**5.3 Level crossings**

176 Electrical Generation and Distribution Systems and Power Quality Disturbances

Once that the problems have been described and the physical phenomena were formulated in the above sections this section will describe how electrical disturbances in already existing railway lines can be lessen or eliminated. In the next subsections a compendium of

Track circuits, as has been shown in this chapter, are safety elements that can be affected by disturbances. Since it has been demonstrated that this kind of devices can be altered by disturbances, the recommendation is to change them for audiofrequency track circuits in order to avoid malfunctioning. In order to determine which track circuits should be replaced simulations are performed in order to determine the induced voltage level between both rails. When this simulated voltage level is above a determined disturbance level, a decision to change them is carried out. The induced level of voltage is function of the current circulating through the overhead wire system in the high speed line, the distance between both lines, the angle between them, the distance between the rails (UIC gauge, Spanish

When direct contact disturbances might occur, usually nearby a gauge changing facility or a electrification changing section isolation rail joints might be used. In order to avoid that train units could shortcut both electrical zones two isolation joints can be placed with a separation bigger than the length of the largest train that will pass by that lines. When isolation joints cannot be deployed due to traction return problems, impedances can be used. In the Spanish example impedances are installed in order to offer a high impedance at 50 Hz and low impedance at zero frequency. This 'high impedance' circuit for 50 Hz and higher frequency is built as a low pass filter to offer maximum impedance at 50 Hz. Thus the 50 Hz current that would flow through the DC rails is dramatically reduced. According to the measures made in some gauge changing facilities the reduction of the flowing current through conventional rails can be reduced from 1/4 to 1/2 of the current that would be present if no impedance was present. These impedances are useless in transitions between electrified and non electrified lines since only isolation joints are needed to avoid the presence of 50 Hz current through the rails. The electrified rail cannot have isolation joints in order to allow the traction current to return to the feeder station. Since these impedances have a direct effect in the safety of the line they should be monitored in order to detect malfunctions. If they are not monitored, no assumption can be made regarding the reduction of the section of the line where 50 Hz track circuits should be replaced by audiofrequency ones. The way to calculate the boundary between affected and not affected zones is to determine the distance where there's a current through the rails equivalent to that of the normal operation of the track circuits. This distance can be as long as 18 km

Wires are used in railway signalling in order to connect the interlocking with the field elements. These wires are spread in parallel along the track and some of them can have a length of several kilometres. The input signals to the interlocking are basically the state of the track circuits (occupied/unoccupied), the position of the points and the current state of the light signals, and the outputs are the orders to the points and the orders to the signals.

**5. How to protect systems and equipment from electrical disturbations** 

solutions is exposed.

**5.1 Track circuits** 

gauge…), and the length of the track circuit section.

depending on the technology.

**5.2 Signalling and communications wires** 

Railway level crossings are particular signalling systems that can be or not be connected to other signalling systems such as interlocking. These systems are in charge of protecting cars and citizens from railways. Level crossings electronic systems can be triggered by the interlocking or by track pedal that gives a train announcement to the system. These facilities have also wires that can have a length of more than two kilometres.

The solutions to immunize these systems from disturbances are the same one that those stated above for signalling and communication wires.

#### **5.4 Accessible metallic elements**

According to EN 50122-1 we can define the following terms:


As was described above, usually DC rails used in traction lines are isolated from earth in order to prevent stray currents. This measure can raise rail potential. This can happen even without tractor trains in the line if there's a parallel high speed line nearby. Passenger might want to cross from one platform to other crossing the rails even if it's forbidden. This is more common in small stations with low traffic density.

Electrical Disturbances from High Speed Railway Environment to Existing Services 179

The use of this shortcut steady state current is of great value in order to assure the repetitiveness of the measures. A real traction unit won't allow a steady state current to be

To measure touch voltage the **EN 50122-1 Annex E methodology** is used. The methodology

• The voltage shall be measured with a voltimeter that has an internal resistance of 1 kΩ. • Each measuring electrode, in order to simulate one feet, shall have a total area of 400 cm2 and shall be pressed on the earth with a minimum force of 500 N. Alternatively, a sensor driven 20 cm into the earth may be used instead of the measuring electrode. • To measure the touch voltages/accessible voltage in concrete surfaces or dry up soils, a wet cloth or a water film shall be placed between the foot electrodes and the earth. The foot electrodes shall be positioned at a distance not less than 1 meter from the accessible

• A measuring electrode, usually a tip electrode, shall be used in order to simulate a

• One clamp of the voltimeter shall be connected to the so called hand electrode and the

In order to perform the measures a quad is used. This quad is loaded in the determined ends with the characteristic impedance of the wire that is usually a value between 500 to 600 Ω depending on the wire. These measures are carried out with the shielding of both ended connected and with one of the ends unconnected in order to determine the disturbance

These measures are usually done by two measurement teams each one of them placed at each one of the ends usually in a station communication facility. In order to comply with the measurement procedure a continuity of the shielding from one end to the other of the wire needs to be achieved. This was not always possible because DC stray currents could be a more important problem than preventing AC disturbances. In these cases, only prevention against electrostatic field was achieved since only one end of the shield was correctly

The differentiation between electromagnetic shield and electrostatic shield needs to be clarified. In order to prevent electromagnetic disturbances both shields must be connected between them and connected to earth according to the standard **ITU K14 Provision of a metallic screen in plastic-sheathed cables.** The electrostatic shield is just a copper conductor. It is enough to earth just one end of the shield to prevent electrostatic

These measures should be carried out shunting the emitter and without shunting it in order to determine the disturbance in both situations. As said before, one of them –occupancy of an unoccupied track circuit- has operational implications and the other one –liberation of an

When measures were carried out in a 50 Hz track circuit the measured values were:

• The transformed track voltage (if transformers were used) in the relay.

• The state of the track circuit (occupied/unoccupied)

occupied track circuit- has safety implications.

• It is enough to carry out such measurements by random checks of an installation. In Figure 8 it is shown the connection scheme to measure the communication wires.

hand. In this case paint coatings (but not insulations) shall be pierced.

other clamp shall be connected to the so called foot electrode.

induced when the shielding in not properly connected.

earthed while the other end was unconnected.

• The track voltage in the receiver.

disturbances.

maintained in order to measure the effects in the determined elements.

described in the standard can be summarized in the following aspects:

conductive part.

There are other elements that have similar problems like the wire fences used to prevent intrusions in high speed lines. Also metal structures in big stations like metallic shelters that are not adequately earthed can have accessible voltages that have to be measured in order to check that there are not over the limits established by the standards.

In order to keep the accessible voltage or the touch voltage below the established limits, and as is reflected in the standard EN 50122-1, voltage limiting devices (VLD) can be connected between earth and rail, fence or structure. These devices keep both parts isolated from each other until the voltage between them exceeds a certain defined value, then it connects both parts so there is an electrical circulation between both systems and after that the VDL opens the circuit and checks the voltage between both parts.

Another solution that cannot be used for rails is to have a good earthing circuit. Metallic shelters or metallic fences should be adequately connected to earth in order to prevent accidents or electrocutions.
