Why raise the rear pantograph?

[gavino200]

I've wondered about this for a while but only got to searching it tonight. I've noticed that electric locos tend to use only one panto at a time. And it's usually the rear one. After a quick search I was surprised to find a lot of disagreement and different answers given. Anyone got the scoop on this?

 

Reasons I found:

 

1. If the front was raised and was damaged, it would hit the rear panto and knock it out as well, leaving the loco with zero. 

 

2. The rear is used because "it's easier to pull than to push".

 

3. Sometimes the front is used in cold climates to scrape frost off the cable

 

4. For single arm pantos the aerodynamic force "Push he panto upwards".

 

5. Using the rear gives the driver "more leaway" when parking the loco at an isolator

 

6. If a rear panto drops it will do less damage to the loco as 90% of the loco has passed out of the way.

 

 

I don't find any of these answers especially convincing.

 

[railsquid]

There was a thread about this not so long ago, which I don't have time to look up, but IIRC the main rationale is (1) from your list.

 

Note this usually applies to AC locos, DC locos (or dual-voltage locos in DC mode) usually use both pantographs to maximise current pickup.

[railsquid]

23 minutes ago, railsquid said:

There was a thread about this not so long ago, which I don't have time to look up,

 

Ah, here we go, though the question was the other way round:

 

[Lina_Blue]

For those of you interessted, an excerpt from the Swiss regulations about pantograph usage.

 

It states that, if two pantographs are available on a single engine or EMU, the last has to be used.

Also if directly following the pantograph is a vehicle with a windshield, like a second loco or steering car, a wagon with panoramic windows or carriages with loaded automobiles, the front pantograph shall be used.

There is a distinction about which side of the pantograph is front, but with speeds until 160 km/h, it doesn't matter.

 

Info: This is for AC electrified mainlines.

 

Edited September 11, 2018 by Waisneed

[Socimi]

In Italy (3000V DC overhead lines), the rear panto is used mainly for the reasons (1) and (6) and also used because you could technically have more breaking space before the pantograph hits a catenary obstruction (not that 20m would change much, but if there's even a small chanche of saving the pantograph, why not?).

 

On pre-2000 EMUs, such as this ALe 582 (1987), where you have a locomotive-like pantograph setting (two pantographs on the same car), the farthest pantograph to the front cab (the one in the direction of travel) is to be used.

 

 

The front pantograph might be used only:

 

1) if the rear pantograph is unavaible or defective

 

2) on multivoltage locomotives that provide a single pantograph for each system (such as the modified E402B locomotives for France-Italy services)

 

3) (on freight trains only) if the first wagon behind the locomotive carries flamable or dangerous goods.

 

4) Togheter with the rear one either for "raschia-giacchio" ("Ice-scraper" in italian) services (reason 3 of your post).

 

These regulations are valid only on the national railway network and on the "concesse " (think japanese "third sector") railways (that still follow the national standards anyway).

 

On other railways such as subways powered by overhead lines (Lines 2 and 3 in Milano, Lines A, B and C of Roma, Lines 1 and 6 of Napoli and the Genova and Catania subway lines) and narrow gauge railways (such as the Trento-Malè-Marilleva and the Genova-Casella lines) there are no such regulations, and the driver usually raises the front one.

[Welshbloke]

I have a feeling most European systems use the front one when hauling oil tankers too?

 

The RhB metre gauge system adds to the rules with its open observation cars. These have to be at least four axles away from the nearest pantograph, so you'll usually see them at the rear of the train, or sometimes in the middle with at least one bogie vehicle either side. I've seen photos of the historic Bernina railcars hauling them with the matching yellow 4w coach providing the spacing too.