Types of MPCBs: Thermal-Magnetic vs Magnetic-Only
What is the difference between a thermal-magnetic MPCB and a magnetic-only MPCB? A thermal-magnetic MPCB pairs an adjustable bimetal overload element, dial-set to the motor's full-load current (FLC), with a fixed magnetic short-circuit trip near 12-13x In in one DIN-rail device per IEC 60947-4-1, while a magnetic-only MPCB omits the bimetal and delivers short-circuit protection alone, relying on a separate thermal overload relay for the overload function. That construction choice determines where overload protection sits in the panel, how many devices the starter needs, and which Type 1 or Type 2 coordination table the panel builder references. This article compares the two constructions, the trip settings each uses, when magnetic-only earns its place over a combined unit, and how Schneider, ABB and Siemens split their ranges between the two types.
What Separates the Two MPCB Types
Every MPCB has a fixed magnetic trip for short-circuit protection. The split between the two types is what happens on the overload side. A thermal-magnetic MPCB builds the overload function in: a bimetal strip heats with current draw and trips the mechanism once it deforms past a threshold set by the FLC dial. A magnetic-only MPCB deletes that bimetal entirely. What is left is a breaker that opens on short circuit and does nothing on a sustained overcurrent below the magnetic threshold.
Schneider labels its magnetic-only line GV2L, ABB uses MO132/MO165, and Siemens builds magnetic-only variants into the 3RV2 family. All three still call the device an MPCB because the magnetic short-circuit function and the manual isolation switch are unchanged from the thermal-magnetic version. Only the bimetal is gone.
Thermal-Magnetic MPCBs: How the Combined Trip Works
The bimetal strip carries line current and bends toward the trip mechanism as it heats. The dial does not change the physics of the strip; it sets the mechanical travel needed to trip, which is calibrated to a current range printed on the housing (for example 9-14 A or 24-32 A depending on frame). Set the dial to the motor's nameplate FLC and the bimetal reproduces the motor's own I²t heating curve closely enough to trip before winding insulation is damaged.
The magnetic trip sits on a separate armature, unaffected by the dial. It is fixed at roughly 12-13x the rated current In of that frame size, high enough that a direct-on-line inrush of 6-8x FLC will not open the breaker during a normal start. Set it wrong and it nuisance-trips on every start; set it right and it only opens on a genuine short circuit or a locked rotor.
Formula: Magnetic Trip Threshold — Source: IEC 60947-4-1, motor starter combination clauses
Im = k × In
| Symbol | Description | Unit |
|---|---|---|
| Im | Magnetic (instantaneous) trip current | A |
| k | Manufacturer multiplier, typically 12-13 | unitless |
| In | Rated current of the MPCB frame (not the dial setting) | A |
Because the bimetal and the magnetic trip are one mechanical assembly, a thermal-magnetic MPCB needs a single device per motor branch: no external overload relay, no extra DIN-rail space, one set of terminals to wire. That is why it dominates simple direct-on-line starters up to a few tens of amps.
Magnetic-Only MPCBs: Short-Circuit Protection Without the Bimetal
A magnetic-only MPCB is, mechanically, the thermal-magnetic unit with the bimetal stage removed. The magnetic trip point (still roughly 12-13x In of the frame) and the manual handle stay the same. What changes is the panel design: overload protection has to come from somewhere else, almost always a dedicated thermal overload relay mounted on the downstream contactor, or occasionally an electronic overload relay for finer class control.
Rated current on a magnetic-only unit is a single fixed value per frame, not a dial range, since there is no thermal element to adjust. Some ranges label this current as the "In" of the device and expect the overload relay downstream to carry the fine current adjustment instead.
What we see in the field: magnetic-only MPCBs get specified less often than thermal-magnetic ones, mostly because the combined unit is cheaper to stock and wire for a single motor branch. Magnetic-only earns its place when the overload function needs to sit somewhere other than the breaker — most commonly a soft starter or VFD bypass branch, or a panel standardized on one electronic overload relay type across many motor sizes.
Why Choose Magnetic-Only Over a Combined Unit
Three situations push a design toward magnetic-only. First, electronic overload relays give finer thermal-image modeling, adjustable trip classes, and sometimes ground-fault or under-current detection that a bimetal cannot replicate — useful on motors that are expensive to replace or run in harsh duty cycles. Second, some panel standards keep the overload function on the contactor assembly rather than the upstream breaker, so replacing a burned-out overload relay does not mean pulling the whole MPCB. Third, retrofit projects sometimes reuse an existing overload relay and only need short-circuit protection from the new device.
The tradeoff is real: one more device, one more set of terminals, one more failure point to wire correctly. Get the CT ratio or current range wrong on the separate overload relay and the motor is unprotected even though the magnetic-only MPCB looks fine on inspection. This depends on how disciplined the panel builder is about commissioning documentation — a labeled, tested overload relay is safe; an unlabeled one inherited from a rebuild is not.
See wiring a magnetic-only MPCB with a separate overload relay for the terminal-level detail, including where the CT or bimetal coil of the overload relay sits relative to the contactor.
Trip Class and Coordination: Where the Two Types Diverge
Trip class (Class 10, 10A, 20, 30 per IEC 60947-4-1) describes how long the overload element tolerates 7.2x its setting from cold before tripping. On a thermal-magnetic MPCB, trip class is a property of the bimetal and is usually fixed by the product line (most general-purpose MPCBs are Class 10). On a magnetic-only unit, trip class becomes a property of the separate overload relay instead — and many electronic overload relays let the class be selected in software, which is the finer control mentioned above. Full detail on the class numbers is in the MPCB trip class comparison for Class 10, 20 and 30.
