Some electric motors need a surge of electricity when they start up. These surges can be up to three times the current the motor uses when running at normal speed. To protect such motor circuits, an overload relay works better than a normal circuit breaker because it withstands these surges without tripping. An overload relay only trips when some other problem arises.
Short in the Wiring
The purpose of any breaker is to protect the wiring in a circuit. A short circuit will trip any breaker, including an overload breaker. Short circuits can be caused by wires overheating and the insulation melting, or by accidents where the insulation is stripped away or the wire is cut by a metal object and shorted to ground. A short circuit will draw current above the overload and surge ratings and cause the relay to trip.
Several types of motor failure can cause the motor to draw excessive current and trip the overload breaker. Bearing failure can cause the motor to freeze up and burn out. Shorts in the armature windings can draw excessive current. Gearbox or drive failure can cause the motor to freeze and burn out. The main purpose of the overload breaker is to protect the electrical circuits in the event of a motor failure.
All motors have a rated load or amount of work they can do. If the work that a motor has to perform exceeds the amount of work it was designed to do, it will draw excessive current trying to do the work. Overload relays can be set up to withstand temporary work overloads, just as they will withstand surge currents at startup. But if the overload condition continues, the breaker will trip.
Heat can cause motors to overwork. If a motor in a hot climate or in an enclosed building is not properly ventilated or cooled, it may heat up over time. When the motor heats up, even though it is not overworked or damaged, the potential for damage exists and increases as time goes by. The overload relay will sense the increased current from the heat and will trip to protect the motor.
Overload Relay Failure
An adjustable overload relay could be improperly set up, causing it to trip with a normal surge or temporary overload. Also the overload relay itself could fail. The sensing device, of a bimetal strip made of two metals bonded together, expands in overload conditions to eventually trip a mechanical device. The device itself could malfunction, causing an unnecessary trip, or damage from shock, water or dirt could trip it as well.