A manual motor starter, also called a motor protective circuit breaker (MPCB), combines in one compact device the functions that used to take a disconnect, fuses and a separate overload relay. It switches a motor on and off by hand, protects it against overload and short circuit, and can serve as the protective device in front of a contactor. Well-known product families include the Moeller / Eaton PKZM, the Schneider Electric (Telemecanique) GV2 and the Allen-Bradley 140M series. This article explains how these devices work and how to set them.
What is inside a manual motor starter
An MPCB is a three-pole circuit breaker designed around the needs of a motor. It has two protection systems:
- Thermal (overload) protection: a bimetal element in each pole heats up with the motor current. When the current stays above the setting for long enough, the bimetal bends and trips the mechanism. The delay lets the motor start, since starting current is several times the running current but lasts only a few seconds.
- Magnetic (short-circuit) protection: a coil in each pole acts on the trip mechanism almost instantly when the current reaches a high multiple of the rating. This trip level is fixed by design; the manufacturer's data gives its value.
Many MPCBs also include phase-loss sensitivity: if one phase disappears, the differential action of the thermal elements trips the device faster than a plain overload would. Check the data sheet of the exact model to see if it has this feature.
What is inside a manual motor starter (one pole).
The adjustable current range
Each MPCB covers a narrow current band, and a dial on the front sets the thermal trip point within that band. A product family is therefore made up of many catalog numbers, each with its own range, so that the whole family covers fractional-horsepower motors up to the largest frame the family supports.
Choose a model whose range includes the motor's full-load current (FLA), ideally with the FLA not at the very edge of the range, then set the dial to the motor nameplate full-load current. Do not set the dial higher to stop nuisance trips: a trip usually points to a real problem such as a mechanical load issue, low voltage, a lost phase or a wrong model. Where the applicable electrical code or the motor's service factor allows a different setting, follow the code and the motor manufacturer's guidance.
Our tables of motor nominal current (UL / CSA) and motor nominal current (IEC, kW) give typical values for guidance, but the motor nameplate always comes first.
How to set an MPCB, step by step
- Read the full-load current on the motor nameplate, at the voltage and connection actually used (a dual-voltage motor has two currents).
- Confirm that the MPCB's range includes that value.
- Turn the dial to the FLA. On most models the scale is marked directly in amperes.
- Check that the magnetic trip level will not trip on the motor's starting current. Motors with high inrush may need a model designed for it; see the manufacturer's selection data.
- Verify the short-circuit rating of the MPCB at your supply voltage. If the available fault current is higher, an upstream device or a current-limiting accessory may be required, as listed by the manufacturer.
- Record the setting in the panel documentation.
Auxiliary contacts and trip indication
Accessories turn an MPCB into a monitored device. Typical options are:
- Auxiliary contacts that follow the ON/OFF position of the handle, often used to signal "motor protector on" to a PLC.
- Trip-indicating (signal) contacts that change state only when the device trips on overload or short circuit, so the control system can tell a trip from a manual OFF.
- Undervoltage releases that trip the device when the supply drops, preventing an unexpected restart when power returns.
- Shunt trips that let a remote signal open the device.
- Enclosures, door-mounted handles and lockable handles for local disconnection.
Accessories are usually specific to one family and sometimes to one series, so note the full catalog number of the base unit before ordering them.
MPCB with a contactor
An MPCB alone is operated by hand. For remote or automatic control, it is combined with a contactor, often with a link module that connects the two mechanically and electrically. The MPCB provides the protection, and the contactor does the frequent switching. The contactor must be rated for the motor load (utilization category AC-3 for standard squirrel-cage motors).
MPCB and contactor combined into a compact starter.
UL 508 Type E and Type F in general terms
In North America, how an MPCB may be used depends on how it is listed. In general terms:
- A device listed only as a manual motor controller needs separate upstream branch-circuit protection (fuses or a branch circuit breaker).
- A Type E self-protected combination motor controller is evaluated to provide branch-circuit short-circuit protection, disconnection and overload protection in one device. Type E use often has extra requirements, such as line-side terminal adapters or spacings and limits on the supply system.
- A Type F combination is a Type E device combined with a specific contactor.
The permitted use, the voltage ratings (some ratings are limited to certain grounded systems) and the short-circuit ratings appear on the product's marking and in the manufacturer's listing data. Always check the listing of the exact catalog number and series, and the current edition of the applicable standards and electrical code; listing rules have changed over the years, and UL 60947-4-1 now covers many of these products. Our article on short-circuit current ratings explains why the combination rating matters for the whole panel.
Common product families
| Manufacturer | Family name (examples) |
|---|---|
| Moeller / Eaton | PKZM |
| Schneider Electric / Telemecanique | GV2 (TeSys) |
| Allen-Bradley / Rockwell Automation | 140M |
Within each family, the current range, short-circuit rating and listing differ by catalog number and series: check the manufacturer's data for the exact model.
Key takeaways
- An MPCB combines manual switching, thermal overload protection and magnetic short-circuit protection.
- Pick a model whose range includes the motor FLA, then set the dial to the nameplate FLA.
- Signal contacts let the control system tell a trip from a manual OFF.
- In North America, the listing (manual motor controller, Type E, Type F) decides how the device may be used: check the marking.
Work on electrical equipment must be done by qualified personnel following the applicable electrical code and the manufacturer's instructions.
Related parts at 3JIndustry
Find used manual motor starters and accessories in our motor control collection and in Eaton / Klockner-Moeller parts.
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