Magnetic-Only MPCB with a Separate Overload Relay
What is a magnetic-only MPCB? A magnetic-only MPCB (e.g., Schneider GV2L, ABB MO132/MO165) is a manual motor starter component that carries just the fixed magnetic short-circuit trip and the manual switching/isolation function — it has no thermal element, so overload protection comes from a separate thermal or electronic overload relay wired downstream to a contactor. Drop the built-in thermal dial and the person choosing the FLC range, trip class, and phase-loss sensitivity is now the overload relay, not the breaker, which shifts where coordination and calibration decisions get made. This article covers what a magnetic-only device actually trips on, how to size and set the paired overload relay, trip-class selection, Type 1/Type 2 coordination for the combination, and when the split makes more sense than a one-box thermal-magnetic MPCB.
A Magnetic-Only MPCB Trips on Short Circuit Only
Strip the thermal bimetal out of a standard MPCB and what remains is a fixed magnetic trip plus a manual disconnect — nothing more. Schneider's TeSys GV2L, ABB's MO132 and MO165, and Siemens 3RV2 magnetic-only variants all work this way: the instantaneous element opens at roughly 12-13x the device's rated current In, fast enough to clear a fault before the panel wiring or the contactor takes damage. Set the dial, where one exists, to bracket the motor's full-load current, and that is the extent of the adjustment — there is no thermal curve to tune. For the baseline device, see what an MPCB is and how it works.
What we see in the field: technicians sometimes assume a magnetic-only device is "half a breaker" and skip specifying the relay, leaving the motor with short-circuit protection and nothing to catch a stalled rotor or a worn bearing. That gap only closes once the relay is actually installed and dialed in.
Why Split Overload and Short-Circuit Protection
Two reasons drive the split. An electronic overload relay reads current on all three phases continuously and adds features a bimetal cannot match — ground-fault sensing, jam detection, communication to a PLC. Some panel architectures also put the overload function on the contactor itself rather than upstream of it, so the relay mounts under or beside the contactor and the magnetic-only MPCB becomes purely the short-circuit and isolation device at the top of the branch.
Retrofits push the same way. A plant standardized on one electronic relay platform across its motor control center can keep using it. Swap in a thermal-magnetic MPCB instead and that consistency breaks — now there are two overload mechanisms with different trip curves in the same building.
Setting the Overload Relay to the Motor's Full-Load Current
The overload relay's current dial gets set to the motor nameplate FLC, exactly as a thermal-magnetic MPCB's built-in element would be. The magnetic-only MPCB's own adjustment range only needs to bracket that same FLC so the manual switch sits close to the operating current — it is not resized for overload purposes, because it has none. See thermal overload relays for the relay side of this pairing.
Formula: Overload Relay Current Setting — Source: IEC 60947-4-1, motor nameplate data
Iset = FLC
| Symbol | Description | Unit |
|---|---|---|
| Iset | Overload relay current dial setting | A |
| FLC | Motor full-load current, from the nameplate | A |
| In (MPCB) | Magnetic-only MPCB rated/adjustment range, selected to bracket FLC for the manual switching function | A |
Get the dial wrong in either direction and the failure mode is predictable: set below FLC and the motor nuisance-trips on normal load swings; set above it and a genuine overload runs long enough to cook the winding insulation before anything opens.
Trip Class Now Lives on the Relay, Not the Breaker
Class 10, 10A, 20 and 30 still apply — IEC 60947-4-1 defines the trip time at 7.2x the setting from cold regardless of where the thermal element physically sits. The difference with a magnetic-only build is that the electronic relay, not the MPCB, carries the class rating, usually selectable in software rather than fixed at manufacture. For the full breakdown of each class, see MPCB trip classes 10, 20 and 30.
Class 10 covers most standard-inertia pumps and fans. A high-inertia load — a large centrifugal fan, a crusher, a blower with a long run-up — needs Class 20 or 30 so the relay tolerates the extended start current without tripping before the motor reaches speed. Pick the wrong class and the symptom looks identical whether the culprit is the relay or a built-in MPCB thermal element: nuisance tripping on every start.
