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Why do locos sometimes use the rear pantograph and sometimes the front pantograph? What is criteria for making this choice?  
2 Answers
Aug 05 2011 (14:44)

Entry# 501     
Charm Totally Vanished*^~
Why do locos sometimes use the rear pantograph and sometimes the front pantograph? What is criteria for making this choice?

Dec 07 2014 (07:46)
Blog Post# 1299136-1     
AKGarg*   Added by: Soumitra Chawathe*^~  Dec 07 2014 (09:51)
During winters, frost is a major problem. When the front panto is raised, it looks like this.. ">".. Now, any frost on the wires will be brushed aside by the panto in the forward direction, and the frost drops/particles will land outside the loco. However, if the rear panto is raised, it looks like "<", and here the frost particles will get deposited on the roof of the loco, which contains DBR and other valuable equipment. There is a risk of them getting damaged due to the frost deposited by the brushing action of the rear panto. For this reason, front panto is used in winter. However, this by no means is a convincing explanation, and it cannot explain why front panto is not used in rainy season. This is a subject of intense debate, and we have not found any satisfactory explanation from even 2-3 loco pilots, with whom we...
have spoken. In fact, one may spot electric locos with rear pantos also in winter, though that's quite rare.

Aug 05 2011 (14:36)
Blog Post# 215482-0     
SMJ   Added by: Charm Totally Vanished*^~  Aug 05 2011 (14:44)
Q. Why do locos sometimes use the rear pantograph and sometimes the front pantograph?
There is in principle no difference between using the front and the rear pantographs for most locos as each is fully capable of delivering the required electric current from the catenary to the loco. (The AC-DC locos are special in that each loco is intended for a different traction supply.) Generally on IR there is no need for both pantographs to be raised at once since there are usually no unusual situations such as frost on the catenary or increased current collection requirements seen with other countries' railways.
it is often seen that there are some definite patterns in pantograph usage. It has been the practice in many areas for locos to always have their rear pantographs up. It is thought that this practice arises from the idea that entanglement of the catenary by the front pantograph may result in damage to the rear pantograph as well as the debris or broken equipment lands on it, and using the rear pantograph lessens the chance of this. However, in recent years, this does not seem to have been adhered to very much. Another pattern that has been seen, especially in northern India, is for the front pantograph to be used extensively in the winter, but not in the warmer months. As a variation of this, it is also known that in certain divisions or zones, orders have been issued for drivers to use the rear pantographs at night. While the reasons for these usage patterns are not entirely clear, it is thought that there is a concern about condensation and the accumulation of dew on the catenary. An adequate technical explanation for the pantograph usage pattern is not known at this time [1/07]. (Please note - theories about falling water from dew on the catenary causing short circuits in loco equipment are implausible considering that locos operate just fine in heavy rain.) It has been suggested that front pantograph use may be a historical vestige from British practice carried over from conditions in the UK where sometimes the front pantograph was raised to scrape ice from the catenary and allow the rear pantograph to collect current fully, but this has not been substantiated either.
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