IEC 61508: Understanding Demand Modes
01 Apr 2026
How End-Product Applications and Standard Requirements Determine Functional Safety
Clients that are new to functional safety may find themselves asking, “what demand mode is my safety function?” The answer depends on both the application and the requirements of the end-product standard.
Demand Modes in IEC 61508
What is the demand mode of a safety system? IEC 61508 defines three modes:
- Low demand mode: where the safety function is only performed on demand, and where the frequency of demands is no greater than one per year.
- High demand mode: where the safety function is only performed on demand, and where the frequency of demands is greater than one per year.
- Continuous demand mode: where the safety function retains the [equipment] in a safe state as part of normal operation.
(Ref: IEC 61508-4:2010 clause 3.5.16)
The demand mode is dictated by the number of times the SIF (Safety Instrumented Function) is called upon. This does not mean that a SIF is inactive or de-energized during the non-demand time—a sensor may very well be powered 24/7, 365 days a year, waiting to detect a hazard. But the hazard itself is not expected to occur within these windows of time.
A common example is a smoke alarm, as found in most homes. The sensor is constantly sensing for smoke and will initiate the warning if it’s detected, but it is not expected to be demanded more than once a year (or ever) in a residential setting.
In some cases, both a low and high demand mode application may be relevant. It will very much depend on the end-product integrator and their application requirements. Certifying to one mode may limit potential applications.
In other industries, one mode is all that is necessary. In industries like the process sector, or oil and gas, low demand rate is practical and expected. These systems may have many other layers of protection which can reduce the burden on any one SIF, and therefore there is no reason to consider high demand mode.
What This Means in Practice
The demand mode ultimately dictates the requirements used to evaluate the safety device or function. The quantitative evaluation approach differs between demand modes in IEC 61508. For example, low demand functions are typically assessed using PFDavg, while high and continuous functions use PFH, and the architectural constraints applied by IEC 61508 may differ accordingly.
The metrics for failure rates differ for low versus high/continuous:
- low demand PFDavg is used: Average Probability of Dangerous Failure on Demand
- high and continuous PFH is used: Average Frequency of Dangerous Failure per Hour
These metrics make intuitive sense, because a low demand SIF may sit idle for years before being called upon. Whereas a high/continuous SIF may be called up frequently, or in some cases continuously. The actual parameters defined in IEC 61508 differ for the low / high-continuous modes, as can be seen in Tables 2 and 3 of IEC 61508-1:2010 for target failure measures for a safety function:
| SIL | Low Demand (PFDavg) |
High/Continuous Demand (PFH) [h-1] |
| 1 | ≥ 10-5 to < 10-4 | ≥ 10-9 to < 10-8 |
| 2 | ≥ 10-4 to < 10-3 | ≥ 10-8 to < 10-7 |
| 3 | ≥ 10-3 to < 10-2 | ≥ 10-7 to < 10-6 |
| 4 | ≥ 10-2 to < 10-1 | ≥ 10-6 to < 10-5 |
Low demand mode also introduces different interval and diagnostic testing requirements based on the low expected operation: proof test intervals and diagnostic test intervals may be relevant to ensure the functionality of the system does not fail in the long dwell periods between action.
Looking back on our smoke detector example, most detectors have a “push to test” button which should be manually initiated on a monthly basis to ensure the alarm annunciates properly.
Application Requirements
The choice of low, high, or continuous demand is not always left to the system developer. Often there are requirements in particular functional safety or end-product standards (such as ISO 13849, Safety of Machinery) which define that the demand mode must be high or continuous, to meet expectations of that industry and equipment. In this case there is no exception to use a lower demand mode than the application standard permits.
How Intertek Assurance Can Help
Intertek Assurance supports organisations at this early decision-making stage through structured standards research and applicability analysis, helping to remove uncertainty before significant design or certification effort begins.
Our support typically includes:
- Design and gap analysis
- Standards identification and scoping
Intertek Assurance helps clients move forward with confidence - knowing that their functional safety strategy is technically sound, proportionate to risk, and aligned with both current and future market expectations.
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