Fuse and Contactor Coordination: IEC Type 1, Type 2 and Fuse Selectivity
Fuse-contactor coordination is easiest to understand when the devices are given their correct jobs. In a conventional motor starter, the contactor switches the motor, the overload relay protects the motor against sustained overload, and the fuse or other short-circuit protective device (SCPD) clears short-circuit current.
IEC 60947-4-1 Type 1 and Type 2 coordination describe how that starter plus its specified SCPD behaves when subjected to a short circuit. They do not describe what happens if the fuse “fails to clear” and the contactor is forced to interrupt the fault. In the coordinated test, the SCPD performs the fault interruption; the Type 1 or Type 2 classification describes the condition in which the contactor/starter is left afterward.
That distinction is important because a contactor is not a circuit breaker. Its normal ratings—AC-3, AC-4, DC utilization categories and rated operational current—describe switching duty. Short-circuit survival is established through the coordinated motor-starter test and the manufacturer’s specified protective device.
The three devices in a motor starter
A fuse or other SCPD is selected for short-circuit protection. It needs an interrupting/breaking rating adequate for the available fault current at the installation point, and its time-current characteristic must allow normal motor starting while protecting conductors and the starter.
The contactor is selected for normal switching duty. IEC utilization categories such as AC-3 and AC-4 matter because they define how severe the making and breaking duty is. AC-3 is the familiar squirrel-cage motor starting and running-duty category; AC-4 covers more severe inching, plugging and reversing duties and can require a larger contactor for the same motor current.
The overload relay provides motor overload protection. It responds to sustained overload or phase-loss conditions according to its trip class and settings. It is not a substitute for the branch short-circuit protective device.
This three-device division is why a motor circuit should not be reduced to “choose a time-delay fuse large enough not to blow on start.” The fuse, contactor and overload relay must work as a system.
IEC Type 1 and Type 2 coordination
IEC 60947-4-1 distinguishes Type 1 and Type 2 short-circuit coordination for contactors and motor starters.
Under Type 1 coordination, the short circuit must be safely cleared without danger to people or the installation, but the contactor or overload relay may be damaged. Repair or replacement can be required before the starter is returned to service.
Under Type 2 coordination, the short circuit must again be safely cleared, but the starter is expected to remain suitable for continued use without repair or replacement. IEC practice permits limited contact welding provided the contacts can be easily separated and the manufacturer’s maintenance requirements are followed. Eaton’s current Type 2 guidance summarizes the practical result as no significant damage to the contactor or overload relay, with light, easily separable contact welding permitted.
The classification belongs to the specific combination. A Type 2 result for one contactor, overload relay, fuse rating, voltage and prospective short-circuit current does not automatically transfer to a larger fuse, a different contactor frame or another manufacturer’s fuse.
This is why the correct design tool is the motor-control manufacturer’s Type 1/Type 2 coordination table. Eaton, Siemens, Schneider Electric and other motor-control manufacturers publish combinations stating the contactor/starter, overload device, SCPD and maximum prospective short-circuit current for which the coordination claim has been verified.
Do not calculate Type 2 from a contactor I²t number alone
The source draft treats the contactor as if it normally carries one universal “conditional short-circuit I²t withstand” that can simply be compared with fuse clearing I²t. That is too simple for an IEC Type 1/Type 2 claim.
A manufacturer can publish let-through, peak current or withstand information as supporting data, but Type 1 and Type 2 are combination test results. Contact condition, overload-relay performance, enclosure behavior, dielectric withstand and post-fault operation all form part of the acceptance criteria.
A fuse with lower let-through energy often makes Type 2 easier to achieve, which is why current-limiting fuses are frequently used in Type 2 combinations. Eaton notes that verified Type 2 combinations often use current-limiting Class J, Class CF, Class RK1 or Class CC fuses. But the presence of a fast current-limiting fuse does not create a Type 2 rating by calculation alone.
