Keeping automation systems stable in the presence of interference

Hello,

Noise immunity, more specifically electromagnetic immunity, is a device's ability to maintain an accepted level of performance when exposed to specified electromagnetic disturbances. Here, noise refers to electromagnetic interference rather than audible sound. False sensor pulses, unstable analogue readings, communication interruptions and controller resets can all be symptoms of an immunity problem.

“High immunity” is not a complete technical specification. Identify the disturbance, its severity, the affected port and the performance criterion. Resistance to electrostatic discharge does not establish equivalent resistance to every radio frequency or every disturbance on the supply.

How does interference reach a device?

Conducted disturbances can travel through supply, signal and communication wiring. Voltage changes across a shared return conductor can also affect several circuits. Radiated disturbances act through electromagnetic fields coupling into cables or circuits. Capacitive and inductive coupling can also occur between nearby parallel wiring.

For example, a PLC input briefly changing state when a contactor releases cannot be explained by PLC software alone. Coil switching, supply changes and the signal cable route should be investigated together. Separating the source, coupling path and affected circuit makes corrective work more targeted than replacing components at random.

Which tests assess immunity?

The IEC 61000 family contains separate methods for different electromagnetic phenomena. IEC 61000-4-2 addresses electrostatic discharge, or ESD, from people to equipment or nearby objects. IEC 61000-4-3 covers radiated radio-frequency fields, while IEC 61000-4-6 evaluates immunity to conducted disturbances induced in wiring by RF fields.

IEC 61000-4-4 covers electrical fast transient bursts, or EFT/burst, associated with switching events. IEC 61000-4-5 addresses surge immunity. ESD, EFT and surge are different tests: their waveforms, application methods and energy characteristics differ. Comparing their kV values directly does not provide a meaningful ranking of immunity.

Supply voltage dips and short interruptions can also disrupt operation. IEC 61000-4-11 addresses these phenomena for equipment within its scope, connected to 50/60 Hz AC supplies and drawing no more than 16 A per phase. The same method should not automatically be assumed applicable to a DC-powered product.

Read the test level together with the performance criterion

A report stating only “8 kV” is incomplete: it should identify contact or air discharge, application points and the outcome. Similarly, a radiated RF immunity level in V/m needs its frequency range, modulation and test arrangement. These numerical examples do not specify a common mandatory level for every product.

In a widely used classification, criterion A means normal performance within specified limits during and after the test; criterion B allows specified temporary effects followed by recovery without intervention; criterion C can require operator intervention to restore operation. Consult the applicable standard for precise definitions and permitted behaviour. Surviving without damage is different from maintaining measurement accuracy and output states.

Which standard applies?

First identify the relevant product or product-family standard. IEC 61131-2 may apply to PLC equipment, IEC 61326-1 to measurement and control equipment within its scope, and IEC 61800-3 to adjustable-speed power drive systems. Where no relevant dedicated immunity standard exists, a generic standard such as IEC 61000-6-2 may apply to an industrial environment.

When considering IEC standards and their European EN adoptions, check the full reference, edition and amendments. The intended environment and connection ports also matter. A standard number in a catalogue does not mean that every port underwent the same test or achieved the same performance criterion.

Why digital signals are not always more immune

A digital receiver interprets input voltage using defined thresholds. Its noise margin relates to the separation between the driver's guaranteed output levels and the receiver's accepted input levels. A disturbance crossing a threshold can create an incorrect bit or an extra count. A Schmitt trigger uses different thresholds for rising and falling transitions: this hysteresis can reduce repeated switching around a threshold.

RS-485 and differential outputs can reject common-mode interference by evaluating the voltage difference between two conductors. This advantage is limited by the receiver's common-mode range and cable balance. Suitable termination and bus layout remain necessary. 4–20 mA analogue sensors can also work well in a properly designed loop; the analogue or digital label alone does not establish an immunity ranking.

Shielding, filtering and connection layout

Shielding helps reduce electromagnetic coupling. Cable shield termination, contact with enclosure surfaces and shield continuity affect the result. Short connections with a large contact area and suitable 360-degree shield terminations become important at high frequencies. There is no universal rule that every shield must be connected at only one end; follow the equipment instructions and requirements of the signal type.

Consider earthing and equipotential bonding together with shielding. The protective conductor's electrical safety role should not be confused with the return path for high-frequency disturbance currents. Physical connection geometry matters: a satisfactory continuity reading alone does not establish good EMC behaviour at every frequency.

EMI filters and EMC filters attenuate particular frequency components at appropriate connection points. Routing a filter's input and output wiring close together again can allow disturbances to couple around the filter. Filtering, shielding and suppression at the source are complementary measures rather than interchangeable universal solutions.

Two practical examples

Temperature readings jump when a drive runs: if a thermocouple cable runs parallel to the motor output cable over a long distance, examine routing, shield termination and measurement input connections first. Separating the cables is a useful diagnostic step. Increasing measurement filtering may reduce displayed fluctuations, but it also slows the response and may leave the coupling mechanism unchanged.

Counts are lost as a motor accelerates: check the encoder supply, compatibility between its output and the receiver, cable routing and connections. Incorrect wiring, reflections or voltage drop can produce similar symptoms. Record the operating event associated with a fault instead of immediately labelling every interruption as EMI. After corrective work, reassess communication, measurement or counting under the same conditions.

Related standards and technical explanations

Explore EN 61000 standards, CISPR 32 for the emissions perspective, the FCC explanation for US radio-frequency rules, IEC standards for PLCs for controller requirements and device standards for the broader framework.

Other application and component topics

Related topics include noise suppression in servo systems, line driver encoders for feedback, drive input filters, optocouplers for signal isolation and power supplies. Their presence alone does not guarantee immunity: appropriate selection, correct installation and application-specific verification are all required.