7.5.2.4 of IEC 61508-1 gives the requirements that apply for the control system not to be designated as a safety-related system. In summary, these are:

• allowing for a dangerous failure rate of the control system higher than the maximum defined by the standard for a safety-related system (ie higher than 10-5 dangerous failures per hour);
• providing an adequate demonstration that the dangerous failure rate allowed for is achieved (7.5.2.4 of IEC 61508-1 contains further details);
• determining all reasonably foreseeable dangerous failure modes of the control system;
• ensuring that the control system is separate and independent from all safety-related systems.

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