Practically speaking, any intermittent device that _can_ run continuously or can fail to a continuous needs to be considered as such for safety purposes.

For example, a fridge should only run the compressor intermittently, but it has two obvious cases where the compressor could run indefinitely:

* An influx of heat, like filling an empty fridge with room temperature cans.

* A door being left open.

In the case of the kettle, it will likely be evaluated against it's "nominal" draw after the initial startup. If the auto-off sensor were to fail, it could run continuously at the tempurature.