Alert no. 402 | 18 November 2021 | Version 1

Introduction, inspection and maintenance of gas detecting and monitoring equipment

What happened?

Incidents have occurred whilst undertaking maintenance tasks associated with the gas monitoring systems.

1. An incident occurred at an underground coal mine where a Longwall CH4 sensor was being relocated from a powered roof support to another location along the faceline. When the sensor was disconnected, the tripping circuit failed to operate.
2. An incident occurred at another underground coal mine where an ERZ/NERZ boundary sensor went into a fault condition due to the failure of the uninterruptible power supply and the tripping circuit failed to operate.

How did it happen?

Investigation on the first incident found that the designated relay in the gas monitoring controller had not been configured to activate for a failsafe condition. The parameter sheets for the gas monitoring installation stated that the relay should have been configured to activate for a failsafe condition.

The second incident investigation found that another digital address module was still active on the signal line from a redundant gas monitoring station and was transmitting the same address. This prevented the boundary sensor from be able to trip on a fault condition.

Key issues

• Commissioning and testing plans provided insufficient detail and were not fully completed.
• Periodic maintenance schedules did not identify the requirement for testing for a failsafe condition.
• Periodic maintenance schedules did not identify the requirement to verify parameter settings.
• Verification methods utilised to test all operational tripping functions were not adequate.
• Decommissioning of redundant gas monitoring installations / stations was not adequate.

Recommendations

Mines should review their safety and health management system to ensure:

• where gas detectors are used to initiate alarms and/or tripping functions remote to the detector itself. A requirement to test alarms should include verification of the actuation of the remote alarms and tripping functions.
• periodic maintenance schedules associated with gas monitoring verify parameter settings. 
• that if a fixed methane detector malfunctions or fails, the fixed methane detector will automatically cause the equipment, or part of the equipment, the detector is monitoring to shut down and give a visible alarm.
• authorised personnel are deemed competent and have the knowledge, skills and experience to effectively and safely carry out activities associated with gas monitoring systems.
• that decommissioning of redundant gas monitoring installations / stations is effective.

References and further information

• OEM Operational Guidelines and Technical Information
• AS/NZS 2290.3:2018 Electrical equipment for coal mines – Introduction, inspection and maintenance Part 3: Gas detecting and monitoring equipment
• Coal Mines Health & Safety Act 1999
• Coal Mines Health & Safety Regulations 2017