The risky side of default alarm settings

17 Feb 2016

Logically, people assume the lower the gas detection alarm level, the safer the working environment, as the body will be exposed to less poisonous gas. However, this is not always the best option! If set too low, they can cause spurious alarms and unnecessary disruption. Worse still, these wolf cries have led to many incidents of detectors being ignored or switched off; with terrible results1.

Don’t default on setting alarm levels

Commonly, personal gas detectors have two instantaneous alarm level settings, one low (alarm 1) the other higher (alarm 2). Many sites default to the alarm levels set by the manufacturer, typically at statutory short term and long term exposure limits. But these limits are not designed to act as instantaneous alarms. They are time weighted averages measured to protect workers over a specified time period from overexposure to toxic gas (for more information, see our previous blog).  An unintended consequence of using these factory settings can be alarms going off more often than necessary, leading to alarms being switched off or ignored. This has led to accidents in the past.

Alarmingly unsafe

Time weighted averages are appropriate when it is predictable that toxic gas will be present. But instantaneous alarms set by default at the short and long term limits will go off more often than safety demands. In areas with high concentrations of target gas it may be safer to have higher alarm set points so they do not alarm constantly. Why safer? Because false alarms result in two separate problems; they are inefficient, and they lead to alarms being ignored. It has been stated that, “too many false alarms can and have led to operators turning off the alarms, sometimes with catastrophic results1”.

Not for everyone

It is not recommended to completely rely on TWA alarm levels by any means. Confined space working is a good example. By their nature, confined spaces are difficult to exit, and so escape from the hazard, quickly. It would therefore be advisable to set alarm levels lower; or, if frequent alarms are an issue in a confined space, then breathing apparatus may be what is required. False alarms can also be caused by other factors relating to the instrumentation, and this must be considered in any risk assessment.


The UK Health and safety research report ‘Review of alarm settings for toxic gas and oxygen detectors’, digs into the issues surrounding this subject in much more detail, and stresses the importance of alarm levels tailored to the circumstances. It also discusses the importance of the right balance “between effective protection to toxic gas and maintaining productivity”.

It is clear that relying on the manufacturers pre-set alarm levels may be neither safe nor operationally efficient. A review of the specific operational conditions to evaluate appropriate alarm setting is advised. As well as removing the frustrations associated with frequent spurious alarms, a more robust risk assessment and mitigation are likely to be achieved.


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