RMPU Troubleshooting & Maintenance — Train Lighting & AC Coaches Notes

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This chapter covers real fault-diagnosis procedures and workshop safety practice for RMPU units — genuinely practical, exam-relevant detail beyond the specifications already covered in the Air Conditioning & Refrigeration chapter.

Water Leakage Inside Coach

A frequent complaint with a clear checklist: ensure return-air filters and cooling coils are clean, the condensation drain pipe isn't clogged, and — critically — that a P-trap or U-trap of minimum 60mm height is actually installed at the condensate drain outlet. Also check that the trough housing the AC unit isn't itself full of water with blocked drain outlets, verified by physically climbing onto the coach roof.

Blower Overload Tripping

Real diagnostic sequence: check fuses/MCBs and confirm 415V AC 3-phase 50Hz supply reaches the blower motor, check supply at the contactor's incoming terminals, verify wiring from panel to AC unit, confirm the overload relay's trip setting is 3.3 Amps, and rotate the blower by hand to check for a jammed bearing or loose blower wheel. After rectification, restart and confirm normal running current is under 2.6 Amps at 415V AC.

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Exam Focus — Two Specific Current Values

The blower motor's overload relay trips at 3.3 Amps, but normal healthy running current should stay under 2.6 Amps — a real, specific pair of numbers worth memorizing distinctly, since exam questions often test whether you can tell a trip threshold apart from a normal operating value.

Air Loss Fault

Indicated by loss of air across the evaporator coil/heating element. Checklist: clean return air filters and cooling coils, confirm 415V AC 3-phase supply to the evaporator motor circuit, verify blower motor voltage is within 5% of 415V across all 3 phases, check the direction of blower rotation, inspect the Air Loss Relay switch and its wiring, and confirm return/supply air openings aren't obstructed.

Workshop Safety — Do's and Don'ts

Key Do's

Tighten nuts and bolts fully with proper torque — never leave half-tightened.
Confirm neutral is available on the 3-phase supply before starting work.
Check continuity ONLY using a series-connected light bulb (100W or higher) — not an ohm-meter.

Key Don'ts

Never start a compressor until its crank case heater has been energised for 2-4 hours beforehand.
Never lift a PAC (Package AC unit) using a forklift — lift only through the top hooks.
Never allow refrigerant gas to escape to atmosphere, and never use the unit's own compressor for vacuuming/pulling a vacuum on itself.
Never run the machine in manual operation for a prolonged period without a qualified technician supervising.