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Two MCCBs sit next to each other on a quotation. Same frame, same 36 kA breaking capacity at 415 V, same pole count, same terminal layout. One is priced 30 to 40 percent above the other, and the line that explains the gap is a single word in the description: trip type.
Thermal-magnetic versus electronic trip is usually presented as old technology against new. That framing is not useful for anyone specifying a panel. Both trip types are current products, both are type tested to IEC 60947-2, and both will protect a cable correctly if the device is sized properly. What separates them is a specific list of things one can do and the other cannot — and a set of cost and failure-mode trade-offs that decide which one belongs in your panel.
This comparison works through the mechanism of each trip type, the parameters that actually differ on site, and the cases where the cheaper device is the correct engineering choice rather than a compromise.
A thermal-magnetic (TM) trip unit is a purely electromechanical assembly: a bimetal strip for overload and a solenoid for short circuit. There is no electronics, no measurement chain and no auxiliary supply — the current passing through the device is the only input it needs.
An electronic trip unit replaces both elements with current transformers, a signal-processing circuit and a low-energy actuator. It measures current, compares it against settings stored in the unit, and releases a latch when a threshold and its associated time delay are exceeded. Because the thresholds are settings rather than geometry, the same physical device can be configured for a range of duties.
Both are governed by the same product standard. IEC 60947-2 requires a device to carry 1.05 times its rated current without tripping and to trip at 1.30 times rated current inside a conventional time — one hour up to 63 A, two hours above it. That is the floor both trip types meet. Everything above the floor is where they diverge.
The overload element is a bimetal strip heated by the load current, often indirectly through a heating coil. Its deflection is a function of the heating it has absorbed, which is roughly proportional to the square of current multiplied by time. When deflection exceeds a calibrated threshold, the strip releases the latch.
Three consequences follow directly from that mechanism, and all three matter at commissioning:
The short-circuit element is a solenoid whose plunger strikes the latch when current exceeds a factory-set multiple of the rating — commonly somewhere between five and ten times In, sometimes with an Im dial to trim it. Above that threshold the device trips with no intentional delay, which is why a TM breaker is a utilisation category A device under IEC 60947-2.
The measurement chain is the defining feature. Current transformers built into the breaker produce a signal proportional to load current; the trip unit samples it, calculates a true RMS value, and compares the result with the settings. When a threshold is exceeded for longer than its delay, the unit fires an actuator that opens the mechanism.
Because the comparison is arithmetic rather than mechanical, the settings can be expressed as multipliers of the rating and adjusted independently. A typical MCCB electronic trip unit exposes most of the following:
| Ortam | What it defines | Typical adjustment band |
|---|---|---|
| Ir — long-time pickup | Overload threshold, the continuous current the breaker will carry | 0.4 to 1.0 × In, in steps |
| tr — long-time delay | How long an overload at six times Ir is allowed to persist | A few tenths of a second up to about 24 s, by curve shape |
| Isd — short-time pickup | Fault current level at which the short delay starts timing | 1.5 to 10 × Ir |
| tsd — short-time delay | Intentional hold time before tripping between Isd and Ii | 0 to about 0.4 s, often with an I²t option |
| Ii — instantaneous | Level above which the breaker trips with no delay at all | 1.5 to 15 × In, sometimes defeatable |
| Ig — earth fault | Residual current threshold, where the function is fitted | 0.2 to 1.0 × In |
Exact ranges differ between manufacturers and frames — always read the specific trip unit data sheet rather than assuming. But the structure is common, and the important point is that the intentional short-time delay turns the device into a utilisation category B breaker. That single capability is what makes time-graded selectivity possible in a distribution board. Our guide to MCCB açma ünitesi ayarları covers how to coordinate those dials against each other.
| Parametre | Termal-manyetik | Electronic trip |
|---|---|---|
| Protection curve | Fixed by construction; narrow Ir band if fitted | Adjustable, independently, per function |
| Kasıtlı kısa süreli gecikme | None — category A only | Yes — category B, enables time grading |
| Ambient temperature influence | Direct; the bimetal is part of the measurement | Minimal in the measurement chain; electronics have their own temperature limits |
| Repeatability after heavy loading | Thermal memory shortens the next trip time | Largely unaffected by prior loading |
| Earth fault protection | Not available | Available as Ig on many frames |
| Measurement and communication | Yok | Current, energy and status where the unit supports it |
| Auxiliary supply | Not required | Not required to trip; often required for display and communications |
| Failure mode | Drift, contact wear, welded latch | Electronics failure, settings corruption, actuator fault |
| Spares strategy | One part number per rating | One adjustable unit can cover several ratings |
| Relative cost, same frame and Icu | Baseline | Higher — commonly a fifth to a half more |
The extra cost of an electronic trip is not margin. The device carries current transformers sized and classed for measurement, a populated control board, a calibrated actuator, and firmware. Every one of those has to survive the same type tests as the TM version, including the short-circuit test that subjects the interior to a fault at full breaking capacity.
