Case study # 1.9 Title Electric railway interference Type of trouble Acoustic noise, degradation, abnormal operation, loss function. Source of trouble Electric railways within a few kilometres of telecom systems. System affected Customer's equipment. Location Customer premises, outdoors. Keywords Electric railway, common mode chokes, screening, filtering. Version date 2004-01-01 System configuration Normal PSTN line. Customers hear noise come and go, coincident with trains coming/going into sections of track. It can be determined from the noise level (and tone) if the train is accelerating or decelerating. If booster transformers are used on the electrification system, then the sudden cut-in/out of the noise is very evident as trains enter and leave the section. Measurement/Searching techniques/Experiment Customers suffer; noise creates audible and data problems, even to the extent that dialling out does not proceed. Railway noise tends to be different from normal power harmonic noise in that it tends to be at a low level when no trains are in the section and then varies dramatically when a train arrives. Having knowledge of the timetable or a view of the line helps in determining if it is due to the trains. The noise is typically due to the multi-pulse rectifiers used in the power conversion of the motor and the spread of the return currents. Very little of the noise can be attributed to airborne effects. Even when good mitigation measures have been employed, noise can sometimes re-occur. In most instances, this has been found to be due to poor track cleaning practices, which have resulted in numerous damages to earth bonds between rail and the return wire, thereby increasing stray currents in the earth. Copyright © ITU 2010 – All Rights Reserved