The correct answer is: A. short-circuits.
Thermal overload relays are used to protect motors against overcurrent due to short-circuits. A short-circuit is a sudden, unintended connection between two points in an electrical circuit that has a low resistance. This can cause a large amount of current to flow, which can damage the motor.
Thermal overload relays work by detecting the temperature of the motor windings. If the temperature of the windings gets too high, the relay will open the circuit and stop the motor from running. This prevents the motor from being damaged.
Thermal overload relays are typically used in conjunction with fuses or circuit breakers. The fuse or circuit breaker will protect the motor from short-circuits and overloads, while the thermal overload relay will protect the motor from prolonged overloads.
B. heavy loads is not correct because thermal overload relays are not designed to protect motors against heavy loads. Heavy loads can cause the motor to overheat, but the thermal overload relay will not open the circuit unless the temperature of the windings gets too high.
C. grounds is not correct because thermal overload relays are not designed to protect motors against grounds. Grounds can cause the motor to overheat, but the thermal overload relay will not open the circuit unless the temperature of the windings gets too high.