Functional Safety and Electromagnetic Interference (EMI) may not often be used in the same sentence. But EMI is just one of the environmental stresses that can stop a system from performing its safety function. It is important for a functional safety system to be immune from the EMI levels that are likely to present.

Unlike other environmental stresses like temperature and vibration, EMI is more difficult to sense and it more likely to be transitory. Still the effects can be catastrophic.

EMI can take many forms: motor switching, lightning strikes and use of handheld radios. The standards for EMI use terms like surge, transient, and electromagnetic fields for these stresses. 

There are numerous standards that deal with EMI and the broader term Electromagnetic Compatibility (EMC). The latter includes limits on the emission of the interference, not just the immunity from the interference. For Functional Safety, it is immunity that is key.

The EMI standards relevant to Functional Safety in industrial environments are:

• IEC 61326-1, Electrical equipment for measurement, control and laboratory use
• IEC 61326-3-1, Functional Safety Industrial Locations
• IEC 61326-3-2, Functional Safety Industrial Locations with Specified EM Environment

A product or system being used for Functional Safety should meet the requirements of IEC 61326-1 and then either IEC 61326-3-1 or IEC 61326-3-2. The choice depends on whether the electromagnetic environment is specified. Annex B in IEC 61326-3-2 has example criteria for a specified environment and consequently lower test levels. The criteria include limiting the use of mobile transmitters and periodic maintenance to prevent corrosion of cable shields.

For a Safety-Related System, EMI is an important consideration and IEC 61508 lists two standards with the option of paying attention to the installation or testing to higher levels.