EMC For Functional Safety

The aim of EMC is to ensure the reliability and safety of all types of systems wherever they are used and exposed to electromagnetic environments. So EMC development is closely linked with the whole field of electrical and electronic engineering, including the design and testing of these systems.

Any electrical or electronic device creates what is called an electromagnetic (EM) environment as electrons are moved around to make the device work. IEC defines the EM environment as “the totality of EM phenomena existing at a given location”. Successful avoidance of these problems illustrates what is called electromagnetic compatibility (EMC).

Electromagnetic compatibility itself is defined as:
“the ability of equipment or a system to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbances to anything in that environment”

For the situations where EM interference (EMI) could lead to a safety hazard, special design and testing procedures need to be applied. For Functional Safety applications, the safety integrity level (SIL) of the equipment must be considered and if the equipment fails, it must be designed to fail safely.

Functional Safety.

Is defined  as part of overall safety of system that depends upon the correct functioning of electrical and/or electronic equipment in response to its inputs. Functional safety is safety achieved by active systems  e.g. Smoke detection by sensors and intelligent activation of evacuation and fire suppression systems. Basic standard that covers Functional safety is IEC 61508

 

The approach to achieve Functional Safety is to carefully evaluate the product design for Functional Safety failure modes in the presence of EMI. There is also a need to consider life cycle design, the operation of equipment and to test the product to ensure that its safety function is maintained or fails safely. This approach is in addition to the normal EMC design and testing practices

 

Functional Safety is not regulated by the  EMC Directive

There exist a misconception across the industry that all that is needed to control EM interference for all purposes in the EU is to manufacture (or purchase) apparatus which is CE marked and declared compliant with the EMC Directive [EU 2014/30/EU]. Here are some basic reasons why this stays incorrect

  • The EMC Directive does not use the word “safety” anywhere in its text

  • The EMC Directive only covers normal operation and does not cover reasonably foreseeable faults, environmental extremes, operator errors, maintenance situations, or misuse – all considerations which are essential for functional safety

  •  The EMC standards harmonized under the EMC Directive either explicitly or implicitly exclude safety considerations

  • The EMC standards harmonized under the EMC Directive (or R&TTE Directive, RED- Latest) cover a restricted number of EM disturbances, and their limits allow a finite probability of incompatibilities

  • EMC Technical Construction Files (TCFs) can include significantly lower EMC performance (or lower confidence of performance) than would have been achieved had the harmonized standards been applied in full, also a Competent Body would not usually assess a TCF for safety

Therefore, complying with the EMC Directive is not necessarily a guarantee of freedom from EM interference in real life operation, or of freedom from safety risks due to inadequate EMC. The fact that the EMC Directive does not address issues of functional safety has been acknowledged by IET

Below listed are examples of safety-critical and safety-related infrastructure systems and equipment which could have their safety performance reduced as a consequence of EMI. This list provides an indication of the types of systems and equipment which the infrastructure controller needs to include in the assessment

  •  Train detection systems (including track circuits and axle counters)
  •   Interlocking systems c) signals and point operating equipment and their controlling circuits
  •  Train warning and protection systems
  • Telecommunications systems (including voice and data transmission, and supervisory control and data acquisition (SCADA) systems)
  • Radio systems (including voice and data transmission, fixed and mobile systems).

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