Determining EMC performance criteria

The effects and consequences of the different electromagnetic phenomena on electronic systems and installations are directly related to the different functions carried out by the various electronic instrumentation and control systems, and to the processes involved.

In the evaluation of these effects and consequences, it is useful to identify the 'main functions' of the apparatus. The following functions are considered of particular relevance to electronic equipment and systems:

• ++ Protection and tele-protection
• ++ On-line processing and regulation
• ++ Counting
• ++ Command and control
• ++ Supervision
• ++ Man-machine interface
• ++ Alarms
• ++ Data transmission and telecommunication (analog and digital)
• ++ Data acquisition and storage
• ++ Metering
• ++ Off-line processing
• ++ Monitoring
• ++ Self-diagnosis

Combinations of different functions are generally present in most items of apparatus.

Depending on the types of electromagnetic phenomena (conducted or radiated, low or high frequency) present in the intended operational environment, the resulting interference may be limited to one function or to an unforeseeable number of functions.

----- The section below summarizes the typical performance criteria for industrial control systems, versus the threats which were discussed earlier. A short discussion follows of the different functions in the context of this type of system, together with their possible degradations due to electromagnetic phenomena. For each function, considerations on the relevance of such degradation, in terms of consequences to the process, are given, taking into account the electromagnetic phenomena typically encountered in heavy industrial apparatus.

Although this example covers heavy industrial apparatus, similar tables and discussions may readily be created for other types of systems and installations using the following example as a guide. This procedure is recommended as a way of helping to determine the performance criteria, always listing what the user reasonably expects to be achieved given the use to which the equipment is put and the quality of personnel who will use it. It is recommended that the performance criteria are contractually agreed with the user (along with the parameters of the user's electromagnetic environment), although this is probably more likely to occur for larger, high-exposure, or otherwise critical projects.

----- Summary of functional performance versus threats (in descending order of criticality) -- heavy industrial application

Notes:

* For the application of the assessment criteria to apparatus with multiple functions, as well as for concurrent functions (e.g. supervision and monitoring), the performance related to the most critical function applies.

** For teleprotection using power line carrier the "normal performance" during the switching of HV isolators may need an appropriate validation procedure.

*** Only where used in automation and control systems as an auxiliary function to other ones, e.g. to implement coordination.

++A delay of a duration which is insignificant compared to the time constant of the controlled process is acceptable.

+++Temporary loss of data acquisition, and deviation in event scheduling time is acceptable, but the correct sequence of events shall be maintained.

With respect to the degree of urgency (not to the process). Temporary bit error rate degradation can affect the communication efficiency; automatic restoration of any stop page of the communication is mandatory.

++ No effect on stored data or processing accuracy.

Protection and teleprotection

Protection is of particular relevance to heavy industrial apparatus, and this function is considered to include safety and security.

Protection involves the detection of abnormal conditions and the taking of appropriate actions. The precision and rapidity of electronic protection equipment shall not be subjected to degradation of performance as a consequence of electromagnetic phenomena, such as:

++ lack of protection function, with the consequences of critical conditions (including damage of power system components)

++ delay in the protection operation, with consequent overstress of power system components

++ spurious operation, with unavailability of the process or discontinuous working conditions, (depending on the type of apparatus). Any degradation of protection function or safety is unacceptable; consequently, the requirement of total immunity against electromagnetic phenomena, with proper margins, is mandatory in achieving compliance with both the EMC and Machinery Directives, as well as a host of other laws and the bylaws of all Professional Institutions.

Where apparatus is intended to perform some safety function it is recommended that IEC 61508 be applied in full. This covers all aspects of apparatus which are involved with functional safety, (including EMC) and requires that the integrity requirements for a safety function be detailed in the manufacturer's specification.

On-line processing and regulation

On-line processing and regulation systems give the process working condition as defined by control/telecontrol systems or by operators. The optimum running of the process is achieved by these functions.

Degradation of on-line processing and regulation could occur due to lack of immunity of the equipment and related input/output interfaces and process instrumentation involved. A possible consequence is unnecessary stress or damage of the process and degradation of performance.

The immunity of on-line processing and regulation systems to electromagnetic phenomena, including transient phenomena with low probability of occurrence, is of particular importance.

Counting and metrology

The function of counting a product being manufactured, or its energy usage, or of measuring its parameters such as weight or dimension, can be of particular significance due to the contractual aspects which may be involved.

This applies for example to traditional watt-hour meters for electrical energy measurement, and to similar equipment based on advanced technology, having a capability for setting operating conditions and storing data, This function is required to be highly reliable, and therefore immunity against all continuous and transient phenomena is mandatory. Separate legal requirements and Directives apply to such apparatus in the EU.

Command and Control

Command and control functions are important to all operating conditions of industrial apparatus, including partial operation of the apparatus or temporary out of service conditions.

Insufficient reliability of the command and control functions due to lack of immunity could result in:

++ improper operation of electrical equipment, involving safety aspects

++ wrong operation sequence or procedure, with possible damage/overstress of the controlled equipment

++ unavailability of the process equipment and thus of part or all of the process.

Command and control units are required to operate properly in actual environmental conditions, e.g. for continuous phenomena or high occurrence transient phenomena.

