How to compile an ‘Incident Sequence’ in a HAZCHEM incident investigation

May 2, 2019 Posted by Walter Ingles

If you’re investigating a workplace incident that involved hazardous chemicals — once you’ve gathered all the physical evidence and interviewed witnesses — you’ll need to evaluate all that data so you can work out what happened. An important part of the investigation process is building an incident sequence that details every event/element that contributed to the final incident. This blog explains the process and uses a real-world example to demonstrate how an incident sequence is actually compiled.

WORKPLACE INCIDENT:  A diluter tank containing sulphuric acid overflowed into a containment area. A worker entering the containment area was overcome by the fumes and collapsed. The worker lay in the sulphuric acid spill for several minutes which caused second and third degree burns — they died a month later from these injuries.

Though the above incident actually happened at a worksite in the USA, through this blog we’ve added some fictional elements to help demonstrate the incident sequence process. It it not our intention to disrespect the deceased worker nor any of their colleagues.

Step 1: Collate and review the evidence and data

If you were conducting an investigation into this incident you would have already collected evidence and data, this needs to be collated by the investigation team. A data collection from the  example above may look something like this:

  • Physical evidence: Photographs of a dark containment area. Sketches that demonstrate the positioning of WORKER A in relation to the diluter tank and the acid spill - these indicate the worker could not have seen the spill. CCTV footage showing WORKER A collapsing a few seconds after entering the containment area. Photographs of burned out lights.
  • Written statements from witnesses: Statement from the Supervisor who claims to have called both WORKER A and WORKER B back to check the containment area. Statement from WORKER B claiming they did not hear the instructions from the Supervisor as they had already left the building.
  • Results of scientific tests: drug and alcohol tests for all workers and the supervisor are within normal range. Machine logs indicate the alarm system and shut-off switch were offline for 4½ minutes.
  • Workplace documentation: copies of workplace procedure for end-of-shift checks in the containment area indicate that anyone entering the area should be wearing full face shields, chemical gloves, and coveralls. Training records of WORKER A, WORKER B and the Supervisor indicate all had received recent training. Last end-of-shift report completed by WORKER A & B with no indication of a spillage or system malfunction.

Step 2: Draft an incident sequence

Once the data has been reviewed your team will want to piece together a draft incident sequence. The incident sequence is really just a list of every event that contributed to the incident — in the order it occurred. To make this process easier, we suggest starting from the time the incident occurred and working your way back. Here is an example to demonstrate what we mean.

DRAFT Incident Sequence

  • WORKER A is transferred to the safety shower and the site is placed in lockdown.
  • The supervisor and WORKER C hear the alarm. They put on PPE and enter the containment area. They see WORKER A lying on the ground.
  • The alarm system comes back online and activates.
  • WORKER A enters the area and does not see the spilled acid. They are immediately overcome by fumes and fall to the ground and lay in the spilled acid.
  • WORKER A bypasses the PPE cabinet and proceeds directly to the containment area.
  • The power outage has cause a malfunction and diluter tank is overflowing with sulphuric acid. The alarm system is not activated.
  • The power outage has taken out the main light in the containment area, so the area is now quite dark. Worker A is unaware of this.
  • WORKER A returns alone to the containment area. WORKER B does not return —  reasons unknown.
  • The Supervisor calls out and asks both workers to return to the containment area and make a quick check to ensure the power outage hasn’t caused damage.
  • As they are walking away there is a power outage for 5-10 seconds but everything seems to reset normally.
  • WORKERS A & B return their PPE to the PPE storage cabinet and proceed toward the staff room.
  • WORKERS A  & B check the area and submit their shift report to the supervisor, they verbally advise that everything is normal in the area.
  • WORKERS A & B enter the containment area to carry out routine checks before finishing their shift. They wear full PPE, breathing apparatus, chemical resistant gloves, and protective coveralls.

Why work backwards?

Why do we suggest starting at the ‘End’ instead of the ‘Beginning’? Because at this point we don’t actually know the origin of the incident. If you start building sequence from the time of the incident itself,  you begin with a known event then focus on finding events immediately preceding that.

Step 3: look for gaps and contradictions

When you construct the incident sequence your list will only include events for which you have evidence or testimony. In this way the incident sequence will bring to light any contradictions in the evidence, or gaps in your understanding of what happened.

In our example above we see that WORKER A returned alone to the containment area without wearing PPE. You might need to re-check CCTV footage, plus interview WORKER B and the Supervisor again to determine why WORKER A entered the area alone. You should NOT speculate about the reasons why.

If you remember from the draft sequence, we didn’t speculate — our entry looked like this.

  • WORKER A returns alone to the containment area. WORKER B does not return, reasons unknown.
  • The Supervisor calls out and asks both workers to return to the containment area before leaving and make a quick check to ensure the power outage hasn’t caused damage.

Step 4: Finalise the Incident Sequence

Once you have a cleared up any contradictions of evidence and testimony, your final incident sequence will now be used to identify everywhere a system failure occurred. Ultimately this will be used to establish the root causes of the incident.

PLEASE NOTE: We will be outlining this process in our follow-up post Finding root causes of an incident involving hazardous chemicals.


Walter Ingles

Walter Ingles Compliance Specialist

Walter is STOREMASTA’s Dangerous Goods Adviser. He loves helping businesses reduce the risk that Dangerous Goods pose upon their employees, property and the environment through safe and compliant dangerous goods storage solutions.

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