Type 2 Coordination Still Has to Be Proven for the Combination
Type 1 and Type 2 coordination testing doesn't disappear just because the overload function moved to a separate relay — it now has to be proven for the specific trio: magnetic-only MPCB, contactor, and overload relay, tested together at a declared short-circuit current. Manufacturers publish these tables for their own combinations; mixing a magnetic-only MPCB from one brand with a relay from another voids the documented coordination and leaves the installer without a tested fault-current rating for the assembly. See MPCB Type 1 vs Type 2 coordination for the full explanation.
This depends on the fault level available at the panel, not the motor size alone — a coordination table valid at 25 kA doesn't automatically extend to a 50 kA installation.
When Magnetic-Only Beats a Thermal-Magnetic MPCB
Three situations favor the split build. A plant standard already specifies a particular electronic overload relay platform across the motor control center, and consistency matters more than compactness. The application needs overload features a bimetal cannot provide — ground-fault detection, remote monitoring, adjustable trip classes without hardware changes. Or the overload relay mounts directly on the contactor for a compact combination starter, with the magnetic-only MPCB serving purely as the upstream disconnect and short-circuit device.
A thermal-magnetic MPCB still wins on simplicity and panel space for a single motor with no special monitoring requirement — one device, one dial, no separate relay to wire and coordinate. For that combination, see MPCB plus contactor.
Brand Notes: GV2L, MO132/MO165, and 3RV2 Magnetic-Only Variants
Schneider's GV2L is the dedicated magnetic-only member of the TeSys GV2 family, built to pair with an LR thermal or LT electronic overload relay ahead of an LC1 contactor. ABB offers MO132 and MO165 alongside its thermal-magnetic MS132/MS165 line, sized to the same frame so the mechanical footprint and accessory range carry over. Siemens' SIRIUS 3RV2 family also includes magnetic-only variants within its S00-S3 frame sizes, intended to pair with 3RU2 or 3RB3 overload relays and 3RT2 contactors through the same link-module system used across the SIRIUS range.
Across all three, the magnetic-only device physically resembles its thermal-magnetic sibling — same frame, same terminals, same manual switch — so a panel builder standardized on one brand's manual motor starters can usually drop in the magnetic-only version without redesigning the mounting.
Frequently Asked Questions
What's the difference between a magnetic-only MPCB and a thermal-magnetic MPCB?
A thermal-magnetic MPCB has both an adjustable bimetal overload element and a fixed magnetic short-circuit trip in one device. A magnetic-only MPCB has just the magnetic trip and manual switching function; overload protection comes from a separate thermal or electronic relay wired into the starter.
Can any overload relay be paired with a magnetic-only MPCB?
Only within a tested and published coordination table. Manufacturers test specific MPCB, contactor, and relay combinations together at a declared short-circuit current; mixing brands or untested part numbers leaves the assembly without a proven Type 1 or Type 2 rating.
Do I still need to set a current dial on a magnetic-only MPCB?
Some magnetic-only devices have an adjustable range that should bracket the motor's full-load current for the manual switching function, though this setting provides no overload protection — that comes entirely from the relay.
What trip class should I use for the overload relay?
Class 10 covers most standard motors with normal start times. Class 20 or 30 suits high-inertia loads with longer starts — a large fan or crusher, for example — where a Class 10 setting would trip during a normal start.
Is a magnetic-only MPCB cheaper than a thermal-magnetic one?
Not necessarily once the separate overload relay is priced in. The combination can cost more than a single thermal-magnetic device, but it buys relay features — electronic trip curves, communication, ground-fault sensing — that a built-in bimetal doesn't offer.
Does Type 2 coordination still apply with a separate overload relay?
Yes. Coordination testing covers the full assembly — MPCB, contactor, and overload relay — at a declared fault current, regardless of whether the overload element is built into the breaker or supplied as a separate relay.
Conclusion
A magnetic-only MPCB is a deliberate trade: give up the built-in bimetal to gain flexibility in how and where overload protection is implemented. Size the relay to the motor's FLC, match its trip class to the actual start time, and confirm the coordination table covers the exact MPCB-contactor-relay combination before it goes in the panel. Get those three right and the split performs the same job as a one-box thermal-magnetic MPCB, with more room to change the relay later without touching the disconnect. For background on the base device and the wider category of motor protection circuit breakers, see the MPCB engineering guide.
