Motor Protection CB Symbol
Definition: The Motor Protection CB (circuit breaker) symbol represents a combined motor protection circuit breaker — also called an MPCB or motor starter protector — that provides simultaneous short-circuit, overload, and phase-loss protection for a motor feeder circuit, depicted as a switching device block with Line input and Load output terminals, per IEC 60947-2 and IEC 60947-4-1.
Also known as: MPCB, motor protection circuit breaker, motor starter protector, motor circuit protector, GV2 breaker, PKZ breaker, MCP.
What the Motor Protection CB symbol means
The Motor Protection CB symbol represents a specialised circuit breaker optimised for motor protection duty. Unlike a standard MCB (miniature circuit breaker), the MPCB combines magnetic short-circuit protection, thermal overload protection, and often phase-loss detection in a single device. The symbol appears in motor control centre (MCC) and DOL starter drawings as the first protective element in the motor feeder.
The symbol communicates that the device simultaneously performs the roles that would otherwise require a separate motor protection fuse and a separate thermal overload relay. The MPCB's overload element is factory-set or field-adjustable to the motor's full-load current rating, protecting against sustained overloads, locked-rotor conditions, and single-phasing.
How to identify the Motor Protection CB symbol
The Motor Protection CB symbol is drawn as a rectangular block (representing the breaker mechanism) with a Line terminal at the top and a Load terminal at the bottom. The symbol may include a small overcurrent trip indicator (zigzag line inside the rectangle) or a bimetal overload element representation. The label 'MPCB', 'MCP', or the manufacturer part reference (e.g. 'GV2-ME', 'PKZ-M') is shown adjacent. Some representations show it as a standard circuit breaker symbol with an additional overload trip indication.
Function in a circuit
The Motor Protection CB protects a motor circuit against: (1) short circuits — via a magnetic trip element that operates in milliseconds; (2) sustained overloads — via a thermal or electronic overload element calibrated to the motor's full-load ampere (FLA) rating; (3) phase loss or phase imbalance — via a dedicated trip mechanism in three-phase MPCBs. The device is manually resettable after a trip event. Many MPCBs also integrate auxiliary contacts (for remote trip indication) and an on/off switch function, eliminating the need for a separate isolating switch.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60947-2 governs low-voltage circuit breakers including motor circuit protectors. IEC 60947-4-1 specifically covers motor starters and motor circuit protectors. The symbol follows IEC 60617 circuit breaker conventions with overload indication. |
|---|---|
| ANSI/IEEE 315 | ANSI/UL 489 governs molded-case circuit breakers in North America; motor circuit protectors (MCPs) are covered under UL 508 (industrial control equipment). ANSI Y32.2 / IEEE 315-1975 uses the standard circuit breaker symbol with the MCP label. |
| Key difference | IEC symbols for MPCBs follow IEC 60617 breaker conventions with an explicit overload element indicator; ANSI/UL symbols use the standard breaker symbol with 'MCP' or 'MPCB' label. Both standards use a switching device rectangle with Line and Load terminals. |
Terminals / pins
| Pin | Name |
|---|---|
| line | Line |
| load | Load |
Typical values
Current setting range: typically 0.1 A–100 A (adjustable within rated range); breaking capacity (Icu): 18–150 kA at 400 V AC depending on frame; trip class: Class 10 or Class 20 (per IEC 60947-4-1); voltage rating: 230/400/690 V AC three-phase.
