This is a standard principle in DC systems. If the positive side of the circuit is shorted to the coach body (earth/ground), it creates a path for current to flow through the metal structure, leading to rapid electrolytic corrosion at the point of the fault and elsewhere. A negative earth fault is less destructive in this regard, hence the temporary allowance.

Section 708a (Temperature Rise Test) describes running the alternator at high speed with full load 'for five hours with a fan cooling the body.' This fan replicates the airflow the alternator would experience when the train is moving, making the test realistic.

Section 707 (and others) promotes the use of crimped sockets with bolted connections, using washers and corrosion-inhibiting grease. Soldering can create brittle joints and is less reliable under vibration, which is why it's discouraged.

Section 705c states, 'Cells showing reverse voltage, zero volts should be withdrawn and replaced by charged cells immediately.' A cell at 0V is a dead short and can affect the performance of the entire battery string and cause overheating.

Section 708f (ii) specifies their use. The grommet protects the cable from the sharp edge of the metal (preventing chafing and shorts), and the 'fire retarding' property helps prevent fire from traveling along the cable into the coach.

Section 709c (Don'ts) clearly states, 'Do not use old and new mixed belts in sets.' Old belts will have stretched differently from new ones, leading to uneven load distribution and premature failure of the new belts.

Section 708h (Testing) links the blade angle check directly to performance: 'Check the air delivery of one or two fans from a batch to ascertain the correctness of the blade angle.' The correct angle is essential for the fan to move the specified amount of air.

Section 703j concludes by saying, 'Coaches which are suspected to have wiring defect either in the underframe or roof shall be subjected to insulation test with 500 V megger.' This provides quantitative data to assess the wiring's health.

Section 708b (Rectifier Regulator Unit) lists this check. In the regulator circuit, a field transformer is typically used to supply power to the field windings. Checking its output voltage ensures the excitation circuit is functioning correctly.

Section 709c (Do's) talks about 'Form grades of the belts... after checking on belt measurement gadgets. The grading of belts should be between 48 and 52 only of matched sets.' This grading system ensures that all belts in a set have nearly identical lengths for even load sharing.

Section 705c mentions this precaution in the context of handling defective batteries. The primary reason is safety (acid is corrosive and hazardous) and preventing damage to the coach or workshop infrastructure.

Section 702a describes the 4.5 kW alternator as 'a three-phase heteroporal inductor type Alternator.' This is a technical description of its internal design, distinguishing it from other types of alternators.

Section 705d states, 'If MCBs are not readily available provide rewirable fuses of appropriate ratings, purely as a stop gap measure.' This provides a temporary, albeit less convenient, solution.

As explained in question 6, a megger's high voltage can destroy semiconductor components. Section 709b (Don'ts) explicitly warns, 'Don't use a megger to test the components. Use multimeter.'

Section 703h states, 'ensure that the nylon cord provided to prevent loss of fan dust cover, is available in position.' This is a small but important detail to prevent parts from going missing.

Section 708a (Overhauling) twice mentions, 'Impregnation of the varnish shall be done in an air circulated oven.' This ensures deep penetration of the varnish and proper curing.

Section 707 explicitly states, 'Strands of aluminium wire cannot serve as tinned copper fuses to protect circuits. Never resort to this habit.' It is a dangerous practice because the aluminum wire will not blow at the intended current, failing to protect the circuit.

Section 708e details the capacity test. The purpose of recording the capacity during discharge is to quantitatively measure how much charge the battery can actually deliver, which is a direct measure of its health. The pass/fail criterion is 80% of its rated capacity.

Section 703g permits a coach with a negative earth fault to run for a maximum of one trip, with proper marking. The note implies this rule was invoked, and the coach is now due for the required attention to clear the fault.

Section 708g (Light fittings) states: 'Anti-theft arrangement for fluorescent light fittings shall be checked as per ICF Drg. No. ICF/SK-7-6-079.' This shows that specific mechanical designs are controlled by detailed drawings.

Section 705c states that for batteries that don't pick up charge, one should 'replace the battery by another set and send the defective battery to Depot/Shop for treatment at the earliest. Sulphation will be the main cause for this and Sulphated cells should be treated for their recovery as specified.'

Section 708f (General Precautions) warns: 'Remove earth fault by isolation method instead of hit and trials and short circuiting of opposite polarity of earth.' Shorting opposite polarity can cause a direct short circuit, creating a dangerous situation.

Section 702d explicitly states that special care is taken to provide locking nuts and split pins 'to avoid any accidental falling of batteries while running.' This is a critical safety measure for the under-gear equipment.

Section 709c (Do's) specifies a gap of approx. 75mm for AC coach alternators. It also states to 'Re-tension newly fitted belts after first trip.' The increase in gap from 75mm to 80mm indicates the belts have stretched and seated, requiring re-tensioning by adjusting the nut to restore the correct 75mm gap.

Section 708a (Overhauling) mentions applying insulating varnish and impregnating it in an oven. This process fills microscopic voids in the winding insulation, improving its dielectric strength, thermal conductivity, and resistance to moisture and contaminants.

Section 707 sets a maximum permissible voltage drop of 3.0 V. A higher drop indicates higher than acceptable resistance in the distribution system (cables, connections, fuses, switches), which needs to be investigated and rectified.

