Why trains must start with two pantographs raised?

[manolollr]

In the newest version of the simulator (22.08), the "Driving aid" window suggest that whe train is stopped you must raise the two pantographs, and when the train run at about 5 km/h you can lower the front pantograph.

Why is this action done? Is it applicable to inverter asynchronous locomtives, EMU's or light passenger trains?

[Rozi]

With high power consumption and low speed, the pantograph can weld to the network (the contact surface gets very hot). Thats why its safer to start with both pantographs in the air. Additionally, during winter the front pantograph may scrape off frost / ice in front of the rear pantograph.

[matek123]

Why is this action done? Is it applicable to inverter asynchronous locomtives, EMU's or light passenger trains?

Program probably don't know what type of locomotive you drive. But when you have turn on heating of train, you should raise two pantographs, when you stop train longer than 5 minutes.

Kolega zbanowany jednak trochę racji ma z tymi błędami.

[DB-47]

Mostly passanger trains need that, because there is signficant power draw around 50 - 150A from 3 kV DC OHL to feed carriages with power to charge their batteries, power A/C or heater and feed electric sockets / lights. This current is quite small consumption at travel speeds, however when standing at one place it can cause local overheat of overhead lines, which can in worst case lead to its rupture and teardown. Halving that current by raising second pantograph reduces waste heating power to 25 % (P = R * I^2) which prevents this from happening.

EMUs and all cargo trains pulled by thyristor / IGBT driven locomotives (like ČD 163, Traxx, Taurus 1216) do not require this procedure as their standstill power consumption is much smaller and ammount of taken current raises gradually with increasing speed instead of burning "surplus" power in resistor bank like in for example EU07/201Eo/ČD 181, and so on. Light cargo trains pulled by locomotives with resistive regulation like ČD 181 also do not need both pantographs raised as their power draw at standstill is miniscule. For instance ČD 181 with all auxiliary devices running (fans, both compressors on and 48V DC network convertor) barely draws 50A. However as for what I heard from ČD Cargo locomotive engineers, you are not supposed to exceed current draw 200A per pantograph below 5 km/h, so they raise both pantographs anyways to be sure.

Here in Czech republic this is being done on 3 kV DC routes. On 25 kV 50 Hz routes this is not mandatory as currents are smaller by magnitude and barely exceed 20A at standstill assuming power factor is near 1.

[Farmadupe]

Why is is preferred to raise the rear pantograph? I assume this is because if the front pantograph is raised asnd there is a pantograph-accident, the rear pantograph may also be damaged... but if the rear pantograph is raised and there is a pantograph-accident, then the front pantograph may not be damaged?

P.S. Can anyone explain why there is additional Polish graffiti in the CD183 cabin for the pantograph lever? My (bad) guess is that to raise the rear pantograph, the switch must be moved in a different direction depending on which cabin is being used (e.g up for cabin A; down for cabin B)... But maybe the polish person with the label printer did not check if they are in cabin A or cabin B?

P.S.2 Does the power controller also move by itself in the real Skoda 3KV locomotives when the driver does not press ctrl? Is this a real feature (automatically move the power controller if it does not agree with the tap changer) or is it a bug?

[DB-47]

Actually also in CZ default pantograph choice is rear one. One of the reasons is to reduce friction as front pantograph tends to be more pushed by air upwards. Under normal circumstances this is not necessary and this only contributes to faster pantograph wear. Secondly if there is pantograph accident it is less likely that rear pantograph damages also front one compared to vice versa scenario.

Front pantograph is being used only when:

1) Behind your locomotive is another locomotive. If there was risen rear pantograph, potentially glowing shards could damage front glass of the locomotive.
2) Behind your locomotive is wagon loaded with cars. Logic is same as in scenario 1
3) Driving through strong rain. Extra contact pressure is less harmful for pantograph than violent arcing.
4) Rear pantograph does not support current voltage system or is damaged. Some Czech classes 380 have contact shoe specialised either for AC system or for DC ones.

Only trams if having option to use single pantograph use front pantograph because in past there used to be electric contacts on overhead lines, which were used to change switch direction and when rear pantograph would be used, then the switch would get locked and ignored command dictated by current uptake from rear pantograph. Secondly in street traffic it is more practical to have pantograph as close as possible to driver so driver can easily tell if they are already in front of / near / past section insulator. With rear pantograph this can be very tricky.