MCC Panels

Protection Relays in PLC & Automation Control Panel

Protection Relays selection, integration, and best practices for PLC & Automation Control Panel assemblies compliant with IEC 61439.

Protection Relays in PLC & Automation Control Panel

Overview

Protection relays in PLC and automation control panels are used to safeguard feeders, process drives, auxiliary systems, and critical plant equipment while maintaining high availability of the control system. In IEC 61439-2 assemblies, the relay must be selected not only for its protection function but also for its electrical, thermal, and communication compatibility with the enclosure, busbar system, and downstream devices. Typical applications include feeder protection for MCC branches, motor and pump protection, generator incomers, transformer secondary protection, and selective protection of automation power supplies. Depending on the architecture, relays may be combined with MCCBs, fuse-switch disconnectors, contactors, soft starters, and VFDs to create a coordinated protection and control scheme. For PLC and automation panels, the most common protection relay functions include overcurrent, earth fault, under/overvoltage, phase failure, phase unbalance, frequency monitoring, differential protection, thermal overload, and reverse power where generator or synchronizing functions are involved. Modern numerical relays with IEC 60255-compliant measurement accuracy, event logging, and Modbus RTU/TCP, Profibus, Profinet, or Ethernet/IP communication are widely used to support SCADA and BMS integration. In process plants, relays are often paired with remote I/O and PLC diagnostics so alarms, trip states, and breaker status can be transmitted to the control room in real time. Selection criteria should start with the system voltage, rated operational current, fault level, and required coordination grade. The relay’s protection curves and pickup settings must coordinate with upstream ACBs or MCCBs and downstream motor starters, ensuring discrimination and minimizing unnecessary shutdowns. Short-circuit performance must be verified against the panel’s declared short-circuit withstand strength, typically expressed as Icw or Icc in kA for the specified duration. For example, a relay protecting a 400 V feeder in an automation panel may be used alongside a feeder MCCB with a 25 kA or 36 kA breaking capacity, provided the complete assembly satisfies IEC 61439 temperature-rise and short-circuit verification requirements. Thermal management is critical because relays installed in densely packed PLC panels contribute to internal heat dissipation, especially when housed near power supplies, servo drives, VFDs, or UPS modules. Panel builders must consider ventilation paths, derating at elevated ambient temperatures, and separation of sensitive low-level signal wiring from power circuits. Forms of separation, such as Form 2, Form 3b, or Form 4, are often applied to isolate functional sections, improve maintenance safety, and reduce the risk of fault propagation. Terminal design, wiring duct layout, and CT/VT routing should also support serviceability and noise immunity. In hazardous or special environments, the broader panel system may also need to consider IEC 60079 for explosive atmospheres and IEC 61641 for arc fault resistance where applicable. For machine automation, IEC 61439-1 and IEC 61439-2 govern the assembly design, while IEC 60947-2 and IEC 60947-4-1 define coordination with circuit-breakers and motor starters. Patrion designs and manufactures PLC and automation control panels in Turkey with protection relays integrated to meet project-specific functional, communication, and short-circuit requirements for water treatment, manufacturing lines, building automation, energy infrastructure, and process skids.

Key Features

  • Protection Relays rated for PLC & Automation Control 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

PropertyValue
Panel TypePLC & Automation Control Panel
ComponentProtection Relays
StandardIEC 61439-2
IntegrationType-tested coordination

Other Components for PLC & Automation Control Panel

PLCs & I/O Modules

Programmable logic controllers, remote I/O, fieldbus communication

HMI & SCADA Systems

Touch panels, visualization, remote monitoring, data logging

Contactors & Motor Starters

DOL/star-delta/reversing starters, overload relays, Type 2 coordination

Moulded Case Circuit Breakers (MCCB)

Branch protection 16A–1600A, thermal-magnetic or electronic trip

Other Panels Using Protection Relays

Main Distribution Board (MDB)

Primary power distribution from transformer to sub-circuits. Rated up to 6300A. Houses main incoming breaker, bus-section, and outgoing feeders.

Power Control Center (PCC)

High-capacity power distribution for industrial facilities. Controls and distributes incoming power to MCC, APFC, and downstream loads.

Motor Control Center (MCC)

Centralized motor control with starters, contactors, overloads, and VFDs in standardized withdrawable/fixed functional units.

Power Factor Correction Panel (APFC)

Automatic capacitor switching for reactive power compensation. Thyristor or contactor-switched, detuned or standard configurations.

Automatic Transfer Switch (ATS) Panel

Automatic changeover between mains and generator/UPS. Open or closed transition, with or without bypass.

Generator Control Panel

Genset start/stop sequencing, synchronization, load sharing, and paralleling controls.

