Contactor Utilization Categories: AC-1, AC-3, AC-4, Overload Relay Trip Classes and Type 1/2 Coordination

Two contactors with the same frame size and the same catalogue current can behave very differently in service: one runs a pump for years, the other welds its contacts within months on a crane hoist. The difference is almost always the duty, and that is what contactor utilization categories in IEC 60947-4-1 describe. This guide explains AC-1, AC-3, AC-3e and AC-4 in practical terms, how to size contactors for motors, heaters and capacitors, how thermal and electronic overload relays and trip classes work, what Type 1 and Type 2 coordination really mean, and the mistakes that cause chattering, welding and nuisance trips.

How a Contactor and Overload Relay Work Together

A contactor is an electrically operated switch designed for frequent operation. A coil energises a magnet that closes the main contacts; when the coil is de-energised, springs open them and arc chutes extinguish the arc. A contactor is not a protective device. It makes and breaks normal operating currents, including motor starting current, but it is not designed to interrupt short-circuit currents.

The overload relay, mounted directly on the contactor or separately, protects the motor against sustained overcurrent, such as a jammed pump or a lost phase. It does not switch the main circuit itself; it opens an auxiliary contact in the coil circuit, which drops out the contactor. Short-circuit protection comes from a third device upstream: fuses, a motor circuit breaker or an MCCB. The three together form a motor starter, and IEC 60947-4-1 is the product standard that defines how they are rated and tested.

Contactor Utilization Categories in IEC 60947-4-1

A contactor’s rated operational current (Ie) is meaningless without its utilization category and voltage. The category defines the current and power factor the contactor must make and break in normal use, which drives contact erosion and electrical life.

DC loads have their own categories, such as DC-1 for resistive loads and DC-3 and DC-5 for shunt and series motors. DC arcs are harder to extinguish because there is no current zero, so DC ratings are often achieved by connecting poles in series.

Why AC-3 and AC-4 Contactor Utilization Categories Are So Different

When a squirrel-cage motor starts direct-on-line, it draws a locked-rotor current that is typically several times its rated current, commonly in the region of six to eight times. In AC-3 duty the contactor must make that current but only breaks the running current once the motor has accelerated, so the arc at opening is modest.

In AC-4 duty, the contactor opens while the motor is still at or near standstill: during inching, or when reversing the motor before it stops. It must then break the full starting current. The arc energy is far higher, the contacts erode quickly and electrical life falls by a large factor. Manufacturers publish separate AC-4 ratings and life curves for this reason. Applying an AC-3 rated contactor to a crane, hoist or conveyor that is jogged frequently is one of the most common causes of premature contact failure.

The practical rules are simple:

• For motors started and stopped normally, select by AC-3 (or AC-3e) rating at the supply voltage, using the motor’s rated current or power.
• For inching, plugging or frequent reversing, use the manufacturer’s AC-4 rating or mixed AC-3/AC-4 life curves, which usually means a larger frame.
• For heaters and other resistive loads, AC-1 applies, allowing a smaller frame than the same current would need in AC-3.
• For capacitor banks, use contactors designed for capacitor switching, often fitted with pre-charge resistors to limit inrush.

Mechanical vs Electrical Life

Mechanical life is the number of no-load operations before the mechanism wears out and is usually in the millions. Electrical life is the number of operations under load before the contacts are worn, and depends heavily on category, current and voltage. A contactor that switches a small fraction of its AC-3 rating may last for a very large number of operations, while the same contactor at full AC-4 duty may need contact replacement much sooner. For high-cycle applications such as packaging machines or compressors with frequent loading and unloading, check the manufacturer’s electrical life curve at your actual current rather than relying on the catalogue headline.

Overload Relays: Thermal vs Electronic

Thermal (bimetal) overload relays

A heater element in each phase warms a bimetal strip; sustained overcurrent bends the strips until a trip mechanism operates. Most are ambient-temperature compensated within a stated range and include differential (phase-loss) sensitivity, which trips faster when one phase carries much less current than the others. They are simple, low cost and do not need an auxiliary supply, but they have a narrow adjustment range and their characteristics are affected by mounting position and nearby heat sources.

Electronic overload relays

Current transformers measure phase currents and a microcontroller models motor heating. Benefits include wide adjustment ranges (fewer variants to stock), selectable trip class, accurate phase-loss and imbalance detection, ground-fault detection on some models, and communication on higher-end devices. Many are self-powered from the measured current. In hot panels, electronic relays are less sensitive to ambient temperature than bimetal designs, although their own electronics still have temperature limits.

