Protection Relays in Soft Starter Panel

The most common functions are overcurrent, earth fault, phase loss, phase unbalance, under/overvoltage, and thermal overload. In soft starter panels, these functions complement the soft starter’s internal protections by covering feeder-level and system-level abnormal conditions. For critical motors, locked-rotor, stall, and start-time supervision are also important. IEC 60947-4-1 addresses motor-control coordination, while IEC 61439-2 governs integration of the relay and its auxiliaries within the assembly. In practice, a multifunction numerical relay is often preferred because it reduces wiring, provides event logs, and supports communication to SCADA or BMS systems.

Selection starts with motor full-load current, starting profile, number of starts per hour, and the prospective short-circuit current at the installation point. The relay must accept CT ratios suitable for the feeder current and provide adequate measuring range for start and overload conditions. It should also coordinate with the soft starter’s current limit, ramp time, and bypass contactor sequence. For IEC 61439-2 compliance, the relay’s heat dissipation, auxiliary supply, and terminal arrangement must be considered in the panel temperature-rise calculation. Where the motor is critical, choose a relay with configurable curves, event recording, and communication ports such as Modbus or Ethernet.

The panel assembly itself is typically designed to IEC 61439-1 and IEC 61439-2. The relay device and its switching/protection coordination are usually aligned with IEC 60947, particularly where control circuits and motor feeders are involved. If the panel is used in explosive atmospheres, the enclosure and installation conditions may also require IEC 60079 review. For installations where arc risk is a concern, IEC 61641 is relevant for internal arc testing of low-voltage switchgear and controlgear assemblies. In all cases, the relay must be integrated so that insulation, clearances, thermal performance, and short-circuit withstand remain compliant at the finished assembly level.

Yes, but the trip method must be engineered carefully. In many panels, the protection relay trips the upstream contactor, shunt trip coil, or the soft starter’s stop input rather than interrupting power directly through the relay contacts. The preferred method depends on the soft starter brand, control philosophy, and fail-safe requirements. Under IEC 61439-2, the control circuit wiring, coil loads, and auxiliary voltage rating must be verified as part of the assembly design. If the relay is part of a safety or critical trip scheme, confirm that the trip logic remains effective during undervoltage conditions and that the bypass contactor opens reliably.

Common options include Modbus RTU, Modbus TCP, Profibus, Profinet, and Ethernet/IP depending on the relay family and site automation standard. These interfaces allow current, voltage, thermal status, trip alarms, and event logs to be sent to SCADA, DCS, or BMS platforms. In soft starter panels, communication is especially useful for remote diagnostics, predictive maintenance, and start/stop event tracking. When designing the IEC 61439 assembly, keep communication wiring segregated from power conductors to reduce EMC issues, and ensure the relay’s auxiliary power supply is stable during start events and voltage dips.

A protection relay adds heat through its electronics, auxiliary power consumption, CT burden, and any associated power supplies or communication modules. While the heat contribution is usually small compared with the soft starter and bypass contactor, it still matters in compact enclosures. IEC 61439-1 requires the assembly designer to verify temperature rise for all installed components at the declared rated current. In practice, relay placement, spacing from power devices, enclosure ventilation, and derating of nearby components should be reviewed. For high-duty soft starter panels, fan-assisted cooling or a larger enclosure may be needed to maintain acceptable internal temperatures.

Although protection relay circuits are low power, they must survive the same installation environment as the main feeder. Wiring, terminals, CT circuits, and auxiliary supplies should be protected and routed so that a fault in the power section does not damage the relay or cause unsafe operation. The assembly’s declared short-circuit withstand rating under IEC 61439-2 must be matched to the panel busbar system and upstream protective device. If the relay is used to issue trip commands during a fault, its outputs should be coordinated so they remain functional long enough to initiate the protective sequence before the fault is cleared by the upstream device.

They are widely used in water pumping stations, HVAC chillers and cooling towers, wastewater plants, compressors, conveyors, crushers, and process drives. In these applications, the relay provides feeder protection, fault recording, and remote monitoring while the soft starter reduces inrush and mechanical stress. For pump systems, functions such as phase loss, underload, and rapid trip logic can help protect against dry running or cavitation-related issues. Patrion integrates these relays into IEC 61439-compliant soft starter panels with upstream MCCBs or ACBs, bypass contactors, and SCADA-ready communication options tailored to site requirements.