The engineering sequence is:
- determine the available short-circuit current at the starter;
- select the motor starter for load and utilization category;
- select the SCPD family and rating;
- find the manufacturer’s tested Type 1 or Type 2 table for that exact combination;
- confirm voltage, prospective current and all table footnotes.
If the exact combination is not in the table, describe it as unverified rather than inventing a Type 2 claim.
Motor starting, fuse characteristic and overload protection
Motor starting current can be several times full-load current, so the fuse characteristic has to ride through the acceleration period. That does not mean every motor needs one specific fuse class.
IEC gG fuses can protect motor circuits when sized and coordinated appropriately. gM is a full-range motor category. aM is a partial-range motor category intended for short-circuit protection and must be used with a separate overload-protection function.
In North American systems, dual-element time-delay Class RK5 and time-delay Class J products are common motor-branch options. Class R and Class J are fuse classes; the exact time-delay behavior belongs to the selected product series. A fast-acting Class J series and a time-delay Class J series can share the same UL class while behaving very differently during motor starting.
Class T should not be described generically as a motor time-delay fuse. Common JJN/JJS Class T products are fast-acting, so a motor application has to be verified against the actual starting current and time-current curve.
The overload relay remains essential where the fuse is intended primarily for short-circuit protection. A time-delay fuse is not a precision motor overload relay.
Fuse-to-fuse selectivity is a separate coordination problem
Type 1/Type 2 starter coordination and fuse-to-fuse selective coordination are related protection topics, but they are not the same test.
For two current-limiting fuses in series, the high-current coordination principle is that the downstream fuse should finish clearing before the upstream fuse reaches its melting condition. In I²t terms, the downstream total-clearing I²t must remain below the upstream minimum-melting I²t for the applicable fault condition.
That principle is useful, but it should not be converted into a universal safety multiplier such as 1.6, 2 or 10 and then presented as an industry standard. Fuse manufacturers publish tested or validated selectivity ampere-rating ratios for their own fuse families.
Eaton’s current Bussmann selectivity guide, for example, states that many Low-Peak fuse combinations achieve selective coordination with a 2:1 lineside-to-loadside ampere ratio, while other family combinations require 3:1, 4:1, 8:1 or another published ratio. Eaton further states that meeting the published ratio can provide selective coordination for all levels of overcurrent up to the stated interrupting range, commonly as high as 200 kA for the documented combinations.
That directly contradicts the idea that fuse coordination must always become only “partial selectivity” above some inevitable crossover current. Partial selectivity certainly exists in some protection systems, but properly selected current-limiting fuse combinations can be fully selective throughout their published range.
The safest approach is therefore:
- use manufacturer fuse-to-fuse selectivity tables where available;
- use time-current curves for the overload/long-time region;
- use manufacturer current-limiting/selectivity data for the sub-cycle region;
- do not mix selectivity ratios from one manufacturer with another manufacturer’s fuse.
AC-3, AC-4 and switching duty
AC-3 and AC-4 are contactor utilization categories, not fuse categories.
AC-3 applies to conventional starting and switching of squirrel-cage motors, with opening normally occurring while the motor is running. AC-4 covers much more severe inching, plugging and reversing duty, with repeated high-current switching.
A contactor may therefore have a substantially lower operational-current rating in AC-4 than in AC-3. If the machine performs frequent jogging or reversing, the contactor must be selected from its AC-4 data rather than from the larger AC-3 number printed on the same product family.
The fuse also sees the repeated current pulses, so its cycling capability and TCC still matter, but AC-4 does not create a universal rule that a particular fuse class must be used. Select the fuse from the actual duty and the manufacturer’s starter coordination tables.
DC contactors and semiconductor circuits
The motor-starter Type 1/Type 2 framework should not be casually extended to every DC converter or semiconductor circuit.