Custom Engineered Panel

Bespoke panel assemblies for non-standard requirements — special ratings, unusual form factors, multi-function combinations.

Soft Starter Panel

Enclosed soft starter assemblies for reduced voltage motor starting with torque control, ramp-up/down profiles, and bypass contactor options.

Harmonic Filter Panel

Active or passive harmonic filtering to mitigate THD from non-linear loads. Tuned LC filters, active filters, or hybrid configurations.

DC Distribution Panel

DC power distribution for battery systems, solar installations, telecom, and UPS applications. MCCB/fuse-based DC protection.

Capacitor Bank Panel

Fixed or automatic capacitor bank assemblies for bulk reactive power compensation in industrial and utility applications.

Frequently Asked Questions

The most common functions are overcurrent, earth fault, phase loss, phase imbalance, under/overvoltage, and thermal overload. In panels controlling motors, pumps, compressors, or feeders, relays are often paired with MCCBs, contactors, and VFDs to achieve coordinated protection. For generator or utility interface applications, reverse power, frequency, and differential functions may also be required. Selection should align with the panel’s intended duty and the discrimination study, in line with IEC 61439-2 for the assembly and IEC 60947-2 or IEC 60947-4-1 for protective devices and motor circuits.
Coordination is achieved by setting the relay pickup values, time delays, and curve characteristics so the downstream device clears faults first while the upstream breaker remains closed for selectivity. In practice, this means verifying the relay against the MCCB or ACB trip unit characteristics and the prospective short-circuit level of the system. The assembly must also satisfy IEC 61439 short-circuit withstand requirements, while the protective devices themselves should comply with IEC 60947-2. For critical processes, time-current studies are essential to avoid nuisance trips and maintain continuity of service.
Yes, but the relay must be selected carefully because VFDs and soft starters change the fault and overload behavior seen by the panel. A protection relay may supervise the feeder side, motor side, or both, depending on the architecture. For VFD applications, it is common to use relay functions for input feeder protection, phase monitoring, and temperature supervision rather than direct motor overload if the drive already provides internal motor protection. The overall design should follow IEC 60947-4-1 for motor control equipment and IEC 61439-2 for thermal and functional coordination within the enclosure.
For modern PLC and automation panels, protection relays typically need Modbus RTU, Modbus TCP, Profinet, Profibus, EtherNet/IP, or IEC 61850 depending on the plant architecture. The key requirement is that alarms, trip status, measurements, and event logs can be read by the PLC or SCADA master without compromising protection performance. Ethernet-based relays are often preferred in new builds because they simplify diagnostics and remote maintenance. Cable routing, EMC segregation, and grounding must also be designed to preserve signal integrity inside the IEC 61439 assembly.
A protection relay itself does not define the panel’s short-circuit rating; the complete assembly does. The panel must be verified for the prospective fault level at the point of installation, usually expressed as Icw, Icc, or short-circuit current withstand in kA. Common industrial automation panels may require 10 kA, 25 kA, 36 kA, or higher depending on the supply network and upstream transformer capacity. The relay must operate correctly within that fault environment and be coordinated with the feeder breaker or fuse system under IEC 61439-1/2 and IEC 60947-2.
Protection relays add internal heat, especially numerical relays with power supplies, communication ports, and display modules. In compact PLC panels, this heat must be included in the overall temperature-rise assessment required by IEC 61439-1 and IEC 61439-2. When relays are installed alongside power supplies, PLC CPUs, network switches, VFDs, or UPS units, the builder may need forced ventilation, air conditioning, or derating. Proper spacing, vertical airflow, and segregation from high-loss components help maintain reliability and extend device life.
The main standard for the panel assembly is IEC 61439-1 and IEC 61439-2. The relay and its protective functions are typically aligned with IEC 60255 for measuring relays and protection equipment, while the associated breakers and motor starters fall under IEC 60947-2 and IEC 60947-4-1. If the panel is installed in a hazardous area, IEC 60079 may apply, and if arc fault containment is required, IEC 61641 should be considered. The final specification should also reflect the plant’s communication, environmental, and coordination requirements.
The best configuration is usually one protection relay per critical feeder or one multifunction relay supervising a group of feeders, depending on the process criticality and budget. For each motor branch, the relay may handle overload, phase failure, and earth fault, while the upstream MCCB provides short-circuit protection. In high-availability plants, individual feeder relays improve diagnostics and reduce downtime because faults are isolated more precisely. The panel should be designed with clear separation, labeled wiring, and coordination studies to ensure compliance with IEC 61439 and reliable operation in manufacturing, water treatment, and process automation applications.

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