In many industrial plants, it is common to find control systems with intermittent faults, unwanted trips, or diagnostic difficulties, even when the software logic was developed without apparent errors.
This happens because a PLC does not operate in isolation. The stability of an automation application depends on a balanced integration between hardware, electrical panel engineering, power supply quality, grounding loops, code structure, and documentation procedures.
Neglecting peripherals, such as thermal dissipation in the enclosure, minimum clearances between power and signal cables, or standardization in variable naming, increases Mean Time to Repair (MTTR) and causes premature degradation of electronic components. Industrial automation demands a systemic view, where controller programming is just one stage of a rigorous lifecycle. Discover, in this article, best practices for your operation.
How to choose the ideal PLC and dimension your operation
Selecting the controller must be based on a technical evaluation of the equipment’s physical characteristics and the application’s specific needs. This prevents both expansion limitations, which can emerge and compromise determinism, and unnecessary over-dimensioning. Here is what should be considered when evaluating your PLC:
Analyzing scan time and processing capacity
Evaluating the CPU’s scan time and processing capacity is one of the most critical factors in selecting a PLC, as it directly determines whether the controller can respond to process events within the required time constraints.
A wrong choice at this stage can compromise the stability of the entire plant, since processes with distinct dynamics demand completely different processing capabilities.
Conventional applications, such as conveyor belts and packaging machinery, operate reliably with cycle times of up to 10 ms. In contrast, high-speed processes, such as synchronized servomotor control or electrical protection in substations under the IEC 61850 standard, require processors capable of maintaining cycle times below 2 ms to 4 ms to prevent severe actuation failures or loss of synchronization.
Beyond the machine’s immediate dynamics, considering scan time when selecting a PLC is fundamental to planning for future system expansion. A frequent mistake is dimensioning the CPU based solely on initial code volume and processing load. Over time, adding new logic, diagnostic routines, alarms, and increased network communication traffic overloads the processor, potentially causing loss of determinism and delayed responses.
For this reason, the ideal controller should be selected with a headroom margin of at least 40% in scan time and 50% free program memory capacity, ensuring the hardware can support plant growth without requiring premature replacement.
Determining I/O count and expansion margin
Accurately determining the quantity and types of inputs and outputs (I/O) is also essential when choosing the ideal PLC, directly influencing the decision between compact (fixed) or modular architectures. Limiting this specification during field instrument mapping is a common engineering mistake.
A proper selection must account not only for existing digital and analog signals, but also for additional channels dedicated to motor starter diagnostics, contactor feedback, and redundant signals for safety interlocks. According to the best practices outlined in the IEEE 1402 standard for control systems, the controller should be specified with a physical reserve margin of 20% to 25% on each module.
Selecting a PLC with this infrastructure headroom avoids lengthy retrofits, whose field costs are typically higher than incorporating additional capacity from the outset, allowing the system to absorb process modifications, new field instrumentation, or contingencies identified during commissioning without the need to replace the chassis or add emergency expansion racks. Here are the details:
| Operation | Recommended reserve | Standard / Reference | Consequence of under-dimensioning |
Processing capacity | 40% headroom | IEC 61131-2 | Loss of determinism and response time delays. |
| Program and data memory | 50% free space | Industry-specific practices | Inability to add diagnostic routines or new operations. |
| Physical I/O points per module | 20% to 25% spare points | IEEE 1402 | Need for chassis replacement or retrofit for expansions. |
| Power supply capacity | 20% to 30% above peak consumption | Electrical design practices | Voltage drop during component startup and CPU reset. |
Evaluating communication, ambient conditions, and safety levels
Choosing the ideal PLC also requires evaluating connectivity with the industrial network, ambient severity, and the plant’s safety requirements. It is necessary to ensure that the controller natively supports the communication protocols used in the factory architecture, such as PROFINET, EtherNet/IP, Modbus TCP, or Fieldbus networks, to prevent integration bottlenecks or the need for protocol converters, which introduce additional points of failure into the system.
From an environmental perspective, the installation site conditions determine whether the selected PLC requires specific ruggedization features. Ignoring temperature, humidity, and mechanical vibration levels (in accordance with the IEC 60068-2-6 standard) or the presence of corrosive atmospheres leads to premature electronic circuit failure.
In these scenarios, the initial specification should guide the choice toward models featuring printed circuit boards with conformal coating and enclosures with an adequate Ingress Protection (IP) rating.
It is also necessary to define safety requirements, an aspect that must be addressed right when selecting the CPU. If the operation involves risks to personnel safety or critical process security, a dedicated safety PLC, certified up to SIL standards, for instance, becomes necessary.
Therefore, conducting market research is valuable, as standard controllers typically lack the continuous internal diagnostics and hardware architecture required by standards such as ISO 13850, IEC 62443, and IEC 60204-1 for safety interlocks.
