Interference Problems with 25 kV AC Traction

The maximum acceptable voltage that can be induced into cable conductors.

Testing the ability of cables to withstand voltage without breaking down.

The maximum allowable current flow through a contact between a cable conductor and the earth or other metal.

A type of coupling between circuits through direct electrical contact.

Coupling between circuits caused by electric fields.

Coupling between circuits caused by magnetic fields.

Maximum permissible current through contact with the earth or other metallic structure arising solely from capacitive coupling.

The voltage induced on a line due to another nearby charged line, especially a high-voltage line.

The current flowing from a line to the ground (earth) due to electrostatic induction. It's proportional to frequency and the length of the lines.

A method used in calculating the effect of the Earth on the voltage induced between parallel conductors. It considers the Earth's surface as a reflective surface.

V = E * (bc / (a+b+c)). V is the induced voltage, E is the inducing line voltage, and a, b, and c are distances/heights.

The induced voltage on a line decreases as the distance (separation) from the inducing high-voltage line increases.

The voltage at which a spark gap (often in a telephone circuit) breaks down and allows current to flow. (The voltage needed to start the discharge)

The voltage induced on other conductors is significant enough to trigger a discharge if those conductors are near high voltage lines.

The capacitance between a conductor and the ground.

The capacitance between two conductors.

The voltage generated in an overhead line or cable due to the presence of a nearby electrified railway.

e = 2π fm L 1 Kr Kc Km, where f is frequency, M is mutual inductance, L is length, Kr is rail current reduction factor, Kc is cable reduction factor, and Km is metal pipework reduction factor.

A measure of the inductive coupling between the two lines.

A factor that reduces the induced voltage due to the current in the rails because it partially cancels out the effect of the contact system current.

A factor that reduces the induced voltage when the line is a metal-sheathed cable due to the effect of the sheath.

A factor that reduces the induced voltage in a line due to the presence of nearby earthed metallic pipework with notable induced current.

The reduction in induced EMF achieved by the presence of earthed conductors (e.g., cable sheaths, metal pipes, earth wires, rails).

Mutual inductance (M) varies according to the separation distance (a) between the lines, soil conductivity (s), and the frequency of the inducing current (f).

At a distance from the track, Kt (another reduction factor) can be approximated as 1 – (It / Ic), where It is the total induced current in the rails and Ic is the catenary current.

The reduction factor Kc improves (reduces interference) as the frequency of inducing current increases.

The effectiveness of a cable sheath in blocking external electromagnetic interference (EMI).

Lower DC resistance in a cable sheath improves its screening effectiveness, reducing EMI.

Higher mutual inductance between the sheath and wires inside enhances shielding against EMI.

Aluminium sheaths offer better EMI shielding than lead sheaths due to lower resistance.

Adding steel tape armouring increases inductance, improving EMI shielding.

Rail currents have very low resistance and high leakance, meaning they readily flow to earth.

Rail currents quickly diminish as they flow through the ground, with higher frequencies attenuating faster.

Rail currents tend to split equally in both directions from feeding and loading points, with some flowing through the earth and cables.

Most of the return current in a rail system flows through the ground and adjacent structures like cables.

A voltage is induced in the rails by the current flowing in the overhead wires, causing a current to flow in the opposite direction.

A measure of how effectively the rails shield nearby communication lines from induced electric fields. It's usually less than 0.6 for well-maintained tracks.

Metal plates used to connect sections of railway track, contributing to resistance in the rail circuit.

A phenomenon where bad fish plate connections exhibit high resistance at low voltages but almost no resistance at higher voltages.

Techniques to minimize induced voltages and protect communication circuits and systems from interference caused by electric railways.

Recommendations by the International Telegraph and Telephone Consultative Committee (CCIT) setting limits for permissible induced voltage to prevent interference from power lines in telecommunications lines.

The maximum acceptable continuous induced voltage on telecommunications lines during normal operation of nearby power lines. It's typically limited to 60 volts RMS for safety.

The maximum acceptable induced voltage on telecommunications lines during a fault on a nearby power line. This limit varies depending on the type of power line and fault.

A metallic covering around a cable that is connected to ground. It helps reduce induced voltage by providing a path for induced current to flow to earth.

Transformers used to electrically isolate communication lines from the power system. They prevent induced currents from passing through the telecommunications lines.

Devices installed at the ends of communication lines to protect them from voltage surges caused by lightning strikes.

The maximum distance between a high-voltage line and a communication line before induced voltage becomes dangerous.

Increasing the distance between a high-voltage line and another conductor reduces the induced voltage.

A conservative formula to calculate the minimum distance between a high-voltage contact wire and a communication line to limit induced voltage.

When a high-voltage line is not perfectly parallel to a communication line, the minimum separation is calculated using the geometrical mean between the distances at both ends.

The induced voltage calculations assume ideal conditions, but in reality, factors like sags, capacitance, and earth roughness affect the actual induced voltage.

Line sags, insulators, and earth roughness increase capacitance to earth, reducing the actual induced voltage, though harmonics can increase the short circuit current.

Measured values of induced voltage are often lower than calculated values due to real-world factors that increase capacitance.

High harmonics, even with small amplitude, can significantly increase the short circuit current induced in conductors.

Unwanted signals or interference in communication lines, primarily caused by electromagnetic induction from nearby sources like electrified railways.

Periodically switching the positions of conductors in a communication line to minimize induced voltage by creating compensation along the line.

A factor that accounts for the reduction in induced voltage due to the current flowing through rails and the earth, partially canceling out the effect of the contact system current.