Stray currents in industrial automation belong to those phenomena that are rarely considered directly, yet very often are at the root of operational problems. In contrast to the classic approach known from power engineering or railway traction, where stray currents are mainly associated with electrochemical corrosion,
in automation, their effects are functional in nature. They manifest as unstable operation of control systems, unpredictable disturbances, and errors that appear only under specific operating conditions of the installation. It is this unpredictability that often leads to the problem being downplayed or attributed to faulty equipment.
The essence of stray currents
In automation, stray currents can be described as electrical currents flowing outside the designed flow path. This means that instead of returning through the ground wire or power supply wire, the current uses other available paths of lower impedance. These can include signal cable shields, protective earth wires, metal equipment enclosures, machine support structures, or equipotential bonding. This phenomenon is particularly noticeable in DC-powered installations, where ground is treated as a common reference point for many devices.
A key factor contributing to the formation of stray currents is the potential differences between ground points. Even small voltages, on the order of single volts or fractions thereof, can cause current to flow through circuits that were not originally intended for this purpose. As a result, installation elements begin to perform roles that the designer did not foresee for them.
Sources of stray currents
Modern automation systems increasingly combine control circuits, measurements, and power supply for high-power devices. A particular source of problems are inverters and other power electronic converters, which generate disturbances with high voltage and current slew rates. These disturbances easily couple with signal circuits, especially when routed in parallel over long distances.
Another significant factor is the extent of the installation. In large industrial facilities, 24 V DC power is often distributed over tens or even hundreds of meters. Voltage drops on ground wires lead to the formation of local reference points that are no longer electrically equivalent. In such a situation, current begins to flow between these points through available metallic connections. Additionally, contactors, relays, and other switching elements introduce interference pulses into the system, which can initiate transient but intense stray current flows.
Paths of stray current flow
Current always chooses the path of least impedance. This means that it does not respect schematic diagrams. Very often, the first victims are signal cable shields, which transform from protective elements into working conductors. Similarly, protective earth (PE) conductors, metal cable trays, or control cabinet structures are used. In extreme cases, the current closes through the ground, especially if the installation has multiple grounding points with different resistances.
Such uncontrolled flow paths are particularly dangerous for low-voltage and low-current circuits. Analog signals and communication interfaces are designed with the assumption of a stable reference potential, and even a slight disturbance to it leads to measurement or data transmission errors.
Effects on control systems
The presence of stray currents in automation most often manifests as system instability. 0–10 V voltage signals can change their value depending on motor load, and 4–20 mA current loops exhibit increased noise or momentary dropouts. Communication buses operate correctly in idle state but begin to generate errors during dynamic machine operation.
Also characteristic are unexplained restarts of PLCs, I/O modules, or peripheral devices. These often disappear after disconnecting one power consumer or after changing the grounding method, which further complicates unambiguous diagnosis. Such problems are sometimes mistaken for hardware defects or software bugs.
Relationship with electromagnetic compatibility
Stray currents are closely related to electromagnetic compatibility issues, although they are not always directly included in EMC analyses. Many of them are common mode currents, which are not effectively suppressed by classic anti-interference filters. This means that an installation that meets standard requirements may still show susceptibility to interference if the grounding and earthing concept has not been properly designed.
Limiting stray currents
Effective limitation of stray currents requires a systemic approach
and consideration of the entire installation, not just individual devices. Galvanic isolation of signal circuits and conscious design of earthing and equipotential bonding are crucial. In practice, significant improvement can be achieved by using current signals instead of voltage signals, proper routing of cable shields, and separation of power and signal cable routes.
Equally important is avoiding treating the protective conductor as part of the working circuit and eliminating ground loops already at the design stage. Many problems that arise during commissioning originate from decisions made much earlier.
Stray currents in automation are a common phenomenon, yet often unnoticed. They do not result from a single assembly error, but from the entirety of design and implementation solutions. A conscious approach to grounding, earthing, and signal routing issues can significantly limit their impact. As a result, the control system becomes more stable, easier to diagnose, and less susceptible to unpredictable disturbances.
However, when prevention fails and a device falls victim to stray currents, quick and accurate diagnosis is key. At PLE Service, we have experience in dealing
with failures caused by these elusive phenomena. We not only restore the functionality of damaged modules but, above all, help identify and eliminate the root cause of the problem in the installation itself, protecting your machinery from recurring failures. If your production line is struggling with unstable controller operation or sudden drive failures, report a repair to our specialists to permanently secure the continuity of processes in your plant.
