Throughout history, total solar eclipses have been a significant event. In primitive societies, eclipses were viewed with fear or as important omens. In the US, the upcoming “Great American Solar Eclipse” is creating much excitement. From buying “official” eclipse-viewing glasses, to paying $1500 or more for a hotel room in the path of the eclipse, the anticipation is building to a frenzy. Some employers have even declared August 21st to be a holiday.

The Great American Solar Eclipse:

The solar eclipse begins in the Pacific Northwest (Oregon) and traces a path southeast across the US to Charleston, SC.  Only a small area along the path will experience a total solar eclipse (complete darkness). This area, called the band of totality (the strip of land in which the sun will be totally obscured), measures only 70 miles across.

Interesting facts about solar eclipses:

  • A total solar eclipse occurs somewhere on the earth every 18 months (it usually occurs over water, which covers 70% of the earth’s surface).
  • On average, a total eclipse is visible from any specific spot on Earth once every 375 years.
  • The sun’s diameter is 400 times larger than the moon, but it is 400 times further away from the earth; this makes the two disks appear the same size to us, making the eclipse possible.
  • The longest period of totality (total darkness) will be 2 minutes and 40 seconds in Hopkinsville, KY.

In today’s society eclipses are more of a spectacle than an event to be feared. The same cannot be said for alarm management.

Eclipsing as an Alarm Management Technique

In alarm management, eclipsing is an application of alarm suppression where a tag’s less important alarm is hidden by a more important alarm. For example, if a “high” alarm (e.g., LSH) is present on the same measurement range as a “high-high” alarm (LSHH), then the “high” alarm can be suppressed when the high-high alarm occurs. According to the EEMUA 191 Guideline, the 2nd level of alarm (the more urgent, priority 1 alarm) masks or eclipses the 1st level alarm because it is less urgent and lower priority (Priority 2).

But is eclipsing a good practice? Is it a necessary and useful function in today’s modern control system?

In the “olden” days alarms were often prioritized based on how far they were from the normal operating range. Thus, a high-high alarm (often used to indicate a trip) would be a higher priority than the high alarm (used as a pre-trip notification).  In other applications, the purpose of the high-high alarm was to “back up” the high alarm in case the operator missed it.

The more modern approach to alarm management advocated by the ISA-18.2 and IEC 62682 standards recommends assigning alarm priority based on the potential (direct) consequences of inaction and the operator’s allowable response time.  This means that the second level alarm (High-High) is not necessarily more urgent (higher priority) than the first level alarm (High).  In fact, by following this methodology, the pretrip alarm (High) would typically be a higher priority than the trip alarm (High-High); thus, eclipsing would suppress the more important alarm!

Today’s alarm management best practices also recommend eliminating redundant alarms if there is not a difference in the cause, consequence, or corrective action (in kind or degree). Since the high and high-high alarms would each serve a different purpose, would it make sense to suppress one of them?  Or is it likely that the information from both alarms is necessary to support the operator’s understanding and situation awareness.

In summary, I believe eclipsing is not as valuable today as it was in the past. It may not be necessary in a system that has been rationalized and prioritized following today’s best practices. Eclipsing, if used, should be implemented on a case-by-case basis (not system wide) to prevent inappropriate alarm suppression (which could be dangerous). Users should understand the eclipsing criteria (is the suppression performed based on alarm type, alarm priority, or other parameter?)…..Lastly, don’t stare directly at the sun during an eclipse. It can damage your eyesight.

Tagged as:     Todd Stauffer     ISA-18.2     IEC 62682     EEMUA 191     Alarm Management  

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