Coordination tables (Type 1 or Type 2, IEC 60947-4-1) are published per combination — MPCB plus contactor, and where relevant plus overload relay. A magnetic-only MPCB, a contactor and an overload relay from the same manufacturer will have a published Type 2 combination; mixing brands across those three devices usually voids that guarantee and forces the designer back to a worst-case Type 1 assumption. Thermal-magnetic MPCBs simplify this because there are only two devices (breaker and contactor) in the coordination table instead of three.
| Criteria | Thermal-Magnetic MPCB | Magnetic-Only MPCB | Magnetic-Only + Electronic Overload Relay |
|---|---|---|---|
| Overload protection source | Built-in bimetal, dial-set to FLC | None — must add separate relay | Separate electronic relay, in the combo |
| Devices per motor branch | 1 (MPCB) + contactor | 1 (MPCB) + contactor, overload missing until added | MPCB + contactor + overload relay |
| Trip class flexibility | Fixed by product line, usually Class 10 | N/A on the breaker itself | Often selectable in software (10/20/30) |
| Coordination table complexity | 2-device table (MPCB + contactor) | Incomplete until relay is specified | 3-device table (MPCB + contactor + relay) |
| Typical use case | Simple direct-on-line starters | Feeding a soft starter/VFD bypass, or standardized relay platforms | Motors needing finer thermal modeling or extra protections |
Brand Examples: Where Each Type Shows Up in Schneider, ABB and Siemens Ranges
Schneider's TeSys GV2ME and GV2P are thermal-magnetic to roughly 32 A; GV2L is the magnetic-only counterpart in the same frame, meant to pair with an LR overload relay. GV3P extends thermal-magnetic coverage to 65 A, and GV4/GV5/GV7 add electronic overload options at higher currents. Browse the range in the motor protection circuit breakers collection.
ABB's MS116, MS132 and MS165 are thermal-magnetic up to 32 A and 65 A respectively, with MO132 and MO165 as the magnetic-only equivalents for pairing with a separate overload relay. Phase-loss sensitive tripping is standard across the thermal-magnetic MS line. ABB pairs these with AF contactors to complete the starter — see the contactors collection for compatible frames.
Siemens builds both constructions into its 3RV2 platform (frame sizes S00, S0, S2, S3): the 3RV2011/3RV2021/3RV2031 series are thermal-magnetic, with magnetic-only variants in the same physical frame for use with 3RU2 or 3RB3 overload relays. Link modules snap the 3RV2 directly to a 3RT2 contactor, which is the basis of the SIRIUS load feeder. For the overload-relay side of a magnetic-only build, check thermal overload relays.
Across all three brands, the magnetic trip multiplier and the manual switching mechanism carry over unchanged between the thermal-magnetic and magnetic-only versions in the same frame — only the bimetal stage is added or removed. That consistency is what lets a panel builder standardize on one frame size and choose the overload strategy per branch. Full construction and sizing detail sits in the MPCB engineering guide.
Frequently Asked Questions
Can a magnetic-only MPCB run without any overload relay?
Mechanically yes, but the motor then has no protection against a sustained overcurrent below the magnetic threshold (roughly 12-13x In). A stalled or overloaded motor can overheat and damage the winding before the magnetic trip ever sees enough current to open. A separate overload relay is required for a complete, code-compliant motor branch.
Is a magnetic-only MPCB cheaper than a thermal-magnetic MPCB?
The magnetic-only device itself is often lower cost, but once a separate overload relay is added the total bill of materials and the labor to wire two devices instead of one usually costs more than a single thermal-magnetic MPCB. Magnetic-only is chosen for functional reasons — finer overload control or a standardized relay platform — not to save money on a single branch.
What do Schneider GV2L, ABB MO132 and Siemens 3RV2 magnetic-only variants have in common?
All three are magnetic-only MPCBs: fixed short-circuit trip, manual on/off/isolation, no bimetal. Each is designed to pair with that manufacturer's own overload relay and contactor to keep the published Type 2 coordination table intact.
Does trip class (10, 20, 30) apply to a magnetic-only MPCB?
Not on the breaker itself, since there is no bimetal to time. Trip class becomes a property of the separate overload relay in a magnetic-only build, and electronic overload relays frequently let the installer select the class rather than fixing it at the factory.
Can I turn a thermal-magnetic MPCB into a magnetic-only one by maxing out the dial?
No. Setting the dial to its highest position delays the bimetal trip but does not disable it, and the resulting current-time curve will not match a manufacturer's tested magnetic-only device or its coordination table. If magnetic-only behavior is the goal, specify the magnetic-only part number.
Which type should a panel builder default to for a simple direct-on-line starter?
Thermal-magnetic, in most cases — one device, one dial set to motor FLC, one published Type 2 table with the matching contactor. Magnetic-only is worth the extra wiring only when the overload function needs to live elsewhere, such as ahead of a soft starter or on a platform standardized on one electronic overload relay.
Conclusion
Thermal-magnetic and magnetic-only MPCBs share the same magnetic short-circuit trip and manual switching mechanism; the difference is whether the bimetal overload element is built in or left to a separate relay. Thermal-magnetic covers the majority of direct-on-line motor branches with one device and one FLC dial. Magnetic-only earns its keep when the overload function needs finer control or a different location in the panel — at the cost of one more device and one more coordination check. Check the manufacturer's Type 1/Type 2 combination table before mixing brands across the MPCB, contactor and overload relay, regardless of which type is chosen. Browse current stock in the manual motor starters collection.