Trip classes

The trip class defines the maximum tripping time from cold at 7.2 times the current setting. Class 10A and Class 10 trip within 10 seconds, Class 20 within 20 seconds and Class 30 within 30 seconds. Most standard motors on pumps and fans are protected with Class 10. Heavy-inertia loads such as large fans, centrifuges, crushers and loaded conveyors may need Class 20 or 30 so the relay does not trip during a long acceleration, provided the motor manufacturer confirms that the motor can withstand the longer stall time.

Setting the Overload Relay Correctly

1. Set the relay to the motor’s rated current from the nameplate at the actual supply voltage and connection, not to the contactor rating or a rounded-up figure.
2. For star-delta starters with the relay in the motor phase circuit, set it to about 0.58 times the motor rated current (1/√3), because it carries phase current rather than line current.
3. If the motor has been derated for high ambient temperature or altitude, set the relay to the derated current rather than the 40 °C nameplate value.
4. Choose the trip class from the measured or calculated run-up time, not by default.
5. Decide between manual and automatic reset. Automatic reset can restart a motor unexpectedly on a two-wire control circuit and is usually avoided where this creates a hazard.

Motors supplied from variable speed drives are normally protected by the drive’s own electronic motor overload function; a separate thermal relay downstream of a drive can misread the non-sinusoidal current. Our guide to VFD selection for hot climates covers drive-side motor protection.

Type 1 and Type 2 Coordination

Coordination describes how the starter behaves when the short-circuit protective device (SCPD) clears a fault on the motor circuit. IEC 60947-4-1 defines two levels, and manufacturers publish tested combinations of SCPD, contactor and overload relay for each, at stated prospective short-circuit currents and voltages.

• Type 1: under short-circuit conditions the starter must not cause danger to people or the installation, but it may be unfit for further service without repair or replacement of parts. Acceptable where downtime after a fault is tolerable.
• Type 2: the starter must remain suitable for further use after the fault. Light welding of contacts is recognised as a risk, provided they can be separated easily and the manufacturer gives maintenance instructions. Required for process plants, critical pumps and continuous operations where a starter must be back in service quickly.

Coordination is only valid for the exact combination tested. Changing the fuse type, the MCCB model or the contactor size, even to a nominally larger one, can invalidate it. Always take the combination from the manufacturer’s coordination tables for the prospective fault current at the switchboard. For upstream protective devices, see our circuit breakers category.

Coils, Control Voltage and Hot-Panel Effects

• Coil voltage and type: AC coils, DC coils and electronic wide-range coils exist. Electronic coils tolerate voltage variation better and draw less holding power, which reduces panel heat.
• Voltage dips: AC contactors drop out below a certain coil voltage. On weak supplies or during large motor starts, this can cause chatter or unwanted stops. Ride-through modules or DC control supplies help.
• Coil suppression: RC modules, varistors or diodes limit switching surges that disturb PLC outputs. Diodes on DC coils noticeably lengthen drop-out time, which matters for safety and reversing interlocks.
• Ambient temperature: catalogue ratings are given at a stated ambient around the device. In GCC panels with internal temperatures well above that figure, apply the manufacturer’s derating for current and check coil operating limits.
• Reversing and star-delta: use mechanical and electrical interlocks between contactors that must never close together. In star-delta starters, line and delta contactors carry about 0.58 times and the star contactor about one-third of the motor rated current.

Troubleshooting Common Failures

Selection Checklist

• Load type and utilization category (AC-1, AC-3, AC-3e, AC-4, capacitor or lamp duty).
• Motor power, rated current, voltage and starting method (DOL, star-delta, reversing).
• Operations per hour and expected inching or plugging.
• Coil voltage and type (AC, DC or electronic wide-range) and auxiliary contact requirements.
• Overload relay type, setting range and trip class; manual or automatic reset.
• Prospective short-circuit current at the board and required coordination type.
• Panel internal temperature and enclosure ventilation.
• Existing part numbers if replacing installed equipment.

Contactors and Overload Relays We Supply

These examples cover the contactor sizes and protection devices discussed above.

Send the motor power, voltage, coil voltage and the utilisation category through our quote request form.

Sourcing Contactors and Overload Relays Through Seven Star LLC

Seven Star LLC supplies contactors and overload relays on request from manufacturers such as ABB, Schneider Electric, Siemens and Eaton, along with motor circuit breakers, motor soft starters and accessories. We help with selection by utilization category and coordination type and cross-check part numbers for replacements; availability and lead time are confirmed at quotation.

Need a replacement contactor or a coordinated starter combination? Send us the motor nameplate, the existing contactor and overload relay part numbers or your starter schedule, and we will prepare a quotation.