Mineral Mines & Quarries Inspectorate | Alert | No.409 V 1 | 26 May 2022

Light vehicle fail-safe brake interlocks

What happened?

An incident occurred in an underground mine when a worker exited a light vehicle to open a barricade into a mobile equipment operating area. After exiting the vehicle it rolled backwards, striking the worker with the open door and continued to travel down the decline for approximately 100m. The vehicle came to rest after striking the decline wall with the tray. The worker sustained injuries as a result of the incident.

How did it happen?

An investigation to determine the nature and cause of the incident is currently underway, however a number of initial observations have been made including:

• The vehicle had been fitted with fail-safe brakes on the rear axle, a dash-mounted control panel, and an interlock between this system and the OEM door switches, however the OEM door switch had been previously damaged in such a way as to prevent the interlock function.
• The vehicle was not in gear, park brake not applied, and wheels not turned into the wall when the uncontrolled movement occurred.
• Pre-start inspections for the light vehicle did not include testing of the interlock functions for the fail-safe brake.

Key issues

• Where administrative and procedural controls are used (such as park-up procedures), regular monitoring and enforcement of these controls is required to ensure they are effective.
• Engineering controls, such as fail-safe systems with interlocks, can experience hidden failures, resulting in the control not working as designed.
• Regular function testing of interlocks can detect hidden failures.
• Where field devices such as switches perform a safety function, the robustness of the device and probability of failure in the usage environment should be considered in the design.

Recommendations

• Sites should, as soon as possible, carry out audits of all in-service light vehicle fail-safe braking systems to ensure correct function of both the brakes and installed interlocks (e.g. doors, seatbelts, engine). Both the brake-apply function and brake-lift inhibit functions of the interlocks should be tested.
• Sites should establish a system of periodic audits for light vehicle fleets fitted with fail-safe brakes to carry out the above tests.
• All persons operating light vehicles must follow correct park-up procedures.
• Sites should ensure administrative and procedural controls are enforced by monitoring and supervision.
• Sites should review training for light vehicles to ensure it includes pre-start inspection requirements for fail-safe brake systems where these systems have been fitted.
• Sites with fail-safe braking systems on light vehicles should ensure the pre-start and maintenance for light vehicles includes inspection and function tests for the fail-safe braking system.