Two effects push back on that price. First, an adjustable unit covers a range of ratings, so a distributor can hold one frame instead of four. Second, adjustability sometimes lets you stay on a smaller frame: if the calculated load sits at 160 A and the next frame up is a 250 A TM unit with a fixed 250 A curve, an electronic 250 A unit set to Ir = 0.64 does not help — but an electronic unit on a 160 A frame that can be trimmed upward later often removes the need to buy a bigger breaker for a design margin that may never be used.
Where the arithmetic genuinely changes is when the alternative is a larger TM breaker plus cable sized for its fixed threshold. A curve you can move is often cheaper than copper. The factors that build an MCCB quotation are worth reading before comparing two offers on price alone.
In a panel, the breaker sits inside an enclosure that is warmer than the room. A TM breaker’s bimetal is already partly deflected at 55 °C ambient before any load is applied, so its effective rating falls. Catalogue derating tables quantify this, and the correction is not small — a device carrying its full rating at the reference ambient may be limited to roughly 80 percent of that at 55 °C, depending on frame and manufacturer.
An electronic trip unit measures current with transformers, so the trip decision is largely independent of the temperature around it. That does not remove the derating question, and it is a common misreading of the datasheet:
So electronic trip removes one variable from the thermal equation. It does not remove the enclosure temperature calculation. If your panel runs hot, you still need the derating exercise described in our guide to choosing the right MCCB.
There is a large part of the market where a TM breaker is the correct specification and an electronic unit is simply more device than the job needs:
The inverse list is shorter and more absolute. Choose electronic trip when any of the following is true:
Both trip types are frequently asked to compensate for errors made elsewhere in the design. They cannot.
A correctly set electronic trip unit will not rescue a breaker whose breaking capacity is below the prospective fault current at its terminals. Icu and Ics selection is a separate calculation with its own inputs. Nor will either trip type protect a cable that is undersized for the load, because the cable’s sustained limit and the breaker’s long-time curve have to be coordinated deliberately.
Conversely, a TM breaker is not automatically less safe than an electronic one. It is a type-tested protective device with a defined curve, and when the curve suits the load it is as reliable as anything on the market — arguably more so in an enclosure that runs at 60 °C.
Before you choose, put these eight items on one line each. They are the questions that decide the answer, and most of them cannot be answered from a price list.
| Ürün | What to state | Why it decides the trip type |
|---|---|---|
| 1. Load and cable | Design current, cable size, installation method | Sets the long-time curve the device must follow |
| 2. Prospective fault current | kA at the device terminals | Sets Icu and Ics, independent of trip type |
| 3. Upstream and downstream devices | Their ratings and trip types | Decides whether time grading is needed at all |
| 4. Required selectivity | Full, partial, or none | Full selectivity upstream normally forces category B |
| 5. Ambient in the enclosure | °C at the breaker, not in the room | Decides how much of the rating survives derating |
| 6. Earth fault | Required at this level, yes or no | Only available on electronic trip units |
| 7. Monitoring | Any need for current, energy or trip cause | Points to electronic, or to a separate meter |
| 8. Maintenance capability | Who adjusts settings, and how often | A curve nobody can set correctly is a liability |
Physically, usually yes if the frame, pole count and terminal arrangement match. Electrically, you must re-check the settings: an electronic unit shipped at Ir = 1.0 will not reproduce the curve of the TM device it replaces unless you set it to the same effective pickup, and the short-time delay must be set to zero or coordinated, otherwise downstream discrimination changes.
Not to trip. Most units are self-powered from their own current transformers above a threshold current, typically a fraction of the rating. Auxiliary supply is normally needed for the display, communications and sometimes for earth-fault functions when the load current is too low to self-power the electronics. Confirm the self-powering threshold with the supplier if the circuit can run at very light load.
The electronic unit is more repeatable, because its threshold is arithmetic rather than mechanical. However its accuracy at very low current depends on the current transformer range, so a unit rated 250 A is less precise at 30 A than a smaller frame would be. A TM breaker is less repeatable but its bimetal responds to real heating, which is the quantity that actually damages a cable.
Rarely on protection grounds alone, because fault levels and discrimination requirements at that size are usually modest. It becomes worthwhile when the same panel also needs earth fault at the incomer, or when the load is genuinely uncertain and you want to defer the final rating decision.
Generally no, and the direction of the mismatch matters. Selectivity needs the upstream device to delay; a category A breaker has no intentional delay to give. The workable arrangement is the reverse: electronic upstream, thermal-magnetic or smaller electronic downstream, with the short-time settings graded. Check the manufacturer’s selectivity tables rather than relying on current ratios alone.
For the range itself, including frame sizes, pole configurations and the trip options available, see our moulded case circuit breaker range.
