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Yes. Two redundant instruments can fail at the same time if they share a cause—such as a power supply, design vulnerability, environment, or maintenance error. Two devices are not necessarily two independent safeguards: redundancy describes the architecture, while independence must be justified for the specific safety function.
What does “one instrument printed twice” mean?
It is an engineering metaphor, not a formal standard definition. It describes a system that appears to have duplicate instruments but can lose both through the same failure. A common-mode failure is a shared failure mode that defeats components intended to compensate for one another.
For example, two sensors may be physically separate yet depend on one power supply. If that supply fails, both readings may be lost. A shared design flaw, environmental event, controller, or procedure can create the same problem. In its US rail-design context, 49 CFR Appendix F to Part 229 cautions that when a common-mode failure exists, analysis cannot rely on the assumption that failures are independent. The guidance applies to parallel redundant elements and monitor/check arrangements.
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Not automatically. A second sensor can provide useful redundancy, but its contribution depends on whether a failure affecting one channel can also affect the other, how faults are detected, and what the system does when readings disagree. No general reliability-improvement figure follows simply from adding another instrument.
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IEC 61508:2010 says sufficient independence in both the design and application of elements should be justified through common-cause failure analysis, considering the safety integrity level of the function. The available text is hosted by StudyLib, rather than an official IEC source; it supports what that 2010 text says, but does not establish the current edition or applicability in a particular jurisdiction. See the IEC 61508 text.
How do you assess whether instruments are independent?
Start with the defined safety function and identify credible ways both channels could be affected. The following questions are useful prompts for an application-specific analysis, not a universal checklist or proof of independence.
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- Shared services: Do both instruments rely on the same power supply, controller, or support system? IEC 61508 identifies common parts and support services as potential shared causes.
- Technology and design: Could a common design flaw, measurement-principle limitation, or component behavior affect both? Functional diversity or different technologies may help address a relevant shared cause, but require engineering judgment.
- Physical routing and environment: Could one event, installation defect, or environmental condition damage both instruments or their signal paths? A product-specific Rosemount 3051T failure-analysis example considers whether channel signal cables are routed separately; that example does not establish independence for other products or installations. See the Emerson/exida FMEDA report.
- Procedures and people: Could one operating, calibration, maintenance, or test procedure introduce the same error into both channels? IEC 61508 identifies common procedures as factors to address.
- Diagnostics and response: What faults can the system detect, what happens when measurements disagree, and which failure modes remain undetected? European Commission Joint Research Centre inspection guidance discusses comparing measurements for diagnostics. Detection alone does not prove that the channels are independent.
- System context and evidence: What are the consequences of losing or misreading the safety function, and what evidence supports the claimed fault tolerance and reliability? JRC guidance treats hardware fault tolerance as an architectural consideration and asks whether common-cause effects are included in calculations. Its page does not establish a universal reliability figure.
Do voting arrangements such as 1oo2 or 2oo3 solve common-cause failure?
No voting arrangement removes a shared cause by itself. Voting logic can affect how the system responds to individual channel faults, but if a common event or vulnerability affects multiple channels, the intended benefit can be reduced or lost. A Rosemount 3051T manufacturer-hosted FMEDA includes a product-specific 2oo3 example; it is not evidence that every 2oo3 installation is independent or safe.
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What can reduce shared failure risks?
IEC 61508:2010 describes approaches such as functional diversity, diverse technologies, avoiding common parts or services whose failure could endanger all systems, and avoiding common operational, maintenance, or test procedures. These are options to evaluate against the actual failure mechanisms—not guarantees or requirements that apply identically to every design.
Physical separation may also matter when a shared event could affect instruments or signal cables. The relevant separation depends on the installation and its hazards; the product-specific cable-routing consideration in the FMEDA example should not be treated as a universal design prescription.
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What the evidence does—and does not—establish
The cited IEC text is from 2010 and is hosted by a third party, so it does not establish the current edition or jurisdictional applicability. The JRC material is government inspection guidance, but a publication date is not established in the page material cited here. The FMEDA is manufacturer-hosted and product-specific. The 49 CFR appendix is US rail design guidance, not a universal instrumentation code. None of these sources establishes a single reliability improvement for adding a second instrument.
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