Railway tracks: overview of electrical interference

Non-electrified railway tracks: signalling and safety

Interference can affect the detection and systems on non-electrified railway tracks, whose signals pass through the tracks, detect the presence of a train and predict its arrival at a railway crossing. Rolling stock can be detected through wheels that short-circuit the rails within a fixed length (block signalling system), defined by insulating joints. These systems also provide instructions (e.g., speed, advance warning) that are transmitted to adjacent zones. The rolling stock position and speed are also used to activate barriers at road crossings, depending on the expected time or arrival. The following figure illustrates the detection principle for a fixed-block railway system (ref.: Electric Power Research Institute [EPRI], Power System and Railroad Electromagnetic Compatibility Handbook).

In Canada and the United States, railway equipment must comply with recommendations from the American Railway Engineering and Maintenance-of-Way Association (AREMA), which ensures good performance under normal operating conditions and during lightning strikes. Moreover, redundancies and the failsafe principles ensure public and worker safety. For example, power is transmitted to the level crossing gates to keep them open. A power outage will cause the gates to close by default.

The key source of electrical interference on a non-electrified railway track is a nearby power system (lines, substations, etc.). If this is the case, then you must check the following:

When the power system is in normal operating condition:

• the voltage induced between the two rails should typically not exceed 5 to 10 VAC, to prevent any impact on signalling equipment operations.
• the rail-to-ground voltage or voltage between two sections of a rail should typically not exceed 25 to 50 VAC, to ensure worker safety.

When the power system is in a fault condition:

• The current flowing in the railway equipment must not cause any damage (lighting arresters, relays).
• The voltage measured by railway equipment must not be higher than its dielectric withstand.
• The step and contact voltages from the rails and other equipment must ensure worker safety.

Electrified railways: various challenges depending on technology and configurations

Interference affecting electrified railway tracks depends on a variety of technology and configurations, which can include the voltage and type of power supply (AC or DC), as well as the type of track (train, tramway, light rail).

Traction systems are usually insulated from the utility power supply, except for a single grounding connection point (bonding). Normally, load current from transformers (AC system) or rectifiers (DC system) feeds the train through a catenary (overhead line) and returns to its source through rails and dedicated ground cables. The rails must be sufficiently insulated from the ground to prevent stray currents from flowing to the earth, accelerating structural corrosion or affecting other adjacent systems. Voltage limiters are connected to the rails at given intervals to ensure the rail voltage does not exceed safe contact values during faults.

Depending on the technology, some of the above-mentioned interference for a power system located near a non-electrified track may also apply to the electrified track. For example, a fault condition on electrified railways may affect nearby power systems. Because the catenary carries electricity, public safety is of particular concern under normal operating conditions and in the event of a fault, especially for stations, rails and catenary structures.

Validation and mitigation of railway electrical interference

Railway interference must be assessed at the preliminary and detailed engineering stages and then confirmed with measurements during commissioning. However, modifying or adding nearby electrical infrastructures (line, substation) should also require further evaluation, usually including 3D modelling of electrical phenomena.

Going further

To learn more about various railway interference phenomena, contact one of our experts. You can count on BBA to provide more advice about AC interference.