Locomotives with two pantographs, one up at a time

 
  Myrtone Chief Commissioner

Location: North Carlton, Melbourne, Victoria
There are some electric locomotives that each have two pantographs, one up at a time, can anyone explain this. Though I must say that modern pantographs are asymmetrical, quite a paradox given that older ones were symmetrical, the only asymmetrical current collectors (largely confined to trams) were trolley poles (which were either turned at each end or came in pairs with one up and the other down) and bow collectors (flipped over when changing direction).
When there are two pantographs, they are always set in opposite directions and maybe if you understand why this is, you can see what I mean if I tell you that symmetry is an advantage of diamond pantographs.

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  justapassenger Minister for Railways

There are some electric locomotives that each have two pantographs, one up at a time, can anyone explain this.
Myrtone
There are four reasons that this may occur, depending on the context.

1. Pantographs with different physical dimensions to run across different networks, e.g. Channel Tunnel and conventional networks or locos equipped to run between Switzerland and Germany or Austria.
2. Different pantographs for different electrical systems, more common on multi-system EMUs but not unknown on locomotives.
3. Redundancy.

Look closely at the pantographs on a TGV locomotive (TGV trains are not EMUs, they have locomotives at each end like a HST/XPT) and you'll see the two pantographs are physically different, because the high speed overhead system has different physical properties to the suburban overhead they use for the run between the terminus and the start the high speed system.

Though I must say that modern pantographs are asymmetrical, quite a paradox given that older ones were symmetrical … When there are two pantographs, they are always set in opposite directions and maybe if you understand why this is, you can see what I mean if I tell you that symmetry is an advantage of diamond pantographs.
Myrtone
The symmetry of a double arm pantograph is not an advantage and there is no paradox, because the modern single arm pantographs are superior in every meaningful measure. They are lighter, simpler, aerodynamically superior, safer, more responsive to the geometry of the overhead and less wearing on it.

That's why lots of locos and EMUs delivered before the 1980s with double arm pantographs have been upgraded to single arm pantographs in mid-life refurbishments.

The top reason to mount them 'back to back' if there are two on the same vehicle is simply that it's the most convenient layout. The mountings are close together allowing for a single feed to go into the vehicle, and the amount of roof space denied to other equipment is minimised.

The aesthetics of single arm vs double arm would be quite a subjective issue. You prefer the symmetry of a double arm design, I prefer the clean and sleek look of a single arm design.
  Myrtone Chief Commissioner

Location: North Carlton, Melbourne, Victoria
The symmetry of a double arm pantograph is not an advantage and there is no paradox, because the modern single arm pantographs are superior in every meaningful measure. They are lighter, simpler, aerodynamically superior, safer, more responsive to the geometry of the overhead and less wearing on it.
justapassenger
Okay, so does that mean that a symmetrical pantograph that were as light, as simple, as aerodynamic, as safe, as responsive to the geometry of the wires with just as much wearing of it would have no advantage.

That's why lots of locos and EMUs delivered before the 1980s with double arm pantographs have been upgraded to single arm pantographs in mid-life refurbishments.
justapassenger
So does the single arm variety actually do a lot better, like many times better?

The top reason to mount them 'back to back' if there are two on the same vehicle is simply that it's the most convenient layout. The mountings are close together allowing for a single feed to go into the vehicle, and the amount of roof space denied to other equipment is minimised.
justapassenger
I didn't know there was any reason for that other than what I call geometrical correctness. Let me give other examples, bidirectional vehicles nearly always have two identical front ends. Also, all bidirectional passenger carrying vehicles (like carriages or motor coaches) seem to have longitudal seating, reversible seats or most commonly, half the seats facing each way. This is geometrically correct, having all seats fixed and most facing one end is not basically because they would all be facing forward when driven from one end and backward when driven from the other.

The aesthetics of single arm vs double arm would be quite a subjective issue. You prefer the symmetry of a double arm design, I prefer the clean and sleek look of a single arm design.
justapassenger

I don't think of it as about aesthetics but about geometrical correctness. If only someone could combine the geometrical correctness of a diamond pantograph with the advantages of a single-arm one.
Oh, I am fine with clean and sleek looks too, how about wing-shaped pantographs once tried by the Japanese? These are even aerodynamically superior to the single-arm variety.
  route14 Chief Commissioner

Reason 4: When overhead catches ice or snow, both pantographs can be raised to ensure a consistent current conduction.  However on AC networks one of them has to be lowered before crossing phase insulators.
  bevans Site Admin

Location: Melbourne, Australia
I think the L Class ran with the second panto up in the direction of travel.
  route14 Chief Commissioner

The difference is minimal, but a lot of systems mandate or the drivers prefer to raise the rear pantograph, because in case the pantograph flips and brings down the overhead, the broken contact wire won't fall on the driver's cab.
  petan Chief Commissioner

Location: Waiting to see a zebra using a zebra crossing!
There were rules in NSW about when their electric locos had either one or two pantographs raised. There were several threads on Railpage about this including the following  
https://www.railpage.com.au/f-t11331231-previous.htm
  route14 Chief Commissioner

So panto bouncing was a concern even in fine weather.  Fortunately it's a DC network so there is no phase insulator to take into consideration.
  kitchgp Chief Commissioner

The SEC Latrobe Valley 900mm light railway locos operated with both pantographs. To quote Light Railways Number 84 April 1984, SEC locos “normally operate with both pantographs in contact with the overhead, to minimise arcing and loss of power”. SEC 122 had both types of pantograph.

Some photos by Andrew Cook on the Trams Down Under website:
https://tdu.to/204297.msg

The VR L & E-classes (except for single-panto 1100 & 1101 obviously) seem to have run most of the time with the leading pantograph up. The Ls apparently used both pantographs in winter.
  ngarner Deputy Commissioner

Location: Seville
The Ls apparently used both pantographs in winter.
kitchgp
That certainly ties in with my memories of working on them in winter

Neil
  skitz Chief Commissioner

The SEC Latrobe Valley 900mm light railway locos operated with both pantographs. To quote Light Railways Number 84 April 1984, SEC locos “normally operate with both pantographs in contact with the overhead, to minimise arcing and loss of power”. SEC 122 had both types of pantograph.

Some photos by Andrew Cook on the Trams Down Under website:
https://tdu.to/204297.msg

The VR L & E-classes (except for single-panto 1100 & 1101 obviously) seem to have run most of the time with the leading pantograph up. The Ls apparently used both pantographs in winter.
kitchgp
When there was frost (VR L).   One to knock to ice and the other for contact.

The light show apparently spectacular.

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