Thermal overloads are an important part of motor protection. When too much current passes through the overload it changes the state of the relay, which in turn prevents the motor from running. Pretty simple to use, yet there's three things that most people get wrong.
Overloads have an adjustable range, for example 4-6A in the picture above. This should be set to the full load current of the motor, so if the motor has a full load current of 5.5A then you turn the dial to 5.5A. Mistake number 1 - many engineers set the overload above the full load current, for example +10%. In fact the overload already has a tolerance built in, so for example Schneider overloads allow for the motor to run up to 7% over its full load current before it trips. So if you add 10% as well, that means 17% over what it should do...enough to do some real damage to the motor.
When the overload trips it cuts power to the pump, right? Actually, that's not how it works. When an overload trips it just changes the state of the relay contacts, which is normally a pair of normally open and a pair of normally closed. The feed to the contactor is run through the normally closed pair, so that when it trips it also cuts the supply to the contactor. The overload itself doesn't prevent power going through it at any time.
Larger motors are often started with star/delta starting, to prevent too much current draw on start up. This requires a main contactor which works in tandem with first the star contactor, then the delta contactor. The main contactor is the only one with a thermal overload. This means that the overload doesn't 'see' the full current going through it, so it should be set to 0.58x the full load current. I.E. for a full load current of 9.5A, the overload should be set to 9.5 x 0.58 = 5.5A.
Remember these three rules and you'll be able to make sure overloads are working properly and protecting motors on all of your installations.