Why do some electric locos have 2 pantographs?

[ECML43]

in the past I've titled my questions 'another random train question' or something like that but from now on I'm going to start trying to fit the question into the title space.

Anyway, why do some electric locos (usually heavy duty ones) have 2 pantographs (like a class 92 for example) while most only have one (for example a class 90).

[Deathbyteacup]

I think in the case of some classes, they have two pantographs as a requirement to run through the Eurotunnel, infact they have an entire redundant electrical system, to prevent a train becoming stranded in the tunnel and disrupting the service.

Certainly this is the case for Class 92. I imagine similar reasons exist for other classes.

[John Hunt]

I recall the Class 76s also had two pantographs and they worked the Woodhead Tunnel. Perhaps it is for redundancy reasons.

[mjobling]

Originally Posted by John Hunt

I recall the Class 76s also had two pantographs and they worked the Woodhead Tunnel. Perhaps it is for redundancy reasons.

I think the 76s ran with both pantographs up, so the second pantograph couldn't be for redundancy.

[Deathbyteacup]

Originally Posted by mjobling

I think the 76s ran with both pantographs up, so the second pantograph couldn't be for redundancy.

Didn't the 76's feed power back into the system?

This might be the reason for the second pantograph here.

[Midland Compound]

Didn't some of the early AC electrics (class 81-5) have two pantographs when built, but had one removed fairly early on in life ?

[Gandalf]

One for going forwards, the other for reverse.
OK I am being silly but someone had to say it.
John (G)
still lighting fires in my locos

[mjobling]

The EM1s were fitted with two pantographs. The original plan was to only use the rear pantograph, and keep the front one incase of damage. The Woodhead route was plagued by serious ice problems, soot from steam engines, and unusual atmospheric conditions; all of which contributed to poor and often intermittent pickup from the overhead line. This experience quickly led to the use of both pantographs simultaneously. This greatly improved the pickup and is thought to have also significantly lengthened the life of the overhead wire.

http://www.lner.info/locos/Electric/em1.shtml

[mjobling]

Originally Posted by Midland Compound

Didn't some of the early AC electrics (class 81-5) have two pantographs when built, but had one removed fairly early on in life ?

From http://www.igg.org.uk/gansg/00-app3-4/ap4-ele.htm:

Following a single prototype (E1000 based on the converted gas turbine engine 18100) there were five classes of contractor-built 1st generation loco designs that entered service. Originally designated types AL1 to AL5, they became Classes 81 to 85, all were broadly similar in appearance and known by enthusiasts as Growlers due to the noise of the original type of rectifiers used. Originally they were fitted with two pantographs but in later years (starting in the later 1960s) one was removed as [part of a major rebuild incorporating new solid state rectifiers (the empty roof space being used for a rack of gas cylinders) and all subsequent designs have only one fitted.

[Flying Pig]

Originally Posted by Deathbyteacup

I think in the case of some classes, they have two pantographs as a requirement to run through the Eurotunnel, infact they have an entire redundant electrical system, to prevent a train becoming stranded in the tunnel and disrupting the service.

Certainly this is the case for Class 92. I imagine similar reasons exist for other classes.

I think you're probably right about this. Nowadays most pantograph fitted stock is protected by an ADD (Automatic Dropping Device). It usually consists of a small air pipe which pushes the carbons (at the top of the pan) against the OHLE contact wire. If there are any problems with the way the pan is riding, or if the carbons become damaged it will bring the pan back down. Once this happens the traincrew aren't allow to try to raise it again until a Fitter has checked it. So when you're in a tunnel under the sea where it would take a considerable time to get a fitter to you, it would make sense to have a spare one.

Different classes of traction may have different reasons of course for having two pantographs, but nowadays it's considered a good thing to minimse the number of pans in the air when travelling at speed, as this can cause resonance on the OHLE.