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Posted on 04 December 2019

EN50155 Compliance to Railway Standards

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The whole process requires dedication and commitment

“Ich verstehe nur Bahnhof” is a common German phase which translates literally into “I understand only train station”, but has the same meaning as “it’s all Greek to me!” I must admit, it was the first thought that popped into my head when I started reading through the European Standard EN50155: Railway applications – Electronic equipment used on rolling stock.

By Steve Roberts M.Sc. B.Sc.; Technical Director; RECOM Development in Gmunden, Austria

In fact, the EN50155 standard is one of the few well-written EN standards as it is simultaneously precise and encompassing. However, the price of this clarity is a multi-layered structure which calls on many other standards (36 in total). This article is an attempt to peel back some of the onion-skin layers to explain how the standard translates into a practical power supply specification.

RECOMs new RPR-Family of DC-DC converter

The first section in the standard after the usual scope and definitions preamble concerns environmental conditions of operation, the first major point being the ambient temperature. The specification is surprisingly relaxed compared to the typical industrial operating range requirements of -40°C to +85°C, as only the highest specification of TX rated parts need to cover this ambient temperature range for just 10 minutes during start up conditions. On the other hand, the expectation that rolling stock electronics function reliably over many years of operation is only realistic if the temperature range is benign. In practice, the operating temperature range means three things for the design of a power supply; firstly, the power supply must work efficiently (85% or higher) to reduce the dissipated power that would be otherwise lost as heat, secondly, this efficiency must remain effectively constant over a wide range of input voltage and load conditions, and thirdly, usually some form of thermal management is required such as a heat sink or baseplate.

Conversely, the shock and vibration requirements are anything but benign, as one would expect in such a hostile environment as rolling stock. The requirements are detailed enough to warrant the calling up of a separate standard, EN 61373: Railway Applications – Rolling stock equipment – Shock and vibration tests, just to explain how to conduct the tests to fulfil EN50155. The shock and vibration requirements are split into three application categories of increasing severity; Body mounted (Class A and B), Bogie mounted and Axle mounted. While the axle mounted requirements are astounding (up to 30g for 5 hours with 1km/s shocks), the body mounting specs are on a more human scale (less than 1g vibration, 5g shocks). As almost all power supplies will be body-mounted in the railcar, shock and vibration compliance is not such a big issue as long as no component hits resonance or is poorly mechanically supported. Obviously the smaller and lighter the power supply, the better for shock and vibration.

The next section in the EN50155 standard covers the power supply requirements. The nominal input voltages are 24, 48, 72, 96 and 110VDC, of which 24, 48 and 110 are the most commonly used. Although not covered in the standard, 36V is also often requested. The standard defines the continuous input voltage range as being between 0.7 and 1.25 nominal, with short-term fluctuations between 0.6 and 1.4 being allowed. In practice, power supplies must work continuously between 0.6 and 1.4 nominal as no “deviation of function” is tolerated.

Input voltage ranges

A basic rule-of-thumb for DC/DC converter design is that a 4:1 input voltage range is the practical limit for most typical designs. Thus all of the nominal input voltages can be covered by just three standard converters (see column 4 above). There are special DC/DC converters available that cover the entire 11:1 range and while an all-in-one design sounds attractive, care has to be taken that high efficiency is maintained across the entire input voltage range. It may be that using three interchangeable converters instead of just one is better in terms of reliability, efficiency and compactness. One final point to mention is that EN50155 supersedes the French railway standard NFF01-150, but for backwards compatibility the old NFF maximum input voltage of 1.6 nominal (176V with 110VDC supplies) is sometimes requested.

The input voltage specification also includes a requirement to deal with supply interruptions. Class S2 requires the power supply to still deliver an output during a 10ms power-outage. The 10ms loss of supply can only be bridged using capacitors to hold up the input voltage. The size of input capacitor required can be calculated using the formula:

c f equation

So for a 50W converter with 88% efficiency running from a 48V nominal supply with an 18V minimum input voltage, the bridging capacitor required for 10ms would be:

c equation

An additional advantage of having an input capacitor is to smooth out the input ripple voltage. EN50155 specifies that power supplies must cope with a DC ripple factor of up to 15% nominal, but as EN50155 also requires a form of reverse polarity protection (usually a simple diode in series with the DC input), the diode/capacitor combination will automatically smooth out most ripple. Care must only be taken that the peak currents in the diode are taken into account and that the inrush current to the capacitors does not overload any input fuses or over-current protection devices.

An extreme example of the layered approach of EN50155 is the section in the standard regarding EMC, surges, ESD and transients. The standard covers these points in just two short sentences by referring to another standard called EN52019-3-2: Railway Applications – Electromagnetic compatibility Part 3-2: Rolling Stock – Apparatus. The catch is that although the EN52019-3-2 standard is only 20 pages long, it calls heavily on the general industry standards EN55011 and EN61000- 4-2, 3, 4, 5 and 6. Talk about Inception! As it turned out for our railways certified converters, the test report just for EN52019-3-2 alone is 176 pages long. We found that although the converters could cope with the DC input voltage conditions without problems, the transient and ESD requirements meant that we had to add additional transient suppressors and input capacitors to absorb the surges and spikes. The complete input filter for our DC/DC converters is shown in Figure 2. For other manufacturer’s power supplies, a more complex or even an active filter network may be required to reach compliance.

The performance and reliability of railway rolling stock is of the highest importance. In this respect it is as dependant on the quality of the electronic equipment used on board as on any other parts. In order to promote good design, the EN50155 sets out requirements for quality management (the manufacturer of the equipment must be ISO9001 certified) and required safety features, such as fault protection and under-voltage lockout. While these features are pretty much standard in most industrial-grade power supplies, EN50155 also sets out other optional (application specific) requirements concerning construction, wiring, connectors, layout and materials as well as providing checklists for hardware and software documentation.

Input filter for our DC-DC converters

The last section of EN50155 sets out a useful checklist of all of the mandatory or optional type approval tests, along with a description of how to carry out the test or a reference to another standard which defines the test and pass/fail criteria. The mandatory tests are a visual inspection, a performance test, a low temperature operation test (minimum operating temperature for 2 hours), a dry heat test (maximum operating temperature for 6 hours), confirmation of operation over the full input voltage range, surge, ESD and transient tests, an electrical insulation test and finally the vibration, shock and bump test. The optional tests (to be agreed with the end customer) are a Damp heat cyclic test (usually required), water-tightness (depending on the final application), Production stress screening and low temperature storage (-40°C for 16 hours). Surprisingly, the EMC test is also given as optional, but as all power supplies must be CE marked, the EMC test is actually also mandatory. Of these tests, only the visual test, performance test and electrical insulation test are required to be carried out on each production batch.

In conclusion, the EN50155 is a standard that needs to be read through a dozen times to pick up on all of the nuances and coded references to other standards, but the demanding application of rolling stock requires that the manufacturing processes must be rigorously controlled in order to ensure consistency of performance. The whole process requires dedication and commitment, but at the end you can say “I really do understand “Bahnhof”!

Contact: s.roberts@recom-power.com

 

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