What is a key characteristic of heat-sensitive thermal switches?

A key characteristic of heat-sensitive thermal switches is that they are typically open in their normal condition. This means that under standard conditions, the circuit is not completed, and electrical flow is prevented. When the temperature reaches a certain threshold, the thermal switch responds by closing the circuit, allowing current to flow through. This behavior is essential in many applications for safety and control purposes, such as preventing overheating by activating cooling systems or alarms.

The other options describe characteristics that do not define heat-sensitive thermal switches. For example, the notion of requiring manual activation contradicts their automatic nature; these devices are designed to operate without human intervention in response to temperature changes. Similarly, describing them as normally closed misrepresents their default state, and claiming they are not sensitive to temperature changes directly opposes their fundamental operation.