Maintenance of rectifier.

Recently loco no.16446  failed on 16/07/2014 due to failure of rectifier (a diode had burnt).

In addition to maintenance instructions and testing procedures of rectifiers issued earlier vide Diesel/ Maint. /HQs & Shed/30, dated: 05-09-2011, Sheds are directed to implement following measures immediately for improving reliability.

1.   Cleaning:

In situ air blowing and gentle wiping of diode should be done in every schedule. Inaccessible area should be cleaned by long stemmed bottle brush in every schedule. Spray cleaning of diodes should be done in M4 and above schedule by opening cover. After spray cleaning, diode should be wiped dry.

2.   Condition monitoring in trip schedule:

a)    Voltage spikes: Diodes are venerable to voltage spikes and spikes if not arrested will cause long term damage to diodes. Snubber capacitors and resistors should be visually checked for deformation due to short circuit, overheating and loose terminals. Capacitance measurement of the capacitors should be measured by capacitance meter. Capacitance value should remain within ± 10% of original value. A healthy capacitor shall give a kick and show very high resistance. A capacitor showing short circuit or low resistance (less than 10 K ohm) should be rejected.

b)    Temperature: Surface temperature of individual diode should be measured and its trend monitoring should be done. Temperature rise beyond 10% of last value is a cause of concern and warrants detailed investigation in section.

c)    Resistance: Forward and Reverse resistance of diode should be measured by 1000V meggar. Forward resistance should be zero and reverse resistance should be in Mega Ohms ( 200-500 MΩ).If any diode is showing less reverse resistance , it should be changed. Care should be taken before conducting Meggaring by isolating circuit wire no 34 and 36 from rectifier out going terminal board to avoid damage to electronic components.

3.      Heat sink: Heat sink should not be opened by shed. It has been learnt that diodes are removed from heat sink during stripping. This practice should be immediately stopped as diodes and heat sinks are factory fitted by applying suitable heat conducting agent.

4.     Creepage path:  Creepage path for current flow should not be permitted. It should be arrested by ensuring regular cleaning and providing additional insulation layer in vulnerable locations.

Sheds are advised to implement the above instructions and to send feed back to HQ.