Trains stabled at Cambridge during strike?

[TW1306]

(As a pre-emptive this is not a political thread so please keep any opinions on the strikes themselves away from here)

I was in Cambridge yesterday and decided to visit the station building, when I crossed the cycle bridge I saw a number of GE and Thameslink units stabled in platforms and along the tracks. However it sounded as though several of them were 'switched on' and actively drawing power from the OHLE; was I imagining things, or was that actually the case and TOCs do that often? And if so, why do they do it, since it would presumably be a waste of power?

 

[Surreytraveller]

They'll just be berthed where they're usually berthed

 

[JN114]

(As a pre-emptive this is not a political thread so please keep any opinions on the strikes themselves away from here)

I was in Cambridge yesterday and decided to visit the station building, when I crossed the cycle bridge I saw a number of GE and Thameslink units stabled in platforms and along the tracks. However it sounded as though several of them were 'switched on' and actively drawing power from the OHLE; was I imagining things, or was that actually the case and TOCs do that often? And if so, why do they do it, since it would presumably be a waste of power?

EMUs berthed will typically do so powered on - the power usage is typically negligible.

They could be fully shut down, but this would significantly increase start up times when they are required

 

[dk1]

Yes it's easier just to leave everything pan up incase of any battery/start issues from being left too long. Power usage is not something that's really taken much into consideration.

 

[D365]

… when I crossed the cycle bridge I saw a number of GE and Thameslink units stabled in platforms and along the tracks. However it sounded as though several of them were 'switched on' and actively drawing power from the OHLE; was I imagining things, or was that actually the case and TOCs do that often? And if so, why do they do it, since it would presumably be a waste of power?

’Sounded’ in what sense? An electric train will typically be ”panned up” while stabled, in order so that the auxiliary batteries aren’t depleted.

If the unit wasn’t panned up, it would require the batteries to be recharged (pan up + aux set) before the unit can resume in service.

As you can see, raising a pantograph takes a few more steps than just plugging an electric vehicle back in to charge!

 

[O L Leigh]

If the unit wasn’t panned up, it would require the batteries to be recharged (pan up + aux set) before the unit can resume in service.

Not really. Much like a mobile phone, an electric train can be operated whilst connected to the power supply and the batteries are charging. What you’re really waiting for is the compressor to build up the main air pressure so that you can get brake release.

Where the auxiliary batteries cause problems is when they fail to run the auxiliary compressor needed to charge the pantograph air system when the main air pressure is depleted. Without this you can’t raise the pan and close the breaker to connect the train to the power supply. This can happen when a train has weak batteries, hasn’t been “auxed off” or has been off the power supply for an extended period of time. The only remedy is to couple an assisting unit and feed main air into the failure via the coupler connector block in order to raise the pan.

 

[D365]

Not really. Much like a mobile phone, an electric train can be operated whilst connected to the power supply and the batteries are charging. What you’re really waiting for is the compressor to build up the main air pressure so that you can get brake release.

Where the auxiliary batteries cause problems is when they fail to run the auxiliary compressor needed to charge the pantograph air system when the main air pressure is depleted. Without this you can’t raise the pan and close the breaker to connect the train to the power supply. This can happen when a train has weak batteries, hasn’t been “auxed off” or has been off the power supply for an extended period of time. The only remedy is to couple an assisting unit and feed main air into the failure via the coupler connector block in order to raise the pan.

That's what I mean - there needs to be enough voltage left in the battery for the pantograph to be raised, and the battery charger (motor-alternator or equivalent) to be set. On a properly designed electric vehicle, the aux loads should be shed before the critically low voltage is encountered.

I'm trying to keep it concise as I'm aware that discussion of train electrical systems can quickly become overly technical!

 

[O L Leigh]

If a train has been “auxed off”, which it should be if it’s been fully disposed as opposed to simply immobilised, there should be no loads at all on the batteries. Any discharge then becomes subject to the characteristics of the batteries themselves, environmental factors and the consequent effects on retaining charge.