This part of EN 50126

• is intended to apply to all safety-related electronic (E/E/PE) railway systems/sub-system/hardware. However, the hazard analysis and risk assessment processes defined in EN 50126-1 and in this part are necessary for all railway systems/sub-systems/hardware, in order to identify any safety requirements. The relevant methods are provided by EN 50126-2. If analysis reveals that no safety requirements exist (i.e.: that the situation is non-safety-related), and provided the conclusion is not revised as a consequence of later changes, this part of EN 50126 ceases to be applicable;

• is applicable to safety-related electronic systems (including sub-systems and hardware) for railway applications;

• is primarily applicable to systems/sub-systems/hardware which have been specifically designed and manufactured for railway applications. It should also be applied, as far as reasonably practicable, to general-purpose or industrial hardware (e.g.: power supplies, modems, etc.), which is procured for use as part of a safety-related railway system. As a minimum, evidence shall be provided in such cases to demonstrate:

- either that the hardware is not relied on for safety,

- or that the hardware can be relied on for those functions which relate to safety;

• applies

- to the specification, architecture, design, construction, implementation, integration, manufacturing, installation, acceptance, operation, maintenance and modification/extension phases of the system /subsystem and hardware, and also to individual sub-systems and hardware within the overall system as determined by the process in EN 50126-1 and supported by the methods in EN 50126-2.

- to generic sub-systems and hardware (both application-independent and those intended for a particular class of application), and also to systems/sub-systems/hardware for specific applications;

• addresses railway specifics;

• does not define

- RAMS targets, quantities, requirements or solutions for specific railway applications

- rules or processes pertaining to the certification of railway products against the requirements of this standard

- an approval process by the safety authority;

• does not specify requirements for ensuring system security.

This part of EN 50126 is applicable

• to the specification and demonstration of safety for all railway applications and at all levels of such an application, as appropriate, from complete railway systems to major systems and to individual and combined sub-systems and hardware components within these major systems, including those containing software; in particular:

- to new systems

- to new systems integrated into existing systems in operation prior to the creation of this standard, although it is not generally applicable to other aspects of the existing system;

- for modifications of existing systems in operation prior to the creation of this standard, although it is not generally applicable to other aspects of the existing system.

- at all relevant phases of the life cycle of an application;

- for use by railway duty holders, railway suppliers, assessors and safety authorities.

Application of EN 50126-4 follows from SIL allocation to system/subsystem/hardware through applying the processes described in EN50126-1 and methods described by EN 50126-2. Given the relative maturity of most electrical systems, this part of EN 50126 is largely applicable to Electronic and Programmable Electronic sub-systems, systems and hardware.

NOTE Guidance on the applicability is given in the requirements of this standard.

Existing systems compliant with any version of former EN 50126, EN 50128 or EN 50129 shall not be subject of reconsideration and are considered as compliant with this standard.

Railway applications mean Command, Control & Signalling, Rolling Stock and Fixed Installations for Railways (e.g. Electric Power Supply).