A DC contactor is still primarily a switching device unless the manufacturer gives it a protective interruption function. The semiconductor fuse is selected to protect the converter or power device using the semiconductor manufacturer’s withstand data and the high-speed fuse’s total-clearing I²t and peak let-through data.
Where a DC contactor, high-speed fuse and converter are offered as a tested combination, use that manufacturer data. Otherwise, describe the protection as a component-coordination study rather than calling it IEC Type 2 by analogy.
High-speed semiconductor fuses such as aR or gR are appropriate when their performance is matched to the protected device. A general-purpose fuse is not automatically suitable for an IGBT, diode or thyristor simply because it has a high interrupting rating.
Common mistakes
The first mistake is assuming the contactor is supposed to interrupt the branch short circuit. In a fused starter, the SCPD is the short-circuit interrupter.
The second is claiming Type 2 from a hand calculation. Type 2 belongs to the tested combination.
The third is changing the fuse rating inside a published Type 2 table without rechecking the table. A larger fuse can materially increase let-through and invalidate the result.
The fourth is treating the fuse as motor overload protection when the system actually relies on an overload relay.
The fifth is assuming all Class J or Class R fuses have one time-current characteristic. The UL class does not erase series-level differences.
The sixth is applying a generic 2:1 fuse selectivity rule to every fuse family. Some combinations use 2:1; others require larger ratios. Use the manufacturer’s table.
The seventh is describing all real systems as only partially selective. Published current-limiting fuse ratios can provide full selective coordination through the stated interrupting-current range.
Official technical references
- Eaton Bussmann Selecting Protective Devices Handbook: https://www.eaton.com/content/dam/eaton/products/electrical-circuit-protection/fuses/technical-literature/bus-ele-an-3002-spd.pdf
- Eaton User’s Guide to IEC Type 1 and Type 2 Coordination: https://www.eaton.com/content/dam/eaton/products/industrialcontrols-drives-automation-sensors/overload-relays/User%27s%20Guide%20to%20IEC%20Type%201%20and%20Type%202%20Coordination.PDF
- Eaton Wiring Manual: https://www.eaton.com/content/dam/eaton/markets/machinebuilding/eaton-wiring-manual-pu08703001z-en-en-us.pdf
- Eaton selective coordination resources: https://www.eaton.com/us/en-us/products/electrical-circuit-protection/fuses/selective-coordination.html
- Eaton Bussmann Class RK5 fuse family: https://www.eaton.com/us/en-us/catalog/fuses-and-fuse-holders/bussmann-series-class-rk5-fuses.html
- Eaton Bussmann Class J fuse family: https://www.eaton.com/us/en-us/catalog/fuses-and-fuse-holders/bussmann-series-class-j-fuses.html
No AI-generated product photo is used in this article.
Frequently asked questions
What is IEC Type 1 coordination?
A Type 1 coordinated starter clears the short circuit safely without danger to people or the installation, but the contactor or overload relay may be damaged and may require repair or replacement.
What is Type 2 coordination?
Type 2 requires the coordinated starter to remain suitable for further use after the short circuit. Significant damage is not permitted; limited easily separable contact welding can be allowed under the standard/manufacturer criteria.
Can I calculate Type 2 by comparing fuse I²t with a contactor withstand number?
Not as a complete IEC Type 2 claim. Use the manufacturer’s tested coordination table for the exact starter, SCPD, voltage and prospective short-circuit current.
Does the fuse protect the motor against overload?
Not necessarily. In many motor starters, the fuse provides short-circuit protection and the overload relay provides motor overload protection. The exact division depends on the fuse category and starter design.
Is fuse-to-fuse selectivity always partial?
No. Manufacturer selectivity ratio tables for current-limiting fuses can provide full selective coordination through the published current range when the specified ratio and conditions are met.
Is 2:1 always the fuse selectivity ratio?
No. Eaton publishes 2:1 for many same-family combinations, but other combinations can require 3:1, 4:1, 8:1 or another ratio. Use the table for the exact fuse families.