Section 703b explicitly states, 'The battery should be isolated while doing this test.' This prevents the test setup from attempting to charge the battery or being influenced by the battery's voltage, allowing a true assessment of the alternator and RRU.

Sulphation on metallic connections creates a high-resistance path. This resistance causes localized heating (as mentioned in section 704a regarding overheated fuses) and reduces the effective voltage available to the load.

The document consistently emphasizes using HRC fuses (e.g., sections 702f, 708f, 709b). HRC (High Rupturing Capacity) fuses are designed to safely interrupt very high fault currents without exploding or causing damage, unlike rewirable fuses which are less reliable for high-energy circuits.

Section 703f states, 'Coaches with discharged batteries which shows less than 100 V on load should be put on charge at double the normal rate...' The first action is to charge the battery, not to immediately identify faulty cells, as the low voltage could be a state of charge issue across the whole battery.

Section 708a (Overhauling) and the note in 708h both specify that 'Only bearing of SKF/FAG shall be provided as per recommendation of RDSO.'

Section 707 warns against using strands of aluminum wire as fuses. Fuses are designed to melt at a specific current to protect the circuit. Using a different material with different electrical and thermal properties would not provide the correct protection and could lead to equipment damage or fire.

Section 703b instructs to 'Check the tension indicator pin at suspension point after every round trip for flat belt drive alternator.' This pin likely moves with the tensioning arm, and its position against a scale or mark indicates whether the spring is compressed to the correct tension.

Section 707b describes the half-yearly IR test for underframe wiring. Shorting the incoming positive terminals (PM & L+) bunches them together so the megger can test the insulation of the entire positive wiring harness against earth and against the negative bus in one measurement.

Section 703h (for noisy fans) states: 'If the noise is due to bearings, replace the fan and send defective fan to shop for replacing.' This highlights a modular replacement strategy at the maintenance line level, with repairs done in a shop.

Section 704a details the hardware for inter-cell connections. The combination of a spring washer (to maintain tension) and multiple fasteners ensures a mechanically and electrically stable connection resistant to the coach's vibration, preventing arcing and voltage drops.

Section 708a (Load Test) defines MFO as the point where the alternator achieves full output (37.5 Amp for 4.5kW alternator) and specifies it should not be more than 600 rpm. It's a key performance parameter.

Section 708f (General Precautions) states: 'In the absence of HRC fuse use correct size equivalent re. wirable fuse.' This is a clear allowance for a temporary substitute.

The compound is described as 'corrosion inhibiting conducting grease.' Its purpose is to prevent the formation of non-conductive oxide films on metal surfaces (especially aluminum) while maintaining a conductive path, as mentioned in sections 707 and 708f.

Sections 705e and 707 specify the minimum IR under adverse (highly humid/wet) weather conditions as 1 Mega-ohm. A reading of 1.5 MΩ is above this minimum and therefore acceptable.

Section 708f (General Precautions) explicitly warns: 'Do not use twisted joints of aluminum cables to avoid oxidation and improper contacts and over heating/earth fault due to loose strands.' Aluminum forms an insulating oxide layer, which is exacerbated by poor joints.

Section 706a states: 'Record hourly cell voltage and specific gravity reading of every cell. Terminate charging when 3 successive readings are constant.' This is the standard definition of a fully charged battery in a controlled charge.

Section 709b (Don'ts) warns: 'Don't use a megger to test the components. Use multimeter.' Meggers apply a high voltage (e.g., 500V) to test insulation, which would destroy low-voltage semiconductor devices (diodes, transistors) inside the electronic RRU.

Section 708e (POH Battery and Battery Box) begins with 'Remove the cells from the battery boxes on arrival of the coach in workshop.' The subsequent testing and reconditioning are done in the shop. The presence of mixed makes would be identified then, and during POH, the goal is to restore the set to a healthy state, which would likely involve replacing the mixed cells as per the principles in section 704a.

Section 705a (Monthly Schedule for Alternators, Rectifier, and Regulators) states: 'Check locking of current setting of regulator, if disturbed it should be reset/locked as prescribed.' This indicates that resetting is a monthly maintenance task.

Section 709c (Do's) specifies, 'The matched set should have belts of one manufacturer only.' The 'Don'ts' section reinforces this: 'Do not use belt of same grade of different manufacturer in a set.' This ensures uniform stretching and load distribution.

Section 703g has a strict rule: 'No coach shall be allowed for service with +ve earth.' This is in contrast to a -ve earth, which can be permitted for one trip. There is no exception mentioned for a positive earth fault.

Section 703f explicitly states, 'Replace missing/defective floats. In case of low level, replenish with distill water.' The float is the guide for the correct level, so it must be present to ensure accurate filling.

Section 703c describes two checks: visual (white mark) and auditory (hammer tap). A dull sound is a direct indicator of looseness, irrespective of the visual alignment of the white mark. The corrective action for a loose pulley is to tighten the bolt to the specified torque.

Section 703b outlines a sequential process. After checking the fuse and loss of residual magnetism, the next instruction is to 'Check the continuity of field and phase winding with the help of test lamp/ multi-meter to ensure that windings are not open circuited.' Flashing is a separate step to cure loss of magnetism, but an open winding is a fundamental fault that must be checked before further action.