (or less when clipped to a wall or buried in insulation), the MCB must be rated at or below 27A.

Common pairings:

Derate for grouping, burial, and ambient temperature as per the installation method factors in your local wiring regulations.

Drawing MCB Circuits in CircuitDiagramMaker

Schematic work often begins with sketching the consumer unit layout -- which circuits get which MCBs, their ratings and curves, neutral bar connections, and RCD grouping. CircuitDiagramMaker lets you place MCB symbols with configurable ratings and annotate trip curves directly on the symbol. You can then draw the downstream circuit schematic connected to the load-side terminal, giving you a complete picture from source to load without switching tools. Sharing the PDF output with an electrician before the installation starts prevents arguments about cable sizing at the panel.

Create Your Own MCB Wiring Diagram

Create your own MCB wiring diagram -- free

Key Takeaways