Checking the need for compliance with standards and manufacturer recommendations
Evaluating and adhering to technical standards and manufacturer recommendations right from the PLC research phase is essential to ensure the feasibility and longevity of the application. Before deciding on the final model, it is necessary to carefully review the manufacturer manuals for power supply voltage tolerance specifications, galvanic isolation levels, maximum current ratings per point, and thermal mounting requirements.
Selecting a PLC that complies with market and industry regulatory standards, such as IEC 61131-2 for hardware, IEC 60204-1 for machine electrical safety, UL 508A for industrial control panels, and NFPA 79, ensures that the equipment will withstand the electrical and environmental demands of the factory without compromising performance or invalidating the manufacturer’s technical warranties.
Choosing a model without considering its mounting constraints or heat dissipation requirements can compromise the control panel layout or lead to premature CPU overheating, rendering the controller selection unsuitable for the plant’s operational reality.
How to organize the electrical panel layout
The assembly of the electrical panel determines the control system’s level of noise immunity and directly impacts the speed of maintenance interventions. Discover the key precautions you need to take before installing your controller in the panel:
Thermal and ventilation conditions
Power supplies, frequency inverters, and transformers are the main sources of heat inside an electrical panel.
The PLC power supply should be positioned in the upper section of the enclosure, respecting the upward flow of thermal convection (the heat transfer process occurring in liquids and gases) while maintaining distance from sensitive electronic components. The internal temperature of the panel must not exceed the limit specified by the PLC manufacturer, with the typical recommendation being to keep it below 50 °C. When internally generated heat or ambient temperature exceeds these specifications, natural ventilation is no longer sufficient.
In such cases, forced ventilation with filters matching the enclosure’s Ingress Protection (IP) rating or industrial panel air conditioners must be adopted. Additionally, maintaining a minimum clearance of 25 mm between the PLC modules and wire ducts is recommended to ensure proper air circulation.
Conductor separation and spatial organization
Electromagnetic coupling between power cables and signal lines is a frequent cause of oscillations in analog measurements and communication faults. To prevent such interference, wiring must be organized in separate wire ducts according to voltage levels and functions:
- Exclusive ducts for AC power conductors (230/400 VAC) and motor output cables.
- Exclusive ducts for DC control wiring (24 VDC), analog signals, and communication networks.
- Physical spacing of at least 100 mm between power and signal routes.
- Mandatory perpendicular crossing (90°) at points where power and signal paths must intersect, minimizing magnetic induction.
Plastic wiring ducts should operate at an occupancy rate of 40% to 50% of their capacity, preventing wire harness overheating and facilitating the addition of new conductors.
The internal layout, in turn, should reserve 20% free space on the mounting plate to accommodate future DIN rail expansions.
Ergonomics and maintenance access
Equipment layout must prioritize safety and ergonomics during field diagnostics. The CPU and I/O modules should be positioned at an optimal eye level, allowing immediate inspection of status LEDs and connection of communication cables without relying on makeshift access tools.
Panel doors must open to a minimum of 90°, and a clearance of at least 1 meter should be maintained at the front and rear relative to fixed obstacles, complying with maintenance standards.
Grounding and electromagnetic compatibility (EMC)
Grounding in a PLC system serves two distinct functions defined by the IEC 61131-2 standard: Protective Earth (PE) and Functional Earth (FE).
Protective Earth (PE) ensures personal safety against electrical shocks by equipotentializing the panel’s non-energized metallic parts with the building’s grounding system. Functional Earth (FE), on the other hand, establishes the zero-volt reference for the PLC logic and provides a low-impedance path for draining high-frequency electromagnetic noise.
To prevent ground loops that cause circulating stray currents, the grounding system must follow a star topology: all ground points from the PLC, power supplies, and metallic structures must converge to a single main grounding busbar inside the enclosure.
The shielding of analog signal cables (4–20 mA or 0–10 V) and communication lines should be grounded at only one end, preferably on the control panel side. Keeping the opposite end floating at the field instrument prevents current circulation caused by ground potential differences between the process area and the control room.
Electrical stability
Instability on the power supply bus compromises the operation of electronic processors, causing spontaneous resets and corruption of memory variables.
Switched-mode power supplies (SMPS) must be sized to supply the current demanded by the CPU, I/O modules, and field devices, applying a safety margin of 20% to 30% over the calculated maximum peak current. This headroom absorbs inrush current surges that occur during the simultaneous activation of solenoids, relay couplers, and sensors.
The use of power supplies classified as SELV (Safety Extra-Low Voltage) or PELV (Protective Extra-Low Voltage) is recommended, as they provide galvanic isolation between the AC mains input and the low-voltage circuits. In locations subject to thermal variations or grid disturbances, installing isolation transformers at the power supply input along with surge protective devices (SPD) helps attenuate the impact of electrical transients.