Electromagnetic phenomena with a low probability of occurrence, having only minor influence, may be observed on a control system and accepted. For example: a delay in the execution of a command may be insignificant compared to the time constant of the controlled process, so that the main function is not affected. But spurious operation of controlled switchgear is never acceptable.

Supervision

Supervisory systems collect data from the process and related equipment for diagnostic purposes, for maintenance program purposes and for evaluation of the process. They generally do not interact with the process itself. The performance degradation or temporary unavailability of the supervision system causes loss of information on the process and deviation in the event scheduling time.

These effects can occasionally be accepted, e.g., in the case of transient phenomena with low probability of occurrence affecting acquisition of cycling measurements; but the sequence of acquisition of event data should not be affected.

Man-machine interface

The man-machine interface function allows the operators to interact directly with the process from operator desks or to manage the information from the apparatus. Control and regulation systems interface with the process, and the manual command to process equipment has higher priority.

This function may be activated by the operator by using this interface; high priority commands to the running process are generally available, and are given by the operator through the use of dedicated devices, The absolute immunity of this function to transient phenomena having a low probability of occurrence is not generally essential, since the operator presence allows manual restoration. Obviously, this begs the question of what is understood by "low probability": manual restoration once a week (or even once a day) may be acceptable, but once an hour would probably not be.

Alarm

The alarm function includes all local or remote indications able to give information on any kind of temporary or extended degradation of the operating conditions of apparatus and systems. Alarms can have different urgency, depending on whether there is the need of immediate intervention or if the apparatus can still operate in an acceptable mode (e.g. thanks to redundancy). in case of self-recovery after temporary degradation, the alarm may disappear, so it is not generally necessary for this function to be completely immune to low-probability transient phenomena. However, whenever a chronological list (trace) of the alarms is created automatically, this alarm function must not be affected by the electromagnetic phenomena.

Data transmission and telecommunication

Data transmission and telecommunication are auxiliary to other functions:

They allow the data acquisition and the remote control of systems installed within an industrial apparatus (the control functions of the process are controlled by local systems). Interference on data transmission and telecommunications can delay the transfer of command and control, affecting the telecontrol efficiency. Depending on the telecommunication link adopted, electromagnetic phenomena can influence the ongoing communication or affect the terminal equipment, producing a bit error rate degradation. Full immunity against electromagnetic phenomena can be obtained only with particular communication technologies, e.g. optical fiber.

Temporary loss of the communication function for a short period is occasionally tolerable, provided that the link is automatically restored within an acceptable time, but the un-corrected receipt of corrupted data is not allowed.

Particular care needs to be taken when the communication link includes a wireless segment. Radio-based communication devices are inherently much more susceptible both to high-level out-of-band transmissions, and to relatively low-level signals, both intentional and unintentional, at or near the operating frequency. It should be assumed that loss of the link will occur on virtually any site under some circumstances, and the safety and operational analyses should be performed under this assumption.

Data acquisition and storage

Data acquisition and storage of relevant parameters from industrial apparatus allows off-line analysis, comparison with reference conditions and computation, etc.

Temporary deviation from precise analogue data acquisition and incorrect time allocation of digital data, due to transient phenomena, are sometimes acceptable due to the possibility of identifying these effects through data validation. No corruption of locally stored data is allowed.

Metering

Metering of parameters of the process gives direct evidence of their values and trend, and is usually carried out by using analogue or digital instruments. These instruments tend to be located on control panels, display panels, or in the proximity of the apparatus being metered.

Temporary deviation of analogue or digital indications as a consequence of a transient disturbance can be accepted, especially where operators can identify the source of the disturbance and hence take it into account; but no degradation outside of normal operational specifications due to continuous electromagnetic phenomena is allowed.

Off-line processing

Off-line processing functions allow simulation of the process, planning of manufacturing, study of models, and analysis of critical working conditions, etc. This function implies the use of data coming from the process or stored data. It does not interact with the on-line process itself.

Temporary degradation of this function due to transient phenomena is often acceptable, on condition that it does not involve the on-line activities and there is no corruption of stored data or processing accuracy.

Monitoring

The process is monitored on displays, showing the entire setting and operating condition of the plants. Information technology equipment with VDU monitors or other devices are generally used to represent the process and its parameters.

Temporary degradation of this function may be acceptable, provided that the consistency of the monitoring with the process condition is resumed. Temporary loss of display with restoration within a given time, e.g. a few seconds, allowing for possible intervention by operators, may also be acceptable. An example would be the distortion of the image on a CRT-type display caused by temporarily high magnetic fields (but note that various Health and Safety legislation applies to the stability of visually displayed images, and the above example must not be taken to imply that EMC considerations can over-ride health and safety issues).

Self-diagnosis

Self-diagnosis capability is increasing in complex electronic systems, and becomes of particular relevance for the credibility of the system itself. Self-diagnostic test cycles are generally assigned low priority in the task sequence.

Temporary loss of the self-diagnostic function can generally be considered as acceptable if it is self recovered within the system working cycle and if it gives rise only to a delay in warning an operator of a system failure condition. Such a loss of function should also not produce spurious alarm conditions which could require attendance at unmanned remote locations.

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