Circuit Breaker (MCB) Symbol
Definition: The Circuit Breaker (MCB) symbol represents a Miniature Circuit Breaker — drawn as a rectangle containing a diagonal trip-bar line and a switching contact, or as a simple switch-in-a-box glyph — that automatically interrupts a circuit when current exceeds a preset rating (in amperes) or when a short circuit occurs, protecting downstream wiring from overload and fault damage; MCBs are designated Q (or CB) in schematics per IEC 60617-07 and ANSI Y32.2 / IEEE 315, and expose a Line terminal and a Load terminal.
Also known as: MCB symbol, circuit breaker symbol, miniature circuit breaker symbol, breaker symbol, Q symbol schematic, thermal-magnetic breaker symbol, overcurrent protection symbol.
What the Circuit Breaker (MCB) symbol means
The Circuit Breaker (MCB) symbol in a circuit diagram indicates the presence of an automatic overcurrent protection device wired in series with a circuit. An MCB combines two protection mechanisms in one device: a bimetallic thermal element that trips on sustained overload currents (above the rated ampere value), and an electromagnetic instantaneous-trip element that trips in milliseconds on short-circuit currents. The symbol therefore represents not just a switch, but a protection device whose state — tripped or closed — affects the safety and operability of the entire downstream circuit. When the MCB symbol appears on a schematic, the designer is communicating that the connected circuit is protected against both overload (gradual heat build-up) and short-circuit (instantaneous high current) faults.
MCB symbols carry critical information: the rated current (e.g. 16 A), the trip curve (B, C, or D — defining how quickly the breaker trips at multiples of rated current), the rated voltage, and the short-circuit breaking capacity (kA). A Type B curve (trips at 3–5× rated current) is specified for residential circuits protecting cable and socket outlets; Type C (5–10×) is used for commercial loads with moderate inrush; Type D (10–20×) is used for highly inductive or motor loads with large starting currents. These annotations alongside the Q designator label on the schematic allow an engineer to verify that the protection is correctly co-ordinated with the downstream load.
How to identify the Circuit Breaker (MCB) symbol
The MCB symbol in IEC 60617 is drawn as a rectangle (the breaker body) with a diagonal line inside — representing the bi-metallic or thermal-magnetic trip mechanism — connected in series with a switch-contact symbol (a short line at an angle representing the moving contact arm). The overall appearance is a breaker box containing a diagonal slash, with a Line terminal line entering from the top and a Load terminal line exiting from the bottom. In ANSI Y32.2 / IEEE 315 schematics, the circuit breaker is often shown simply as a switch symbol with a square or rectangle around it, sometimes with a diagonal line or the letter 'B' inside to indicate the breaker. Both versions are placed in series on the line conductor, making the series-insertion position within the circuit diagram a key identifying feature alongside the Q designator.
Function in a circuit
An MCB (Miniature Circuit Breaker) protects electrical circuits from damage caused by overload currents and short-circuit faults. Its thermal element — a bimetallic strip that bends when heated by excess current — trips the breaker after a time-delay proportional to the overcurrent magnitude, protecting cables from insulation degradation. Its magnetic element — an electromagnetic solenoid — trips the breaker instantaneously (within one half-cycle, typically < 10 ms) when short-circuit current flows, preventing arc flash and wiring fires. When tripped, the MCB mechanically isolates the Line terminal from the Load terminal; the breaker can be manually reset (unlike a fuse, which must be replaced). MCBs are rated by current (A), voltage (V AC), breaking capacity (kA), and trip curve (B/C/D).
