Why do some electric locos have 2 pantographs?

[NicholasNCE]

In Europe, more than one pantographe is fitted to some locos or emus to cope with different OHLE systems. For example, in France, which has both 1.5kV DC and 25kV 50 Hz AC, most locos have one pantographe optimised for high intensities under 1.5kV, and one pantographe built to withstand high voltage (25kV).

[markb846]

I understand the the reason why the class 81-85 had 2 pantographs fitted when new was because the aerodynamic forces on the pan were not fully understood at the time. However itg was soon found that 1 pan worked as well which ever direction the loco was running.
The second pan were removed as part of a life extention program, when this, and other redundent electrical equipment was either removed, or isolated. The roof space was used to house the additional air recievers needed for the locos to be able to work trains formed of air braked stock.
Dual voltage machines do normally have a pan for each of the voltage systems they are required to work through. Eurostar power cars have 2 pans fitted, 1 is for the 25Kv ac, and the other for the 3000v dc supply on the classic Belgium routes. That said they can also use the dc pan to work on 1500v dc routes sothern France, and Holand.
Some French, and Belgium locos were built with 4 pans, and could work on upto 6 electrtical supply systems.

[hairyhandedfool]

Originally Posted by Flying Pig

.... Once this happens the traincrew aren't allow to try to raise it again until a Fitter has checked it....

From my knowledge of the ADD on 319s, the driver would be unable to raise the pan if it activated, nevermind being allowed to raise it!

The air is fed directly from the main air resevior, the air pressure is what keeps the pan raised, if the carbons on the pan are knocked lose/out of position, air pressure is lost and the pan falls, air pressure can't be re-instated because the carbon is not in place. It also works if the pan is raised too high (if the pan goes 'off the wires'), using a device in the 'elbow' of the pan arm, which works in a similar way.

[markb846]

Not sure about the rules for units, but the rules for locos after ADD dropping the pan were if the driver could see that the carbons were not damaged. He could seek the authority of Locos home depots maintenance supervisor to isolate the ADD, and raise the pan. The location of the pan dropping had to be reported so that the OHLE people could inspect for wire damage.
I was told that when the 86's first started to work over the GE routes a number of ADD induced failures were caused by the height of the contact wire being above the normal limits of the pan ADD, so they may have run about with the ADD's isolated
Today checking that the ADD is not isolated is part of the drivers prep, and the loco cannot be moved if it is found isolated.

[Flying Pig]

Originally Posted by flying pig

.... Once this happens the traincrew aren't allow to try to raise it again until a Fitter has checked it....

Originally Posted by hairyhandedfool

...The air is fed directly from the main air resevior.... From my knowledge of the ADD on 319s, the driver would be unable to raise the pan if it activated, nevermind being allowed to raise it!

Not so. That may be the simplified version that the Training School puts out, but it's rather more involved than that. It really wouldn't be a smart idea to have the Main Res Pipe venting directly to the world. Main Res Air is far too precious !

On a 319, a leak in the Air Gallery in the pan head, or operation of the Height Limit Valve effectively causes a leak in the feed to the Servo Piston Assembly, which operates the Pan Air Motor. As a result the Pan Air Motor doesn't function and the Pantograph Dropping Springs quickly bring the pan down.

However if you turn the ADD Isolating Cock it shuts off the 'leak' to the Height Limit Valve and Air Gallery line, allowing the Servo Piston to resume operation. Therefor operating the ADIC will cause the Pan Air Motor to function again. This is considerably more powerful than the Pantograph Dropping Springs, so provided that the pan isn't too mangled from its previous encounter it will go back up.