Can you trust the reading when electrical noise is present?

A controller can remain powered while its measurement drifts or its output changes unexpectedly. IEC 61326-1:2020 addresses electromagnetic compatibility for measurement, control and laboratory equipment and associated accessories. It considers both the emissions a product generates and its immunity to external electromagnetic interference. Accuracy on a quiet bench is only part of the picture.

This is a different assessment from IEC/EN 61010 equipment safety. Safety addresses hazards such as electric shock; EMC asks how equipment functions in its electromagnetic surroundings. A safety test result does not establish EMC performance. For equipment selection, ask for evidence covering the intended installation and the functions that matter to your process.

Match the environment before comparing products

The standard distinguishes basic, industrial and controlled electromagnetic environments. Basic environments can include homes, offices and light industrial premises. Industrial environments feature conditions such as heavy load switching and high currents. In a controlled environment, users or installation design identify and control EMC threats. A room does not qualify simply because it appears quiet.

Class A and Class B describe emissions. Class B includes residential locations and premises connected directly to the public low voltage network supplying homes. Class A excludes those locations. These classes are separate from the A/B/C immunity performance criteria. If a specification says “Class A,” establish which classification it means before drawing a conclusion.

Performance criteria describe the response to a disturbance

Criterion A: Performance stays within defined limits during and after exposure. Criterion B: Temporary degradation may occur, followed by automatic recovery, without unintended changes to operating state or stored data. Criterion C: Specified temporary loss of function may be recovered automatically or through operator intervention.

These are explanations of the concepts, not permission to choose any response for any test. The applicable standard provisions determine the required criterion. A useful test plan translates the requirement into observable behaviour: allowed measurement deviation, communication continuity, output response and recovery.

Relevant disturbances include electrostatic discharge, radiated and conducted RF, electrical fast transients, surges and supply voltage dips. Test methods belong to the IEC 61000 family. Test applicability, severity and acceptance depend on the port and operating conditions. Applying one arbitrary test level to every connection does not create a meaningful assessment.

A supply input, analog input and communication port expose different paths into the equipment. Monitor the relevant functions of a PLC throughout testing. A live display does not tell you whether the input value is correct, a control output has changed or communication has stopped.

Turn measurement deviation into an engineering number

Consider a pressure sensor scaled from 0 to 10 bar with a 4–20 mA output. The signal span is 16 mA. An assumed input error of 0.16 mA represents 1% of span, or 0.1 bar: 0.16 / 16 × 10 = 0.1 bar. This is an illustrative calculation, not an IEC 61326-1 acceptance limit.

In a PLC PID control loop, that apparent pressure change can alter the controller output. Record more than the displayed value: check whether the alarm operates, how the output responds and how long the reading takes to recover. The result becomes useful when you compare those observations with what the application can tolerate.

Installation details belong in the assessment

Suitable separation of power and sensitive signal cables, sensible cable lengths and correctly terminated shields reduce interference coupling. Selecting an Ethernet cable type is only part of the connection design. Follow the equipment instructions for shield termination and assess grounding and equipotential bonding together. There is no single shield connection rule suitable for every circuit.

An EMC filter and a harmonic filter target different problems. The useful question is which disturbance is reduced, and along which path. If the installed cable arrangement differs from the tested arrangement, review whether the evidence still represents the application. “A filter is fitted” is not a complete explanation of EMC performance.

Read the configuration behind the pass result

Look for the standard edition, intended environment, emission class, tested ports, cable arrangement, firmware and operating mode. Check the monitored functions and acceptance limits, then examine behaviour during exposure and after it ends. A report is much easier to assess when it shows the actual measurements and recovery behaviour rather than offering only a final pass statement.

IEC 61326-1 compliance does not establish SIL or functional safety. Safety related functions can require additional applicable IEC 61326-3 provisions. Edition references also deserve attention: IEC 61326-1:2020 is the international designation, while the EVS catalogue lists EN IEC 61326-1:2021+A11:2026. Check the edition and amendments actually cited by the product documentation.