What Is the Latent Fault Metric?

One part of hardware diagnostic coverage is the latent fault metric. Defined in ISO 26262, this metric helps engineers protect drivers against random failures over the lifetime of the hardware. Here’s a closer look at the latent fault metric, what it means, and why you need it.

The Basics of ISO 26262

The latent fault metric is derived from Part 5 of ISO 26262 (Road Vehicles – Functional Safety), which covers hardware-level engineering developments and fault metrics. This standard is the automotive industry standard and is derivative of the more generalized IEC 61508 standard.

ISO 26262 hinges on the definition of functional safety, which is the “absence of unreasonable risk due to hazards caused by malfunctioning behavior of electrical/electronic systems.” Malfunctions are classified by two types of failures:

1. Systematic failures
2. Random failures

Systematic failures are failures induced in a deterministic way, typically due to process causes in development, manufacturing, or maintenance. Random failures appear during the lifetime of the hardware due to random malfunctions.

From this, the standard derives safety mechanisms and technical solutions to address hazards and failures. The effectiveness of a solution to detect random failures is determined by three metrics to detect fault and failure in time, or FIT. These metrics are:

1. Single-point fault metric (SPFM)
2. Latent fault metric (LFM)
3. Probabilistic metrics for hardware failures (PMHF)

This brings us to the question of the latent fault metric, one of the key metrics in understanding the robustness of hardware design.

What Is a Latent Fault Metric?

Latent faults are multiple-point faults not detected by a safety mechanism or perceived by the driver. Typically, latent faults can only be detected during an accident or a detailed proof test.

For this reason, the latent fault metric is a hardware metric determining whether coverage by safety mechanisms is sufficient to protect against risk from latent faults in the hardware architecture.

It can also be understood as the robustness of a safety mechanism’s function against latent faults by design, fault coverage by safety procedures, or the driver’s recognition of fault.

You have a higher latent fault metric if you have many safety faults, many single-point faults, many residual faults, or many detected multi-point faults.

Putting Your Metrics Into Action

The latent fault metric is just one of the many critical safety metrics at your disposal. The question is not how many metrics you have, but you effectively you can use the metrics at your disposal. Otherwise, your metrics are little more than numbers.

Make sure to check out our blog for more tips to put your safety metrics to good use, like this post on why it’s time to move your safety metrics dashboard beyond Excel. And if you need a smarter tool to start utilizing your safety metrics, our safety software is here to turn your raw data into real results.