Frequency avoidance & amplitude minimisation for railway safety cases

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RAILWAY SAFETY CASE BACKGROUND

This page addresses the functional safety of railway rolling stock engineering equipment. For information on general railway safety issues refer to personal track safety.

In the UK the duties of Her Majesty's Railway Inspectorate, part of the Health and Safety Executive, which handled the regulation of health and safety on railways was transferred to the Office of the Rail Regulator, ORR. Since the year 2000 the UK Railways (Safety Case) Regulations require all operators to produce formal railway safety assessment for acceptance by the safety authority. The case must be submitted and approved prior to operations commencing. There are some situations where a full and rigorous railway safety case is not required to assure that a railway organisation is operating procedures that ensure safety for the travelling public, workers, and others incidental to rail services. In these instances the ORR may issue an exemption from submitting a safety case, although this has become a less frequent occurrence. Guidelines on compliance are available via the recommended ORR link.

The Rail Regulator determines if to accepted the railway safety case (under the Railways (Safety Case) Regulations Act 2000), or issue a safety case exemption. The Rail Regulator proceeds to seek all necessary advice on any safety issues raised by advisor. Only when satisfied does the Rail Regulator approve either a licence or an exemption. The process of railway safety case acceptance is detailed and exacting, and frequently extends for longer than anticipated in pursuit of rigorous approvals. The ORR advise applicants to discuss at an early project stage the requirements of a railway safety case with members of HM Railway Inspectorate. For engineering systems a prime concern highlighted here is functional safety. The IEC defines functional safety as the "part of the overall safety that depends on a system or equipment operating correctly in response to its inputs". For example, the train and railway infrastructure electrical compatibility issues are best summarized diagrammatically in the form of a technical EMC demonstration of compliance process.

TRACTION RETURN CURRENT COMPATIBILITY

Traction return currents flow back to the supply stations via the running rails. The rails are periodically bonded to earth, but despite this a significant proportion of the current leaks into the earth, eventually returning to the supply earth via a multiplicity of paths. Dependent on the bonding strategy the dispersed return current can cause danger to personnel from a significant rail to earth touch voltage. Furthermore, stray return current gives rise to corrosive effects on submerged metallic pipes as well as possible interference with other track and lineside signalling and communication systems.

The calculation of earth potential and current depends on the electrical properties of the ground, in particular its resistivity. This problem has been appreciated from early days of electric railway systems when in 1932, J Riordan wrote a classic work describing the propagation of current in railway systems. This was soon followed in 1936 by E D Sunde's paper on "Currents and potentials along leaky ground return conductors". This explained the idea of track admittance, where the track to ground admittance is amalgamated with the rail to rail conductance into a two layer homogeneous model. The upper layer representing the rail coupling components and the lower layer the earth admittance. This model of stray earth currents has proved reliable, although generally pessimistic, as established by Mellitt et al in a 1990 paper, "Computer based methods for induced voltage calculations in AC railways".

The classical two layer model for stray currents is still used and indeed recommended by NR/GN/SIG/50018, the NR Code of Practice. The coupling factors between the elements of the two layer model are determined from the reduced form of the Pollaczeck equation and accounts for both conductive and inductive effects. This has been used effectively across Europe for the past 50 years to ensure the effect of stray return current on both AC and DC electrified railways is accounted for.

EE&CS

Electrical Engineering and Control Systems (EE&CS) safety is a critical area dealing with the risk of signalling wrong side failure as a result of interference or multiple failures. The scope for interference between train electrical subsystems and lineside infrastructure including track circuits (and visa versa), is identified in Railway Group Standard GE/RT8015, Electromagnetic Compatibility between Railway Infrastructure and Trains. Modern traction rolling stock with high power switching devices represent the main interference source, with potential to couple into vital signalling systems via conducted, inductive or radio frequency interference. Traction techniques to combat generated interference are based on two underlying principles:

  1. Frequency Avoidance
  2. Amplitude Minimisation

It is always preferable, by frequency avoidance, to eliminate vital signalling frequencies from the harmonic footprint created by a vehicle. However, this is often not feasible, and countermeasures to restrict the amplitude of interference are necessary. This is termed amplitude minimisation. Harmonic elimination of signalling frequencies can be achieved by careful selection of switching frequencies combined with strategic PWM switching schemes. Amplitude minimisation of interference invariably also involves filtering, by passive or active methods. The latter technique applies feedback to achieve the desired spectral envelope shaping. Filtering may also take the form of electrical screening, or physical layout optimisation to reduce external fields.

STAGING

For new build rolling stock compliance requires following a sequence of safety case "stages" to justify and then validate the compatibility approach. For instance, this may include design, manufacture, testing, interim, interim with service, service, and fleet stages. A process invariably covering a period of years. A demonstration of satisfactory completion of each stage is a precursor to advancing to the next. Therefore the railway safety case is necessarily an integral component of any vehicle build project plan.

SUMMARY

Recognizing these issues requires vehicle concept and design stage decisions to ultimately achieve compliant solutions. A systems approach is essential, simultaneously blending together several engineering disciplines to achieve functional safety by optimised train emissions for all known railway system susceptibilities.

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