Radiocommunications Agency  EMC Awareness

Railway systems

Description

A report from York EMC Services for the RA has looked at the potential for interference from the various parts of the railway system. The following quotes summarise their conclusions:

“It is well known that the railway electromagnetic environment is much more severe than that found in most commercial and domestic premises. However, in many instances the railway runs very close to such premises. In fact, in the example of an inner city light rail scheme the railway effectively runs along public roads, which brings it into close proximity to non-railway premises and potential victim systems.

There are concerns about radio frequency emissions from railways and their potential to interfere with the operation of commercial radio services and other equipment, such as information technology equipment.

There is concern amongst CISPR and the radio community that the emission levels and measurement techniques set out in EN 50121 [the railways emissions standard] do not provide adequate protection to radio services.… Some evidence has been found showing that such emissions are capable of interfering with electrical or electronic equipment and radio services operating adjacent to the railway lines… The findings of this study have implications for planned or existing buildings in which IT equipment will be used, where the buildings are situated very close (i.e. less than 10m) to electrified railway lines. There is a significant probability that the passing trains will interfere with PC monitors that are only a few metres away from the lines.”

Commentary

There are two particular EMC threats from railways to the outside world: first, the use in modern trains of high-frequency power inverters for the traction power, which creates high levels of harmonics of the switching frequency. These harmonics have significant content up to and beyond 1MHz, and they are injected directly onto the overhead line, which acts as an efficient long-wave antenna and may be resonant. Long and medium wave broadcast receivers can suffer interference from these harmonics, which are usually but not inevitably correlated with the passage of the trains. Filtering of the inverter supply on the train would reduce the threat, but at the expense of cost and weight. Arcing between the current collector and the overhead line or third rail conductor is also a potential source of noise, particularly for the higher voltage systems.

The second problem is that a high DC or AC current flows in the overhead line to supply the traction power. This creates high transient magnetic fields in the neighbourhood of the line, which although they fall off rapidly with distance, can be high enough to upset magnetically-sensitive devices; standard CRT-type VDU monitors are likely to be susceptible, though the flat-panel LCD displays are not. Re-orienting the VDU is a possible solution, but otherwise, offices and other premises using such monitors should not be very close to electrified railways.

References and links

“Potential Electromagnetic Interference to Radio Services From Railways” by T Konefal et al, York EMC Services for the Radiocommunications Agency, RA report ref: AY 4110. Download from http://www.radio.gov.uk/topics/research/topics.htm#emc/

Links to Mitigation Techniques

	Installation	Design & Development	Resources
Filtering
Power harmonic filtering
Ground bonding