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

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 solutions is exposed.

#### **5.1 Track circuits**

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

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 depending on the technology.

#### **5.2 Signalling and communications wires**

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. These orders are usually 50 Hz signals. They can be of a different range of voltages that can vary between 50 to 220 volts. So there is a chance that induced voltages can either turn off one aspect of the signal, which would only affect operation of the line, or can turn it on what can affect safety.

Communication wires are laid along the line from one station to their collaterals. Voice services are transmitted via these wires and also data. 50 Hz disturbances in these wires can affect communications, but are not likely to have an influence in safety issues.

Block wires connect collateral stations. This is a coded signal that is not likely to be affected by 50 Hz disturbances, but can be affected by harmonic disturbances caused by the rolling stock.

In order to protect these services from disturbances several actions can be carried out:

