---
title: "Electric traction; current collection arrangements"
subject: "Utilization of Electrical Energy"
chapter: 4
key: "semester_7/utilization_of_electrical_energy:electric-traction-current-collection"
source: https://www.bctengineer.com/q/semester_7/utilization_of_electrical_energy/electric-traction-current-collection
site: BCT Engineering
---

# Electric traction; current collection arrangements

Utilization of Electrical Energy · Chapter 4: Electric Traction

- Asked in: 2079 Bhadra, 8 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/utilization_of_electrical_energy/electric-traction-current-collection
- Whole chapter: https://www.bctengineer.com/md/semester_7/utilization_of_electrical_energy/chapter-4

## Question

What do you mean by electric traction? Discuss compare various arrangement of current collection used in electric traction.

## Short answer

Electric traction employs electric motors for propulsion. Current collection methods include the third rail with collector shoe for low-voltage DC metros, and overhead systems: trolley pole, bow, and pantograph. The pantograph is the standard for main-line service due to its reliability, high speed and high-voltage handling.

## Answer

**Electric traction** is the system of moving vehicles (trains, trams, trolley buses, electric locomotives) using electric motors, with energy supplied from a fixed distribution system along the track or from an on-board source.

For vehicles fed from a distribution line, a **current collector** keeps a sliding contact with the supply conductor while the vehicle moves.

#### 1. Third (conductor) rail system
- An insulated steel rail laid beside or between the running rails; a **collector shoe** slides on it (top, side or bottom contact).
- Used for 600–750 V dc metros and suburban lines (e.g. London Underground, many older metros).
- It can collect high current, but only at low voltage (for safety).

#### 2. Overhead system
An overhead wire is hung above the track; a collector on the roof presses against it.

**a) Trolley collector (trolley pole)**
- A pole with a grooved wheel or slider at its end, pressed upward by springs.
- Used in tramways and trolley buses (two poles, since the return is not through rails).
- Suitable only for low speed (about 30 km/h); it can leave the wire at junctions and must be reversed at route ends.

**b) Bow collector**
- A light metal strip (bow), about 1 m wide, on a frame, sliding on the wire.
- Used in tramways; speeds up to about 30–35 km/h; needs reversing with direction (or a reversible type).

**c) Pantograph collector**
- A hinged, diamond- or half-diamond (Z) frame raised by springs or air pressure, with a carbon/copper contact strip.
- Keeps uniform pressure over a wide range of wire heights; works in both directions and at high speeds (over 300 km/h); can collect large currents.
- Used on all modern ac and dc main-line locomotives and EMUs.

```text
   ===== contact wire ==========
        \____/    pantograph head
         /  \
        /    \   frame (springs/air)
   ____/______\____ vehicle roof
```

#### Comparison
| Feature | Third rail | Trolley pole | Bow | Pantograph |
|---|---|---|---|---|
| Speed | Medium | Low (~30 km/h) | Low | Very high |
| Voltage | 600–750 V dc | ~600 V dc | ~600 V dc | Up to 25 kV ac |
| Current capacity | High | Low | Medium | High |
| Reversal needed | No | Yes | Yes (simple type) | No |
| Dewirement risk | None | High | Medium | Low |
| Use | Metros | Trolley buses, trams | Trams | Main-line trains |
| Safety | Live rail at ground | Good | Good | Good |

The pantograph is preferred today for its reliability, high-speed performance and ability to handle high voltage; the third rail remains common for underground metros where tunnel height is small.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is electric traction?
   A. Any vehicle using a petrol engine
   B. Only electric trains on third rail
   C. Moving vehicles using electric motors supplied from a fixed distribution system or on-board source
   D. Braking by electric means
   Answer: C. Electric traction is propulsion of vehicles by electric motors with energy from a line or on-board source. (Key point: Definition)

2. Why is the trolley pole suitable only for low speeds?
   A. It carries too much voltage
   B. It can leave the wire (dewirement), especially at junctions, and must be reversed at route ends
   C. It is too heavy
   D. It needs two rails
   Answer: B. A trolley pole has high dewirement risk and must be reversed, so it is limited to about 30 km/h. (Key point: Trolley pole limits)

3. Why is the pantograph preferred for main lines?
   A. It keeps uniform pressure over wide wire-height range, works both directions, high speed and high current
   B. It works only at low speed
   C. It is the cheapest low-voltage collector
   D. It runs on a live ground rail
   Answer: A. Its hinged frame and spring/air pressure give reliable contact above 300 km/h with large currents and high voltage. (Key point: Pantograph)

4. Which describes the third rail system?
   A. Insulated rail beside the track with a collector shoe, low voltage 600-750 V dc, common in metros
   B. 25 kV ac overhead
   C. Pole with grooved wheel
   D. Bow collector used on main lines
   Answer: A. Third rail uses a shoe, is limited to low voltage for safety, and suits tunnels with small height. (Key point: Third rail)

## Related in Utilization of Electrical Energy

- [Advantages of electric traction; trains, trolley, tramways](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aadvantages-traction-trains-trolley-tramways) — Utilization of Electrical Energy, chapter 4
- [DC vs AC electric traction systems](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Adc-vs-ac-traction-systems) — Utilization of Electrical Energy, chapter 4
- [Electric traction in Nepal; self-contained vehicles](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aelectric-traction-nepal-self-contained-vehicles) — Utilization of Electrical Energy, chapter 4
- [Key components of electric traction system](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Acomponents-electric-traction-system) — Utilization of Electrical Energy, chapter 4
- [Electric braking in AC and DC traction drives](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aelectric-braking-traction-drives) — Utilization of Electrical Energy, chapter 4
- [Traction motors: applications and characteristics](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Atraction-motors-applications-characteristics) — Utilization of Electrical Energy, chapter 4

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