Where the Motor Protection CB symbol is used
- Motor control centres (MCCs): MPCB provides combined circuit protection and overload protection for each motor feeder in a multi-motor switchboard
- DOL (direct-on-line) motor starters: replaces the combination of motor fuse + overload relay as a single device upstream of the contactor
- Pump and compressor panels: MPCB protects single or three-phase pump motors against short circuit, overload, and phase loss
- HVAC fan and chiller motor feeders: MPCBs are used in air handling unit electrical panels to protect fan motor circuits
- Machine tool panels: MPCBs protect individual axis and spindle drive motor circuits in CNC machine electrical cabinets
- Industrial packaging and conveyor lines: each motor in a production line receives individual MPCB protection in the motor control panel
Example
In a 3-phase pump control panel drawing, the Motor Protection CB symbol appears at the top of the single-line diagram with Line terminals connected to the three-phase bus. The MPCB (Schneider GV2-ME14, rated 6–10 A) is set to the 5.5 kW pump motor's full-load current of 11 A at 400 V. Its Load terminals feed a three-pole contactor (K1); on motor overload or phase loss, the MPCB trips and opens the circuit, preventing motor burnout.
Key facts
- The Motor Protection CB (MPCB) symbol represents a single device that combines short-circuit protection, thermal overload protection, and phase-loss protection for motor circuits per IEC 60947-2 and IEC 60947-4-1.
- The two standard terminals are Line (supply side, top) and Load (motor side, bottom); three-phase MPCBs have three Line and three Load terminals.
- The MPCB eliminates the need for a separate motor protection fuse and overload relay, reducing panel space and simplifying motor feeder design.
- Current setting range is typically adjustable from 0.16 A to 630 A across the MPCB product range; specific devices are selected to match the motor's full-load ampere (FLA) rating.
- Tripping characteristics follow IEC 60947-4-1 Class 10 or Class 20: Class 10 trips within 10 seconds at 7.2× setting; Class 20 within 20 seconds — Class 20 suits high-inertia loads with long acceleration times.
- Common MPCB product families include Schneider Electric GV2-ME, Eaton PKZM0/PKZ-M, Siemens 3RV, and ABB MS series.
- Phase-loss protection in an MPCB detects when one of the three supply phases is absent, preventing single-phase running which causes excessive winding current and motor burnout.
Frequently asked questions
What does the motor protection CB symbol mean in a wiring diagram?
The motor protection CB symbol means the diagram includes a combined motor protection circuit breaker (MPCB) that protects the motor circuit against short circuits, overloads, and phase loss. It is the primary protective device in the motor feeder circuit.
What does the MPCB symbol look like?
The MPCB symbol is a rectangular block with a Line terminal at the top and a Load terminal at the bottom. It resembles a standard circuit breaker symbol but may include an overload element indicator (zigzag or bimetal representation) inside. It is labelled 'MPCB', 'MCP', or with the product reference.
What is the difference between an MPCB and a standard MCB?
An MCB (miniature circuit breaker) protects against short circuits and overloads for general wiring circuits; it is not calibrated for motor starting current. An MPCB is specifically designed for motor duty: it allows motor starting inrush current without nuisance tripping while still protecting against sustained overloads and single-phasing.
What is the difference between an MPCB and a contactor + overload relay combination?
A contactor + overload relay combination uses separate devices: the contactor for switching and the overload relay for thermal protection; a separate fuse or MCB provides short-circuit protection. An MPCB integrates all three functions — short-circuit, overload, and phase-loss protection — in a single device, saving panel space.
What IEC standard covers the motor protection circuit breaker?
Motor protection circuit breakers are governed by IEC 60947-2 (low-voltage circuit breakers) and IEC 60947-4-1 (contactors and motor starters). IEC 60947-4-1 defines performance requirements for motor circuit protectors including tripping classes and phase-loss protection.
How is the MPCB current setting selected for a motor?
The MPCB current setting is selected to match the motor's full-load ampere (FLA) rating from the motor nameplate. The adjustable setting range of the selected MPCB frame must encompass the motor FLA, allowing fine adjustment after installation to match the actual measured running current.
Does an MPCB replace an overload relay in a motor starter?
Yes — an MPCB with a built-in thermal overload element replaces the need for a separate overload relay in a motor starter. The MPCB is wired directly in series with the contactor, with no additional overload relay required between the contactor load side and the motor terminals.
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