CASE STUDY

This case study focuses on the requalification study of approximately 200 AC and DC circuit breakers for Queensland Rail. These devices — predominantly Sensata Airpax, Downer and Merlin Gerin units — serve critical protection functions across rolling stock subsystems including HVAC, battery systems, lighting and communications. IMTRAM was engaged to independently verify their electrical and mechanical integrity and determine their ongoing serviceability.

The work draws on the extensive experience IMTRAM has built servicing and overhauling electrical protection equipment, including Sécheron AC & DC High-Speed Circuit Breakers and contactors. This same discipline — rigorous test methodology, calibrated equipment and full traceability — was applied to give Queensland Rail a clear, evidence-based picture of their in-service breaker population.

The study was initiated following observed test failures across multiple voltage and current ratings in the field. The primary objective was to assess each unit against OEM specifications and the relevant AS/NZS and IEC standards, identify non-compliant units, and provide the data Queensland Rail needed to make informed repair, replacement and fleet-management decisions.

Every unit was first subjected to a detailed visual inspection — assessing physical condition, checking for cracks, burns and deformation, verifying rating-plate legibility, and confirming terminal integrity against the master records. Each breaker’s make, model, rating and delay curve were recorded and cross-checked before any electrical testing began.

IMTRAM then performed a full sequence of electrical tests on each unit: continuity and contact-resistance checks, insulation resistance and dielectric testing, and — the core of the study — trip-current verification at 100%, 125%, 150%, 200%, 400%, 600%, 800% and 1000% of rated current, with tripping time captured at each point. Earth-leakage trip operation was verified on applicable RCBO units by injecting a simulated leakage current and measuring the trip response.

The high-current nature of this testing demands purpose-built equipment. AC breakers were tested using the OMICRON CPC100 primary injection system, while high-range DC trip testing was carried out with a Gillam IDC 20K current injector — configured with two 2500A modules for a combined capacity of 5,000 Amps — injecting well over 3,000 Amps through the units to confirm correct tripping behaviour. Lower-range DC trip testing used an APM MS250VDC DC supply paired with a Siglent digital oscilloscope for precise trip-time capture. Continuity, contact resistance and insulation were verified with a PROVA 710 micro-ohmmeter and a SEW analogue megger tester.

Each breaker was mounted on the test bench and connected to the injection equipment via heavy copper busbars and flexible braided leads, allowing the very high currents required to be delivered safely and repeatably. All test equipment was maintained under IMTRAM’s calibration regime, with calibration certificates provided as part of the deliverables.

Results were recorded in a detailed test spreadsheet capturing every unit’s ID, model, rating, test values at each load condition, and a clear Pass/Fail outcome — with a colour-coded system flagging varying levels of performance and fault severity for the client’s review. This was accompanied by a comprehensive Engineering Test Report documenting the methodology, the calibrated equipment used, and a full failure analysis.

The testing was completed successfully. While most units performed well, the study identified a spread of minor faults — such as tripping speeds slightly outside the acceptable band on individual poles — and major faults, including mechanical damage to protective housings and terminals, and units that failed to trip within the acceptable range or showed signs of overheating under load. In every case, IMTRAM gave Queensland Rail the specific data and rationale needed to decide each unit’s fate, ensuring these safety-critical protection devices continue to perform as designed.