Magnetic trip unit (circuit breaker)

01

Understand how the magnetic element trips the breaker

The US Department of Energy's electrical science handbook explains that a circuit breaker trips automatically when current exceeds a set value. In lower current ratings, a thermal trip element does this: a calibrated bimetal deflects under abnormally high current and releases the linkage holding the breaker shut, and springs open the breaker. Because the bimetal responds to heat, it has an inverse-time characteristic: very high current trips it quickly and moderate overload more slowly. [1]

The handbook adds that molded-case breakers with much larger current ratings also have a magnetic trip element to supplement the thermal one. The magnetic unit uses the magnetic force around the conductor to operate the breaker's tripping linkage. [1]

Eaton's FAZ-NA datasheet shows how the two elements divide the work on one miniature breaker. Its time-current chart lists a specified non-tripping current of 1.13 × In for more than 1 hour and a specified tripping current of 1.45 × In within 1 hour; at 2.55 × In the trip time is 1–60 s for ratings below 32 A and 1–120 s above 32 A. These points belong to the thermal (overload) element. The magnetic element takes over at the curve's instantaneous band, described in the next section. [4]

Eaton describes the FAZ-NA breakers as current limiting: they interrupt fault currents within one half cycle of the fault, which reduces the peak let-through current and energy. The same datasheet lists the series as thermal-magnetic, UL 489 listed, rated up to 63 A, and fulfilling IEC 60947-2. [4]

Ambient temperature acts on the thermal side. Eaton's load-capacity table for the FAZ-NA, referenced to 40 °C, lists a 16 A device carrying 17.6 A at 15 °C and 14.7 A at 60 °C. The table gives no ambient correction for the magnetic trip point, so ask the supplier if the panel runs hot or cold. [4]

Give the supplier the breaker type and frame, rated current, number of poles, the magnetic trip setting or curve required (as a multiple of rated current), whether it must be adjustable, the standard (for example UL 489, UL 1077 or IEC), the interrupting rating, the voltage and whether the load is a motor.

02

Choose the trip point and curve

Miniature breakers express the magnetic trip point as a curve letter. Eaton's FAZ-NA datasheet defines the bands as multiples of rated current (In): B curve 3 to 5 times In, C curve 5 to 10 times In, D curve 10 to 20 times In. Its chart notes give the test points: for type B, 3 × In must not trip within 0.1 s and 5 × In must trip in under 0.1 s; type C uses 5 × In and 10 × In; type D uses 10 × In and 20 × In. [4]

Eaton ties each curve to a load. It lists B curves for resistive loads such as conductors or heaters; C curves for medium inrush such as small transformers, lighting, pilot devices, control circuits and coils; and D curves for high inrush, where the high magnetic trip point prevents nuisance tripping on motors, transformers and power supplies. [4]

c3controls, a control component maker, states that its Series 1100 miniature circuit breakers include both thermal and magnetic trip elements, for overload and short-circuit protection. They come in ratings from 0.5 to 63 A, as UL 489 branch circuit breakers or UL 1077 supplementary protectors, with B, C and D trip curves. It describes C curves as having a low thermal trip delay and a mid-range trip function, and D curves as having a high thermal trip delay and a very high magnetic trip point. [2]

Editorial calculation: for a 16 A breaker, Eaton's bands put the instantaneous trip between 48 and 80 A on a B curve, 80 and 160 A on a C curve, and 160 and 320 A on a D curve. The fault current available at the load end of the circuit has to exceed the upper figure for a guaranteed instantaneous trip.

For motors, c3controls offers magnetic-only motor protection circuit breakers. It explains that these have a fixed trip point at 12 times full-load current, so an overload of 6 to 10 times full-load current will not trip them. Plan overload protection separately when using such a breaker. [3]

Editorial calculation: on a 10 A full-load current, a 12× magnetic-only trip point means about 120 A is needed to trip instantly; 60–100 A would not trip it.

03

Set the magnetic trip on a molded-case breaker

Molded-case breakers (MCCBs) offer the magnetic trip as a fixed or adjustable setting rather than a curve letter. ABB's Tmax catalogue for the Japanese market lists breakers from 15 A to 1200 A, compliant with IEC 60947-2, UL 489, CSA C22.2 and JIS C 8201-2-1. It names three thermal-magnetic and magnetic options: the TMF thermomagnetic trip unit with fixed rated current; the adjustable TMA thermomagnetic trip unit, set at 0.7–1.0 × In for overloads and 5–10 × In for instantaneous trip, available up to 800 A; and an MCP with a magnetic-only trip unit (MA) for motor protection. [5]

In the same catalogue the thermomagnetic (TM) versions span 15–125 A on the XT1 and XT2 frames, 60–225 A on XT3 and 25–250 A on XT4; T6 frames list 300–400 A and 600–800 A TM ratings. Electronic Ekip trip units, which add short-circuit protection with delay, are the alternative where a fixed or 5–10 × In instantaneous band is not enough; they are outside this page. [5]

Editorial calculation: a 100 A TMA unit set to the bottom of its range trips instantaneously from about 500 A; at the top of the range, from about 1,000 A. The same unit's overload setting covers 70–100 A.

A magnetic-only unit leaves the overload job to another device, as c3controls explains for its motor breakers; ABB positions its MA magnetic-only MCP for the same motor-protection role. [3] [5]

04

Magnetic and thermal elements, trip linkage, curves and magnetic-only breakers

The first three rows are the physical parts the DOE handbook describes; the last three are the settings and breaker types by which a magnetic trip is ordered. [1]

05

Rated current, trip multiple, curve, standard and fault clearing for a magnetic trip

Each value names the breaker it comes from — Eaton FAZ-NA, ABB Tmax or c3controls — because every trip multiple is stated against that maker's own rated or full-load current. [4] [5] [2] [3]

06

Send the circuit, load and standard data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Breaker: [type; frame; poles]
Rating: [rated current; voltage; interrupting rating]
Trip: [magnetic multiple or curve; adjustable?]
Load: [motor; resistive; inrush]
Overload: [thermal element or separate relay]
Standard: [UL 489; UL 1077; IEC]

See manufacturer records Send an enquiry through CNFX