How do I size a soft starter for a Soft Starter Panel?

Select the soft starter based on motor full-load current, starting duty, overload class, and the number of starts per hour, not only motor kW. In IEC 60947-4-2 terms, verify the utilization category and thermal capability for the application. In a Soft Starter Panel built to IEC 61439-2, the device’s heat losses must be included in the enclosure temperature-rise assessment, especially if the panel also contains PLCs, power supplies, or network equipment. For high-inertia loads, choose a soft starter with sufficient current margin and a bypass arrangement if permitted by the process.

What short-circuit rating is required for a Soft Starter Panel?

The panel must be rated for the prospective short-circuit current at the installation point, with verification of the complete assembly under IEC 61439-1 and IEC 61439-2. Depending on the protection scheme, this may be a verified short-circuit withstand rating or a conditional short-circuit current rating using a specified MCCB or fuse. Common practical ratings are 25 kA, 36 kA, 50 kA, or higher, but the exact value depends on the network fault level and upstream device selection. The soft starter itself must also be compatible with the protective coordination method.

Do soft starters need bypass contactors in panel assemblies?

Not always, but bypass contactors are common in Soft Starter Panel designs because they reduce heat dissipation after the motor reaches full speed. This improves enclosure thermal performance and can allow higher current density in compact panels. Many modern soft starters offer built-in bypass, which simplifies wiring and reduces component count. For IEC 61439 compliance, the thermal losses of the chosen arrangement must still be calculated and validated. External bypass is often preferred for large motors, frequent starts, or when ambient temperature and ventilation are limited.

Can Soft Starters be used with MCCBs and motor protection relays?

Yes. A typical feeder includes an MCCB or fused switch-disconnector for isolation and short-circuit protection, followed by the soft starter and motor protection functions. Depending on the model, thermal overload and phase-loss protection may be integrated in the soft starter, or an external protection relay may be used for more advanced diagnostics. Coordination should follow IEC 60947 device ratings and the panel’s IEC 61439 assembly verification. This approach is common in pump stations, HVAC units, and conveyor systems where dependable feeder protection and controlled starting are both required.

How does a Soft Starter Panel differ from a VFD panel?

A Soft Starter Panel is designed for reduced-voltage starting and stopping, while a VFD panel provides continuous speed control. Soft starters are generally simpler, generate less harmonic distortion, and are often smaller and cheaper for constant-speed loads such as pumps, fans, and compressors. VFD panels require different EMC, thermal, and harmonic considerations, and often additional filtering. Under IEC 61439, both panel types must be verified for temperature rise, short-circuit withstand, and clearances, but the component selection and application intent are different.

What is the best panel form of separation for soft starters?

The best form of separation depends on maintainability, safety, and footprint. Form 2 is often sufficient for compact industrial panels, while Form 3 or Form 4 may be preferred where outgoing soft starter feeders need better segregation and service continuity. Under IEC 61439-2, the form of separation must be consistent with the internal layout, cable access, and maintenance strategy. For facilities with critical motors or multiple feeders, higher separation can reduce the risk of touching live parts and limit the impact of a fault or maintenance intervention.

Can soft starters in a panel connect to SCADA or BMS systems?

Yes. Many modern soft starters include Modbus RTU, Modbus TCP, Profinet, Profibus, or EtherNet/IP interfaces for status, fault codes, current monitoring, and remote control. In Soft Starter Panel assemblies, this enables integration into SCADA, BMS, or PLC architectures for alarms, trending, and predictive maintenance. Ensure that communication wiring is segregated from power conductors in the panel and that EMC practices are followed. The control circuit design should also align with IEC 61439 and the soft starter manufacturer’s installation guidance.

What thermal issues are most important in a Soft Starter Panel?

The key thermal issue is the heat generated by the soft starter semiconductors during acceleration and, if no internal bypass is used, during continuous operation. This heat adds to losses from MCCBs, contactors, control power supplies, and communication devices. Under IEC 61439-1 and IEC 61439-2, the assembly must be verified for temperature rise at the declared rated current and ambient conditions. Good practice includes allowing airflow around the soft starter, using fan-filter units where needed, applying derating at elevated ambient temperature, and keeping power and control sections properly arranged.

Panel + Component

Soft Starters in Soft Starter Panel

Soft Starters selection, integration, and best practices for Soft Starter Panel assemblies compliant with IEC 61439.

Overview

Soft starters in a Soft Starter Panel are used to limit inrush current, reduce mechanical stress, and improve process reliability during motor starting. In IEC 61439-2 low-voltage switchgear assemblies, the soft starter must be selected not only for motor horsepower or kW, but also for its operational current, overload class, starting duty, and thermal contribution inside the enclosure. In practical panel engineering, that means verifying the device’s continuous current rating, start current capability, bypass arrangement, and the coordination between semiconductor elements, internal cooling, and the panel’s outgoing feeder protection. Typical Soft Starter Panel architectures include one soft starter per motor feeder, usually combined with an MCCB or fused switch-disconnector upstream, a contactor for isolation or bypass, and motor protection via electronic overload or protection relay functions depending on the application. For larger drives, panel builders may use internally bypassed soft starters or external bypass contactors to reduce heat dissipation after ramp-up. This is especially important when the assembly also contains PLC I/O, HMIs, network switches, and control power supplies, because temperature rise must remain within the limits verified under IEC 61439-1 and IEC 61439-2. Panel ventilation, spacing, and derating are therefore part of the design, not an afterthought. Coordination with upstream and downstream protective devices is critical. The soft starter must withstand the prospective short-circuit current at the installation point, and the assembly must be documented with an appropriate short-circuit rating such as 25 kA, 36 kA, 50 kA, or higher, depending on the prospective fault level and the chosen protective device combination. Where specified, the panel may also require conditional short-circuit current ratings linked to a particular fuse type or MCCB model. For motor circuits, coordination should align with IEC 60947-4-2 for semiconductor motor controllers, while the panel assembly itself follows the structural and thermal verification requirements of IEC 61439. Soft Starters are commonly applied in pumps, fans, compressors, conveyors, crushers, and HVAC systems where reduced starting torque and controlled acceleration are desirable. On process plants, communication-ready models with Modbus, Profibus, Profinet, EtherNet/IP, or Ethernet-based diagnostics enable integration with SCADA and BMS platforms for status, fault history, current trending, and remote reset. In more demanding environments, engineers may also consider enclosure protection class, contamination level, altitude derating, and segregation strategy. Forms of separation such as Form 2, Form 3, or Form 4 can be selected to improve serviceability and reduce the impact of maintenance on adjacent feeders. For hazardous areas, the panel design must also consider IEC 60079 requirements when installed near explosive atmospheres, while EMC and immunity considerations may influence cable routing and shield termination. In industrial machine applications, the panel may be part of a larger control system incorporating VFDs, contactors, ACB incomers, MCCB feeders, and safety relays. The result is a Soft Starter Panel that delivers controlled motor starting, compliance with IEC 61439, and dependable integration into modern low-voltage distribution and automation systems.

Key Features

Soft Starters rated for Soft Starter Panel operating conditions
IEC 61439 compliant integration and coordination
Thermal management within panel enclosure limits
Communication-ready for SCADA/BMS integration
Coordination with upstream and downstream protection devices

Specifications

Panel Type	Soft Starter Panel
Component	Soft Starters
Standard	IEC 61439-2
Integration	Type-tested coordination

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