RAILWAY SAFETY CASE BACKGROUND
The duties of Her Majesty's Railway Inspectorate, part of the Health and Safety Executive, which handles the regulation of health and safety on UK railways has been transferred to the Office of the Rail Regulator, ORR.
Since the year 2000 the UK Railways (Safety Case) Regulations require all operators to produce thorough railway safety cases 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 unnecessary for assurance that a railway organisation is operating procedures that ensure safety for the traveling public and workers, and others incidental to the services.
In these instances the ORR can issue an exemption from submitting a safety case, although this has become a less frequent occurrence. Guidelines on exemption are available via the recommended ORR link.
The Rail Regulator determines if it should 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 advisors. 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 initially anticipated.
The ORR advise applicants to discuss at an early project stage the requirements of a railway safety case with HM Railway Inspectorate. The train and railway infrastructure electrical compatibility issues are best summarised 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 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:
- Frequency Avoidance
- 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.
This may include design, manufacture, testing, interim, interim with service, service, and fleet stages for instance. 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 optimised train emissions for all known railway system susceptibilities.
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