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-07 (switchgear, controlgear, and protective devices) defines the circuit breaker symbol as a rectangle with a diagonal trip-bar line in series with a moving-contact switch. The standard designator is Q (from the German Querschnitt/switch convention) or sometimes QF (for feeder breaker). IEC 60898-1 governs the electrical performance, trip curve, and test requirements for MCBs used in residential and commercial installations. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2-1975 (R1989) / IEEE 315-1975, section 3.5, defines the circuit breaker as a switch symbol enclosed in a square or rectangle, optionally with an internal diagonal line. UL 489 and ANSI/UL 489 govern the performance of MCBs sold in North America. The designator CB or Q is used depending on the drawing standard. |
| Key difference | IEC 60617 uses a distinctive rectangle-with-diagonal-line glyph making the MCB visually distinct from an ordinary switch. ANSI Y32.2 drawings often use a simpler switch-in-a-box glyph or the standard switch symbol with a box around it. The IEC version more clearly conveys the thermal-magnetic protection nature of the device in the symbol itself; the ANSI version is more schematic-abstract. Both use Line and Load as the two terminal points. |
Terminals / pins
| Pin | Name |
|---|---|
| line | Line |
| load | Load |
Typical values
Rated current: 1 A, 2 A, 4 A, 6 A, 10 A, 16 A, 20 A, 25 A, 32 A, 40 A, 50 A, 63 A (standard DIN-rail MCB range). Rated voltage: 230 V AC (single-pole), 400 V AC (three-pole). Breaking capacity: 6 kA (Type 6kA, domestic) or 10 kA (Type 10kA, commercial/industrial). Trip curves: B (3–5× In, residential cables and sockets), C (5–10× In, commercial motors and fluorescent lighting), D (10–20× In, transformers and heavy motor loads). Poles: 1P, 2P, 3P, or 4P (number of switched conductors).
Where the Circuit Breaker (MCB) symbol is used
- Consumer unit (distribution board) schematics — MCB symbols in each outgoing circuit position, protecting ring-main socket circuits (32 A Type B), lighting circuits (6–10 A Type B), and cooker circuits (32–40 A Type C)
- Industrial control panel schematics — MCB symbols protecting individual motor branch circuits, with Type C or D curves co-ordinated with motor starting current
- Sub-panel feeder circuits — high-rated MCB symbols (40–63 A) on the incoming supply to a sub-distribution board
- PV solar array and battery system schematics — DC-rated MCB symbols on string combiner outputs and battery disconnect positions
- Single-line diagrams — MCB symbols on incoming service panels showing the main breaker protecting the entire installation
- EV charging station wiring plans — 32 A or 40 A Type C MCB symbols on the dedicated EV charger supply circuit
- Data centre and UPS schematics — MCB symbols on each PDU (power distribution unit) output branch protecting server rack circuits
Example
In a domestic consumer unit single-line diagram, a Type B 32 A MCB symbol labelled Q3 is shown in series with the ring-main circuit feeding the first-floor socket outlets; the Line terminal connects to the 230 V AC busbar, and the Load terminal connects to the live conductor of the ring-main cable, with '32A B 6kA' annotated beside the Q3 label to communicate rated current, trip curve, and breaking capacity to the installing electrician and building inspector.
Key facts
- The Circuit Breaker (MCB) symbol is designated Q in IEC 60617-07 schematics (or CB in some ANSI/North American drawings); individual breakers are indexed Q1, Q2, etc.
- An MCB symbol has two terminals: Line (supply side, pin id 'line') and Load (protected circuit side, pin id 'load'); the breaker interrupts the Line-to-Load current path when tripped.
- MCB trip curves define how quickly the breaker responds to overcurrents: Type B trips at 3–5× rated current (residential cables and sockets); Type C at 5–10× (commercial loads with moderate inrush); Type D at 10–20× (motors, transformers with high starting currents).
- IEC 60617 draws the MCB as a rectangle with a diagonal trip-bar line in series with a moving contact; ANSI Y32.2 uses a switch symbol enclosed in a square or rectangle.
- Standard MCB rated currents follow the IEC 60898-1 preferred series: 6 A, 10 A, 16 A, 20 A, 25 A, 32 A, 40 A, 50 A, 63 A.
- MCB breaking capacity (the maximum fault current the breaker can safely interrupt without damage) is 6 kA for domestic grade and 10 kA for commercial/industrial grade, per IEC 60898-1.
