Three-Phase EMU — Fault Troubleshooting — EMU and MEMU / Metros Notes

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Modern 3-phase EMUs have two automatic protective features on the pantograph — Auto Drop Device (ADD) and Over Reach Detection (ORD) — plus a well-defined set of procedures for brake binding, air leakage, and electrical failures, all diagnosable through the DDU rather than physical inspection alone.

Pantograph Protection — ADD & ORD

ADD (Auto Drop Device) — automatically lowers the pantograph if its collector strip breaks or is damaged, to prevent further damage to the pantograph and catenary. Detected via a pressure switch: healthy status is above 3.0 bar; below 2.7 bar triggers automatic lowering, VCB opening, and a fault message to CCU.
ORD (Over Reach Detection) — lowers the pantograph automatically if it rises beyond its set height limit. Healthy status reads about 0 bar; above 1 bar triggers lowering and VCB opening. If ORD triggers, the motorman must confirm no entanglement/OHE damage before re-raising.

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Exam Focus — ADD vs ORD

ADD protects against a damaged/broken collector strip (pantograph too low/faulty contact); ORD protects against the pantograph rising too high (e.g. at a section transition). Both drop the pantograph automatically, but the underlying fault — and pressure-switch threshold — is opposite in character.

Brake Binding — Isolation Procedure

Indicated by a pink-background brake icon on the DDU, the MIN-1 BRAKE APPLIED lamp, and a 'stuck brake detected' message naming the affected coach. First, the motorman applies and releases the EP brake 2–3 times to try clearing it; if that fails, EP brake of the affected coach is isolated directly through the DDU's maintenance menu (password 888888) — a genuinely software-driven fix that didn't exist on older EMU stock, which required a physical isolating-cock walk-through.

Isolating one coach's brake reduces V-max by 9 kmph per coach isolated — e.g. one coach isolated on a 110 kmph rake shows V-max 101.
In maintenance-login mode generally, V-max is restricted to 15 kmph — a distinct, separate speed cap from the 60 kmph cap used for air-suspension failure or RDM.

Parking Brake Isolation

Common causes: MR pressure below 2 kg/cm², a tripped parking-brake MCB in a DTC, or a closed parking-brake cock. First try releasing via the green PB RELEASE push button in the DTC; if unsuccessful, isolate manually by pulling the release handle on the affected coach.

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Memory Aid — Parking Brake Rule

Parking brake can be released ONLY from the driving cab where the master key is switched ON — but it CAN be applied from any driving cab or motor coach without the master key ON. Applying from a motor coach applies it only for that basic unit.

Electrical & Miscellaneous Failures

Common Failures — Cause → Action

DDU display dark: usually the DDU supply MCB tripped — OFF/ON the MCB. Train can often still work normally even with a dark display.
Train moving sluggishly: check MR/BP pressure first, then check for maintenance-mode login (caps speed at 15), brake binding, or air suspension failure (caps speed at 60).
Head light failure: switch to auxiliary head light immediately; operate the ON/OFF and dimmer switches 2-3 times; check MCBs before assuming total bulb failure.
Air suspension failure: speed is automatically restricted to 60 kmph and indicated on the DDU; close the main isolating cock for that coach and proceed at restricted speed.

Fire & Train Parting

On any sign of smoke or fire, immediately trip the VCB, lower all pantographs, stop the train, switch on the flasher, and inform the guard and TLC (Train Line Control) while protecting the train from both directions. Fire specifically on the Motor Coach requires switching off the battery supply before opening the compartment to investigate — never inspect an energised compartment.

In a train-parting event, BP drops to zero and MR drops gradually, with heavy jerks and possible light/fan failure in the DTC. After applying emergency brakes and protecting the train, never touch exposed jumpers/couplers — they may carry live high-voltage wiring.