INTRO: Failure to establish clear contractual standards for electromagnetic compatibility is leading to costly delays in commissioning rolling stock and electrification schemes. Differing national requirements still present barriers to European procurement in an open market
BYLINE: Dr Phil Dixon, PhD SenMIEEE *
ELECTROMAGNETIC compatibility is achieved when electrical and electronic systems designed for different purposes function effectively and safely, despite interacting in ways that are not necessarily desired. EMC requires the designers of all the systems involved to anticipate this interaction, and collaborate to eliminate or mitigate undesirable consequences.
As soon as electric traction began to be introduced, EMC issues were recognised and addressed in the railway environment. Early examples were earth leakage of DC traction return currents from the running rails into the ground which damaged buried cables and pipes, and the noise induced in nearby telephone wires or cables by AC traction.
As the amount and complexity of equipment installed on trains increased, interactions between apparatus designed by different suppliers who did not understand the EMC environment in which it was to be used became more of a problem. The replacement of tap-changers and starting resistances by solid-state switching devices complicated matters enormously, because they are capable of injecting a wide range of variable frequencies into traction power circuits which can compromise the safe operation of signals.
In terms of testing for EMC, national standards were developed, for example by the Railway Industry Association in Britain. Special requirements would be developed by railway operators in the light of their own experience.
Their suppliers had to ensure that these requirements could be met, and this often resulted in acceptance difficulties. On occasion, preferred supplier procurement became the only option. So there was no level playing field for manufacturers producing equivalent apparatus, resulting in barriers to trade in Europe and higher costs for the railways.
In an attempt to address this situation, ENV50121 Parts 1 to 5 was published as a ’draft for development’ in 1996. An ENV is generally a pre-standard drafted and released to fill a void; clearly, at that time such a void existed for EMC in the European rail industry.
Part 1 is the ’structure’ document providing an overview of the set. Part 2 attempts to describe the railway environment, while Parts 3-1 and 3-2 deal respectively with trains and complete vehicles, and with the apparatus on rolling stock. Part 4 covers ’emissions and immunity of the signalling and telecommunication apparatus’. Part 5 describes fixed power installations.
This is a very useful set of documents. It is expected that many of these ENVs will achieve Euro Norm (EN) status and appear in the Official Journal of the European Communities shortly. Only at this point will they be fully recognised appropriate standards that can be used to demonstrate EMC compliance.
In summary then, a raft of new standards characterising the railway environment and its apparatus, including rolling stock and signalling, isnow available. Either in ENV form, or as ENs awaiting OJEC publication, these standards can still be used through the Technical Construction File route to demonstrate compliance. We should therefore use them.
Having these standards, and knowing that most suppliers are now working to them, gives us a good starting point from which to pursue best practice from the outset through a skilfully crafted EMC management plan. I will return to this point later.
Topical EMC issues
Although general EMC issues are covered by EN50121, there are safety related issues which are intentionally not covered by these standards because they are specific to the route or network of the railway concerned.
At the moment, the main problems of concern to train operators, manufacturers, infrastructure managers and safety case review boards are: