This section explains the standard approach to FTA.

  The most events we see on FTA are "undesirable events". These are categorized in two. One is "undesirable condition was continued" and another is "encountered to the undesirable situation".

The first one is continuity for a significant time.  And the other is in a moment event and most of the time, it is related to human error.

  Did the undesirable condition existed from the first?  Most of the cases, it was made by some event(s). It was not like that at the first time.  The condition of "oil on the floor" was made from "no oil on the floor" by the event "spilled oil on the floor".  When a low risk condition was changed to high risk condition, there must be an event which raised risk at least.  The high risk condition was maintained and became a cause of incident.  If there was an even to lower the risk, high risk condition was not there.

  Now we can describe on FTA as "Dangerous Condition" was made with "Event to raise the risk" and "Nor event to lower the risk".

  This means that if there was an "event to lower the risk", there was not the "Dangerous condition" and the event to lower the risk might be a candidate to the preventive measure of similar incident.

  In case of sample incident of spilling oil on the floor, if someone cleaned the floor before the incident, the risk could be lowered but actually not.

  Here we need to pay attention that the event which could lower the risk may not be one.  On the sample case, if someone poured sands on the oil, we could lower the risk too.  How about placing a board over the oil?  Depend on the situation, we can consider many measures to lower the risk.  When we need to decide the preventive measure, we have to place as many measures on the table and select the most effective set of measures.

  Our action can be successful only when all "Acknowledgement", "Judgment", "Transmission" and "Execution" were successful.  Only one failure on these four can make the action to be an error.  These four come on this order.  If a former step was in fail, all the rest cannot be successful.  Let's see them one by one.

  The error on acknowledgement means the person did not realize an important fact.  We usually need to realize some facts through five senses.  The most important sense is "sight" but "hearing" is also important.  We sometimes realize a failure of equipment by noise.  Sometimes others give us important information by words.  We can feel the "smell" of smoke or burning something which will make us possible to think of fire.  We feel vibration, temperature, moisture, and others by "touch".  We may not have much chance to use "taste" in plant operation but there are some possibility in food factory.  Besides five senses, we have a "sense of passing time".  Sometimes we simulate the status change unconsciously and feel "it must be the time to have done".  This is a precious feeling to detect an anomaly, but this feeling must be polished up by experiences and trainings to feel correctly.  In plants there are many devices to make visible on a screen which can't be detected by human sense.  For example, we can't feel the "radiation" by five senses, but Geiger counter tell us the value.  Plant operators detect the condition of their plant using many data collected through sensing devices and control equipment like DCS.

  The difficulty on acknowledgement error is how we can make people aware of the important facts.  Sometime we hear the words like, "it was dark to realize it", "it was too bright to see" or "the letters were too small to read".  These could be true to them but not for all.  Under this condition what we have to do is to make all people can detect it.   Also people need the knowledge of what to feel from the process.  We know "red light" means "to stop" but if we do not have the knowledge, it is just a light of red color.  The knowledge is inevitable to convey the information of "acknowledgement" to "judgment".

  On DCS screen of a plant, what does red color of a pump mean? This is sometimes bothering problem. Depending on the design or rules, "red means stop and green is on operation" and "red means on operation and green is stopped".  The meaning can be totally different by plant.  To obtain the right information, operators need a good initial training and refresh trainings.
When we have an interview for incident investigation, we have to be careful that the interviewee may tell a lie to hide his error on judgment.  There was a case that interviewee said he did not aware of the bump, other people were surprised by the sound of bump.  Unfortunately this was a case of Hit-and-run. It was not an "error of acknowledgement" but an "error of judgment".

  We all know that red light is to stop.  But actual behavior of people is not always true.  On a TV screen I saw many people ignore the red light and walk across a street.  Not all of them did not realize the red light, but most of them ignored it.  They looked around the street and if they felt safe, they crossed the street.  Here I would discuss if the decision is right or not, but we sometimes encounter the truth of judgment error during incident investigations.

  Then when do we make an error on judgment?  The pedestrians who ignored the red light will not ignore red light when they are driving.  The surrounding conditions may effect on our judgment.  When we are in a hurry, we sometimes increase the speed when we saw the yellow light.  The safety level of the person's mind can also effects on the decision.  The composure level of people cannot be stable all the time.  To prevent an error on judgment, we all need "high safety standard", "high composure level" and "sufficient knowledge".

  To maintain the "high safety mind level" and "knowledge", we need safety education and safety activities.  The "composure level" is depend on the person and situations.  Some are strong against a pressure but some are not.  We know a good education and training is effective to make a right decision under a difficult condition.  It is important to simulate as many difficulties and lead the right decision when we are not in difficult situation.

  This error is a little difficult concept.  In the past, I thought of this as "error on memory" but realized that this should be "error on transmission".  This error means that "acknowledgement" and "judgment" were successful but could not reach to "execution".  The typical case I explained before was an error on posting a letter which was to pass by the post box with the letter in the pocket.  In this case, judgment was correct but myself at the time failed to transmit this decision to myself in the future.  This could be called as "error on memory".  But "error on transmission" can occur between different persons.  On a warship movie, I saw a commander shouted to the voice tube to a steering crew to turn to the right.  If the voice tube was broken, commander's voice will not reach to the crew and the ship will not turn to the right.  This is also an "error on transmission".

  Then how can an error on transmission happen?  Generally speaking, I observe this error when there was a disturbance.  When I failed on posting a letter, I was walking some other thing in my mind.  If there was any to draw attention, any of them can be a disturbance.  Just before the post box, "I saw a beautiful lady", "I saw a very cute pappy", "My mobile phone rang" or "I saw a fight of dog and cat".  So many variety of disturbance is possible and it is hard to eliminate all.  But it does not mean that there is no measure to prevent.  We can strengthen our transmission level.  I decided to hold a letter in hand instead of putting it in pocket.  By this way I can sometimes remember of the letter.  Even I was drawn my attention to some other thing, the letter in my hand can remind me of the letter.  This is an improvement measure on activity.  We can think of many ideas to strengthen our transmission against disturbances.  When we encountered to "error of transmission", we can say that we got a chance to think of good ideas.

  While we were successful on "acknowledgement", "judgment" and "transmission", we sometime fail at the "execution" stage. A luck of training may cause this kind of error.  The errors we see on baseball game are typical error of this kind.  We are confident that those players need more training.  But there are some other causes on "error on execution".  If the person's ability was not enough to the execution, this kind of error can occur.  Maybe some need more training but people like very old or sick may not proper to have a hard training.  We have to think the most effective and adequate measures to prevent "error onexecution".

  Here I explained about "continuity of undesirable condition" and "human errors".  These are the problems we often need to think on FTA.  Please remember of these concepts when you faced to "continuity of undesirable condition" and "human error".

  Once you completed FTA charts, the next task is to read the truth from FTA charts.

Continue to What to Read from FTA

©IA&CI (Japan) 2016 All rights reserved.

It is free to download the files in this homepage for non-commercial, educational purpose.

However, reproducing for any commercial purpose without written consent of IA&CI (Japan) is strictly prohibited.