Best practices for PLC architecture and programming
The quality of software developed for an industrial controller is measured by its stability, clarity, and ease of maintenance by other engineering professionals. Confusingly structured programs further increase plant downtime during emergency interventions. Learn more below about the most recommended practices when programming your controller:
Logical structuring and compliance with IEC 61131-3
Software architecture should adopt the standard languages defined by IEC 61131-3, such as Ladder Diagram (LD), Structured Text (ST), Function Block Diagram (FBD), and Sequential Function Chart (SFC/Grafcet), utilizing the strengths of each according to the application requirements.
The process logic must be divided into reusable function blocks (FB) and functions (FC), avoiding the creation of monolithic programs. Applying the finite-state machine concept in sequential control simplifies tracking process phases while eliminating cross-dependencies between memory bits.
The ideal program structure includes:
- – Dedicated blocks for handling and filtering digital and analog inputs.
- – Blocks containing process rules and operational interlocks.
- – Specific blocks for output driving, diagnostics, and alarm management.
Tag naming conventions and the ISA-5.1 standard
Generic variable identifiers, such as I0.0, M0.1, or Auxiliary_1, make code reading difficult. The recommended approach is to use a structured, self-explanatory naming convention based on the guidelines of the ISA-5.1 standard for industrial instrumentation:
- – Define tags containing the plant area, equipment, and signal function, such as AREA01_PUMP01_CMD_START, AREA01_PT001_PRESSURE_MEASURED, or AREA02_VALV01_LIMIT_SWITCH_OPEN.
- – Maintain a consistent pattern throughout the entire codebase by sticking to a single convention, such as UPPER_SNAKE_CASE (used in the examples above), camelCase, or snake_case.
- – Avoid using special characters, spaces, or accented characters in variable names to ensure seamless data export across industrial networks and OPC UA servers.
Comments and alarm management
Comments embedded within the program should contextualize the intent of business logic and process conditions, avoiding redundant descriptions. Detailed explanations regarding stabilization times, safety interlocks, and sequencing facilitate the work of maintenance teams.
The logic must also incorporate routines for handling alarms and equipment faults. When a discrepancy occurs, such as a lack of feedback confirming a valve has opened or a circuit breaker trip, the program should generate a specific error code and record it for display on the HMI.
This practice enables rapid root-cause identification without requiring a programming computer to be connected to the PLC.
Conductor color coding standard according to IEC 60204-1
| Circuit type | Wire color (IEC 60204-1) | Panel application and function |
| AC and DC power supply | Black | Single-phase or three-phase main power circuits. |
| AC command / control | Red | Contactor coils, relays, and indicator lights operating at 110/230 VAC. |
| DC command / control | Blue | Digital/analog I/O signals and 24 VDC power supply lines. |
| Protective earth conductor | Green/Yellow | Exclusive connection to the protective earth (PE) grounding busbar. |
| Interlocked / external voltage supply | Orange | Wiring energized by an external supply that remains live even with the main circuit breaker open. |
All conductors must be crimped with ferrules (tubular terminals) and identified at both ends using alphanumeric wire markers corresponding to the electrical schematic.
Software governance and update management
PLC programming must adhere to strict code change management procedures. Every modification performed should be documented in a version control table maintained directly within the project file, detailing the date, the responsible engineer, the reason for the adjustment, and the resulting version number.
Updated code backups must be stored on access-restricted servers. Additionally, controller firmware updates should be executed in response to security advisories issued by manufacturers, addressing potential vulnerabilities in alignment with the IEC 62443 standard guidelines.
The importance of testing, commissioning, and fault validation
Before releasing the system for full operation, the installation must undergo structured testing procedures capable of validating both hardware and software.
The first step is I/O loop testing (point-to-point testing), which consists of individually activating each field instrument and verifying whether the corresponding signal is received in the correct PLC variable. For analog channels, a signal generator is used to simulate levels of 4 mA, 12 mA, and 20 mA, ensuring that module parameterization and scaling blocks within the program display accurate readings.
Fault simulation and fail-safe logic
Commissioning must include simulating extreme operating conditions to evaluate the control system’s response. Tests should encompass:
- – Abrupt disconnection of network communication cables.
- – Interruption of the 24 VDC bus power supply.
- – Wire breaks in field sensor circuits.
In accordance with the requirements of IEC 60204-1 and ISO 13850, the electrical circuitry for emergency stop buttons and safety switches must be wired in a Normally Closed (NC) configuration.
This wiring approach enforces the fail-safe concept: if a physical cable breaks or control voltage is lost, the PLC digital input transitions to a LOGIC FALSE state, forcing a safe process shutdown.
Achieving higher operational availability in PLC-based installations relies on integrating technical factors that extend beyond code development.
A well-structured program can still fail due to power supply voltage drops, noise induced by inadequate cable segregation, or thermal overheating caused by poor enclosure ventilation.
The lifecycle of a sustainable automation application demands rigorous hardware selection, strict adherence to manufacturer guidelines, standard-compliant panel assembly, careful grounding implementation, source code standardization, up-to-date technical documentation, and comprehensive testing protocols.
Adopting these best practices minimizes maintenance costs, enhances operator safety, and ensures industrial process predictability without unwanted downtime.
