Main Components of an MPCB: Bimetal, Magnetic Trip, Contacts

What are the main components of an MPCB? An MPCB packages three functional blocks in one DIN-rail housing per IEC 60947-4-1: an adjustable thermal (bimetal) overload element dial-set to the motor's full-load current (FLC), a fixed magnetic trip that opens on short circuit at roughly 12-13x the rated current In, and a manual on/off mechanism with isolation contacts rated for lockout. Get any one of these three wrong and the symptom shows up in the field as a nuisance trip, a breaker that won't reset, or a coordination failure with the downstream contactor. This guide covers the bimetal element, the magnetic trip, the main and auxiliary contacts, the trip-free mechanism, the accessory ecosystem, and how all of it acts together during a fault.

The Adjustable Thermal (Bimetal) Overload Element

Current from the motor circuit passes through a bimetal strip inside each pole, or through a heater coil wrapped around it on some frame sizes. Two metals with different thermal expansion rates are bonded together; as I²t heat builds, the strip bends toward the low-expansion side and pushes a trip bar. The dial on the front of the device sets how much current the strip needs to see before it bends far enough to trip, which is what "setting to FLC" means in practice. A GV2ME dialed to 2.5 A trips near that current after enough time at temperature, not instantly.

Time to trip is not fixed. It follows an inverse-time curve: higher overcurrent trips faster, and the standard reference point is trip time at 7.2x the dial setting, measured from cold. That reference point is what separates Class 10 from Class 20 or Class 30 — same bimetal principle, different curve shape. What we see in the field: a pump motor with a 12-second start on a Class 10 breaker will nuisance-trip on every start, even with the dial set correctly to FLC. The fix is a slower trip class, not a higher amp setting.

The Fixed Magnetic Short-Circuit Trip

Behind the bimetal sits a second, independent path: a magnetic element built from a coil and plunger, or a fixed-armature relay in smaller frames, that reacts to instantaneous current rather than accumulated heat. On standard thermal-magnetic MPCBs this path is not adjustable; the threshold is fixed at the factory as a multiple of the frame's rated current In.

That 12-13x figure is set deliberately high. A direct-on-line motor pulls 6-8x FLC at the instant of start; an MCB with a C-curve (5-10x) can read that inrush as a fault and trip on start-up. The MPCB's magnetic element sits above the highest realistic inrush so only a genuine short circuit, not a start, opens it. Response is in milliseconds, well under the seconds-scale response of the bimetal.

Main Contacts, the Trip-Free Mechanism and Manual Operation

Three poles of silver-alloy main contacts carry the motor current and open under either trip signal, bimetal or magnetic. Both trip paths act on a common bar connected to a spring-loaded, trip-free mechanism: trip-free means the operator cannot hold the contacts closed against an active fault by forcing the handle to ON. That single design detail is why an MPCB can serve as protective device and manual switch in the same housing without one function compromising the other.

The handle — a rotary knob on some Schneider and ABB frames, a toggle on others — gives three states: ON, OFF, and TRIPPED, usually shown by handle position or a separate flag indicator. After a thermal trip the mechanism needs a cool-down period before reset; after a magnetic trip it usually resets as soon as the fault clears, though good practice is to inspect the branch before re-energizing regardless of trip type.

Padlockable isolation is built into the handle on most frames, letting the MPCB double as the lockout-tagout point for the motor branch, a function a plain overload relay does not provide on its own.

Auxiliary Contacts, Magnetic-Only Variants and Terminal Options

Beyond the three main poles, an auxiliary contact block, commonly 1NO+1NC, signals ON/OFF/TRIPPED status to a PLC or indicator lamp and in many installations feeds an interlock into the contactor coil circuit. Add-on modules extend this further: alarm contacts, undervoltage releases, or shunt trips for remote tripping on larger frames.

Some frames drop the bimetal entirely. Schneider's GV2L, ABB's MO132/MO165, and magnetic-only variants of the Siemens 3RV2 keep only the magnetic short-circuit trip and the manual/isolation function, pairing with a separate thermal overload relay for current-dependent protection. That split makes sense where an electronic overload relay gives finer current and phase-loss detection than a bimetal dial, or where the overload relay is meant to sit on the contactor rather than upstream of it; see magnetic-only MPCB with overload relay for how that pairing is wired.

Terminal choice is a construction detail with real installation consequences. Screw terminals are the default across most ranges; spring (cage-clamp) terminals are offered on several ABB and Siemens frames and cut panel-wiring time on high-volume builds. Frame width and DIN-rail depth vary by amperage, which is why panel builders check dimensional drawings before laying out a section for higher-amperage motor protection circuit breakers.

How the Components Act Together During a Fault

On a sustained overload, a jammed conveyor or a partially blocked pump impeller, current rises modestly above FLC and stays there. The bimetal heats, bends, and trips within the time set by its class; nothing else in the device reacts because current never reaches the magnetic threshold. On a genuine short circuit downstream, current spikes past the 12-13x In threshold within a cycle or two, and the magnetic element trips the same trip bar in milliseconds, bypassing the slow thermal path entirely.

Downstream, a contactor normally handles the switching for motor starts and stops while the MPCB stands by for the fault case; together they form the MPCB plus contactor motor starter. Manufacturer coordination tables define whether the contactor survives a fault serviceable, Type 2, or needs parts replaced, Type 1, a rating that depends on both devices together and not the MPCB alone; it is covered separately in the Type 1 vs Type 2 coordination guide. Trip-class selection interacts with this too: a breaker set to Class 20 for a long start tolerates more thermal stress before tripping, which changes how the coordination table applies; see the trip class guide for the detail.

Conclusion

Every MPCB reduces to the same three blocks: an adjustable bimetal for overload, a fixed magnetic element for short circuit, and a trip-free manual mechanism that switches and isolates. Auxiliary contacts, terminal style, and link modules change how it wires into a panel, not what it protects against. Match the bimetal setting to FLC, respect the fixed magnetic threshold against the motor's actual start current, and check the coordination table before assuming any one component is at fault. For the broader protection picture, see the MPCB engineering guide, and browse current stock across manual motor starters and motor protection circuit breakers.