Table 17 List of STPA causes and suggested mitigation measures

Safety sensors & alert

Sensor circuit failure

Audible and visual alert function checking in SAT and maintenance plan

Sensor fault diagnosis and alert function

Gas detection & smoke detection

Physical damage to collector, probe or cables

Data cables protected in appropriate conduit

Sensor contamination or corrosion

Humidity control as part of BESS room HVAC function

Inadequate sensor coverage

Adequate and strategic placement of sensors

Gas composition measurement probes at low levels to detect stratification of combustible gas mixture

BMS monitoring sensor circuits

Sensor/communications circuit failure

Sensor fault diagnosis and alert function at control room

Temperature

Thermal imaging feed of BESS room available in control room

Voltage

BMS to isolate affected module/rack and enter sensor failure safe mode

Current

Physical damage to collector, probe Data cables protected in appropriate conduit or cables

Data cables protected in appropriate conduit

State of charge

Electrical isolation and replacement procedures for faulty sensor

Vent state

Hysteresis leads to lagging measured value

Temperature sensor integrated with hysteresis feedback control loop

Cooling system

Coolant leak

Coolant compound to be detectable by gas sensor

Selection of chemically inert coolant compound

Cooling capacity mismatch

Adequate design of cooling system (coolant thermal conductivity and circulation rate)

Pump failure

Insulated piping for coolant delivery

Redundant back‐up pumps to be installed

Pumps to follow scheduled maintenance plan

Auto fire suppression system

Water supply failure

Adequate design to provide sufficient water pressure, compliant with local fire code

Pump failure

Back‐up pumps to be installed

Pump scheduled maintenance plan

Inadequate sprinkler head coverage

Adequate sprinkler head placement and coverage

Human error: operator turns off sprinkler system prematurely

Operator trained on all known abnormal situations

Ventilation system

Damaged HEPA filter

Maintenance plan to include filter check and replacement

Fan failure

Ventilation fans to follow scheduled maintenance plan

Airflow, quality, and humidity monitoring at operator control centre

Introduction of oxygen to Positive pressure emergency ventilation system combustible mixture

Positive pressure emergency ventilation system using inert gas to displace combustible/toxic gas mixture

Fire and rescue team

Firefighters unaware of status inside portable multi‐gas meters, colorimetric tubes, BESS room (gas composition, thermal runaway status)

Portable multi‐gas meters, colorimetric tubes,thermal imaging equipment

Glass panel(s) on BESS room to provide visual feed

Live feed of data from BMS control server available for firefighters on site

BESS Gas detection system to require gas composition measurement and relation to safety limits (toxicity ppm, explosivity limit, flammability limit)

Firefighters inadequately trained on all ESS hazards and escalation factors

Complete ESS hazard scenarios and mitigation plans to be included in training curriculum

No remote option to manually ventilate BESS room/ open BESS door

Deflagration vents incorporated into design of BESS

Water or inert gas supply inlet for firefighters to be located at safe distance from BESS structure

Remaining stored charge in battery modules

Decommissioning process to disconnect and discharge battery modules in safe environment before

Dept of standards

Inadequate details for cascading thermal runaway prevention requirements for manufacturers & Set clear numerical limits on safety systems (ventilation rate, coolant volume, etc.) and suggested methods BESS system owners (NFPA855/IEC62933)

Set clear numerical limits on safety systems (ventilation rate, coolant volume, etc.) and suggested methods

Further research into prevention of cascading thermal runaway