- Unlike a fuse, an MCB can be manually reset after tripping — a property indicated in descriptive text beside the symbol, though not encoded in the glyph itself.
- MCBs are available as 1-pole, 2-pole, 3-pole, and 4-pole devices; multi-pole MCBs are shown as multiple linked single-pole breaker symbols with a mechanical interlock line in IEC drawings.
Diagrams that use this symbol
- house wiring diagram
- house electrical wiring diagram
- home wiring diagram
- home electrical wiring diagram
- dol starter diagram
- electrical installation diagram
- fuse box diagrams
- earthing diagram
Frequently asked questions
What does the MCB symbol look like in a circuit diagram?
In IEC 60617-07 schematics, the MCB (Miniature Circuit Breaker) symbol is drawn as a rectangle containing a diagonal line (representing the thermal-magnetic trip element), connected in series with a switch-contact symbol (an angled line representing the moving contact). Two terminal lines extend from the symbol: Line at the top (supply side) and Load at the bottom (protected circuit side). In ANSI Y32.2 drawings, it appears as a switch symbol enclosed within a square or rectangle.
What does the MCB symbol mean in a wiring diagram?
The MCB symbol means an automatic overcurrent protection device is wired in series at that point, protecting the downstream circuit from both overload currents (sustained excess current causing cable heating) and short-circuit currents (instantaneous high fault current). The MCB trips — mechanically opening the Line-to-Load contact — when current exceeds the rated value by the amount and duration defined by its trip curve (B, C, or D).
What is the designator letter for a circuit breaker?
The standard designator letter for a circuit breaker in IEC 60617 is Q; individual breakers are labelled Q1, Q2, Q3, etc. In North American ANSI schematics, the designator CB (circuit breaker) is also widely used. The rated current (e.g. 16 A), trip curve (B, C, or D), and breaking capacity (e.g. 6 kA) are annotated alongside the Q or CB label.
What is the difference between MCB trip curves B, C, and D?
MCB trip curves define the instantaneous (magnetic) trip threshold as a multiple of the rated current (In). Type B trips instantaneously at 3–5× In and is used for residential circuits protecting cables and socket outlets where inrush currents are low. Type C trips at 5–10× In and suits commercial loads such as fluorescent lighting and small motors. Type D trips at 10–20× In and is specified for highly inductive loads such as transformers and large motors with high starting currents.
What is the difference between the IEC and ANSI MCB symbol?
IEC 60617-07 uses a distinctive rectangle-with-diagonal-line symbol in series with a switch contact, which visually communicates the thermal-magnetic protection function. ANSI Y32.2 / IEEE 315 typically depicts the circuit breaker as a standard switch symbol enclosed within a square or rectangle — more abstract and less visually descriptive. The IEC symbol is the international standard for modern schematics; the ANSI style is common in North American electrical drawings.
What is MCB breaking capacity and why does it matter?
Breaking capacity (also called interrupting rating) is the maximum prospective short-circuit current (in kA) that the MCB can safely interrupt without being destroyed. Standard domestic MCBs have a 6 kA breaking capacity (IEC 60898-1 class 6kA); commercial/industrial MCBs are rated 10 kA. If a fault current exceeds the MCB's breaking capacity, the breaker can fail catastrophically and cause fire. The breaking capacity must equal or exceed the maximum prospective fault current at the point of installation, calculated from the supply impedance.
What is the difference between an MCB and a fuse in a circuit diagram?
An MCB (Miniature Circuit Breaker) symbol shows a resettable device that can be manually restored after tripping; a fuse symbol (a rectangle with a line through it, or a rectangle representing the fuse element) represents a one-time protection device that must be replaced after operation. Both are shown in series in a circuit diagram, but the MCB symbol conveys that the device provides both overload and short-circuit protection in a resettable package, whereas the fuse symbol indicates a sacrificial element that clears a fault by melting.
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