A double check backflow device should not be used to protect against which of the following?

A double check backflow device is specifically designed to prevent the backflow of non-health hazards into the potable water supply. These devices are particularly effective against contaminants that could potentially be harmful but do not pose immediate health risks. This is why they are not suitable for applications requiring protection against health hazards, which would necessitate a higher level of protection, such as that provided by a reduced pressure zone device.

In this context, the other options presented include various types of dangers. Non-health hazards can be mitigated by a double check device, as it addresses situations where improper substances might enter the water supply without immediate health implications. Clean water is inherently not a hazard, thus falls outside the scope of devices designed for protection against backflow. Physical hazards, which could pose serious health risks, also require more robust protective measures than a double check backflow device can provide.

In summary, while a double check backflow device effectively addresses non-health hazards, its limitations in providing adequate protection against contaminants that pose serious health risks inform us that it should not be relied upon for those specific situations.