Introduction
Selecting the right motor protection device is one of the most consequential decisions in designing a motor control center (MCC). Underprotect a motor and you risk costly burnouts and unplanned downtime. Overspecify the protection and you waste budget on features you will never use.
This guide compares two categories of intelligent motor protection devices: the ST570 series intelligent motor protection controllers and the ST420 series microprocessor line protection relays. We cover the protection functions each provides, communication capabilities, installation requirements, and how to match the right device to your application.
Understanding Motor Failure Modes
Before selecting a protection device, it is important to understand what you are protecting against. The most common causes of motor failure are:
- Overload (thermal overload): Sustained overcurrent causes winding insulation to degrade. Accounts for approximately 30% of motor failures.
- Phase loss (single phasing): Loss of one phase causes the motor to draw excessive current on the remaining phases, rapidly overheating the windings. One of the most destructive fault types.
- Phase imbalance: Unequal voltages on the three phases cause negative sequence currents that produce additional heating in the rotor.
- Locked rotor (stall): Motor fails to start or stalls under load, drawing locked rotor current (typically 6–8× full load current) continuously.
- Undercurrent (loss of load): Sudden drop in current indicates loss of mechanical load (broken coupling, pump cavitation). Protects the driven equipment.
- Ground fault: Insulation breakdown causes current to flow to earth. Detected by residual current measurement.
- Thermistor trip: PTC thermistors embedded in motor windings provide direct winding temperature measurement.
ST570 Series: Intelligent Motor Protection Controllers
Overview
The ST570 series are dedicated motor protection controllers designed specifically for three-phase AC motor protection in MCC applications. They combine comprehensive motor protection functions with integrated current measurement, display, and communication in a single compact unit.
ST570 Models
- ST570B1: Standard motor protection controller for 380–440V systems. Integrated current measurement via built-in CTs. Protection functions: overload, phase loss, phase imbalance, locked rotor, undercurrent, ground fault, thermistor.
- ST570: Modular design version. Separate current input module allows use with external CTs for higher current ratings. Suitable for motors up to 6300A.
- ST570L: Integrated LCD display version. Full parameter display and setting via front panel without additional HMI. Ideal for standalone MCC drawers.
Protection Functions — ST570 Series
| Protection Function | Standard | Trip Class |
|---|---|---|
| Thermal overload | IEC 60947-4-1 | Class 5, 10, 20, 30 (selectable) |
| Phase loss | IEC 60947-4-1 | Instantaneous or delayed |
| Phase imbalance | IEC 60947-4-1 | Adjustable threshold (5–30%) |
| Locked rotor / stall | IEC 60947-4-1 | Adjustable time delay |
| Undercurrent | — | Adjustable threshold and delay |
| Ground fault | IEC 60947-4-1 | Adjustable threshold |
| Thermistor (PTC) | IEC 60034-11 | Direct winding temperature |
| Starts per hour limit | — | Prevents thermal damage from frequent starting |
Communication and Integration
- RS485 / Modbus RTU: Standard on all ST570 models. Full parameter read/write and status monitoring via SCADA or PLC.
- Modbus register map: Fully documented. Compatible with Siemens, Schneider, ABB, and other SCADA systems.
- Digital inputs: Start/stop, reset, external trip, motor running feedback.
- Relay outputs: Trip relay (NC/NO), alarm relay, ready relay. Configurable output logic.
ST420 Series: Microprocessor Line Protection Relays
Overview
The ST420 series are microprocessor-based protection relays designed for feeder circuit protection in LV and MV distribution systems. Unlike the ST570 which is optimized for motor protection, the ST420 is designed for line (feeder) protection — protecting cables, busbars, and transformers rather than motors.
ST420 Models
- ST420B1-1: Standard feeder circuit protection relay. Overcurrent (50/51), earth fault (50N/51N), and directional protection functions. For LV and MV feeder protection in switchgear and MCC panels.
- ST420H: Advanced feeder protection relay with additional functions: voltage protection (27/59), frequency protection (81), and power protection (32). For more complex distribution systems requiring comprehensive feeder protection.
Protection Functions — ST420 Series
| Protection Function | ANSI Code | ST420B1-1 | ST420H |
|---|---|---|---|
| Overcurrent (instantaneous) | 50 | ✓ | ✓ |
| Overcurrent (time-delayed) | 51 | ✓ | ✓ |
| Earth fault (instantaneous) | 50N | ✓ | ✓ |
| Earth fault (time-delayed) | 51N | ✓ | ✓ |
| Directional overcurrent | 67 | ✓ | ✓ |
| Undervoltage | 27 | — | ✓ |
| Overvoltage | 59 | — | ✓ |
| Underfrequency / Overfrequency | 81 | — | ✓ |
| Directional power | 32 | — | ✓ |
ST570 vs ST420: Which Do You Need?
| Requirement | Recommended Device |
|---|---|
| Protecting a three-phase AC motor | ST570 series |
| Protecting a feeder cable or busbar | ST420 series |
| MCC motor starter drawer | ST570 series |
| LV switchgear feeder protection | ST420B1-1 |
| Complex distribution with voltage/frequency protection | ST420H |
| Thermal overload protection with trip class selection | ST570 series |
| Directional overcurrent for ring main or parallel feeders | ST420 series |
| Undercurrent / loss of load detection | ST570 series |
| Modbus RTU SCADA integration | Both series |
Installation and Wiring Considerations
CT Connection
Both the ST570 and ST420 series require current transformer (CT) inputs for current measurement. The ST570B1 has integrated CTs for direct connection to motor cables up to a certain current rating. For higher current motors, external CTs (such as the WLCT series) are required. The ST420 always requires external CTs.
Power Supply
Both series are available in AC 220V and DC 24V auxiliary power supply versions. Specify the correct auxiliary supply voltage for your panel design. DC 24V versions are preferred for MCC applications where a common 24VDC supply is available.
Setting and Commissioning
Protection settings must be calculated based on the motor or feeder parameters:
- Overload setting (ST570): Set to motor full load current (FLC). The thermal model then calculates the trip time based on the actual current and the selected trip class.
- Overcurrent setting (ST420): Set the instantaneous pickup (50) above the maximum load current including inrush, and the time-delayed pickup (51) above the maximum continuous load current.
- Earth fault setting: Set below the minimum earth fault current but above the maximum normal residual current (CT mismatch). Typical setting: 10–20% of rated current for ST570; 5–10% for ST420 with dedicated ZCT input.
Multifunction Power Meter — ST490
For applications requiring energy monitoring in addition to protection, the ST490 Multifunction Power Meter complements the ST570 and ST420 protection relays. The ST490 measures voltage, current, power (kW), reactive power (kVAR), apparent power (kVA), power factor, energy (kWh), and harmonics — providing complete electrical parameter visibility via Modbus RTU.
Wanlong Electric Motor Protection Products
Wanlong Electric's ST series protection devices are designed for industrial MCC and switchgear applications:
- ST570B1 — Motor protection controller, 380–440V, integrated CTs, Modbus RTU
- ST570 — Modular design, external CT input, up to 6300A
- ST570L — Integrated LCD display, standalone MCC drawer installation
- ST420B1-1 — Feeder protection relay, overcurrent + earth fault + directional
- ST420H — Advanced feeder protection, voltage + frequency + power protection
- ST490 — Multifunction power meter, full electrical parameter monitoring
All products manufactured under ISO 9001. OEM/ODM available. For technical specifications, setting guides, or a quotation, contact our engineering team. English-language support, response within 24 hours.
Related: Air Circuit Breaker (ACB) | Molded Case Circuit Breaker (MCCB) | Current Transformer (CT)
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