What device opens its contacts when a motor heater coil uses too much current?

The device that opens its contacts when a motor heater coil uses too much current is typically an overload protector. An overload protector monitors the current flowing to a motor and is designed to interrupt the power supply when it detects that the current exceeds a predetermined limit. This is crucial for protecting the motor from damage due to overheating, which can occur if the motor operates under excessive load or if there is a fault in the system.

While thermal relays can respond to excess current by opening their contacts as well, they are often part of a broader overload protection scheme rather than acting independently. Thermal relays specifically utilize heat generated by high current to trigger their operation, and while they are effective, overload protectors are more directly associated with opening contacts in high current situations.

Magnetic starters, on the other hand, are primarily used to control the starting and stopping of motors and may include overload protection but do not directly perform the function of sensing and interrupting excessive current solely on their own.

Lastly, relay switches function primarily as electrically operated switches and do not inherently include protective functions related to current overloads unless combined with other protective components. Thus, in the context of sensing and reacting to excessive current levels specifically, the overload protector is the most accurate choice.