is there some sort of light on the contactor? or are there external lights wired to the contactor, which become lit when power to the freezers is lost?
luke, contactors are absolutely vital for industrial circuits.
lets say you have a circuit carrying, say, 1000A at 1000V, by using a contactor you could control that circuit using 12V dc at 100mA. From miles away if you wish.
imagine a 3 phase motor. If you strapped it accross the supply, it would just run continously. By using a contactor (or a configuration of them) you can start it and stop it at will, and again, remotely.
Regarding the light, a contactor will frequently have 'normally open' (N/O) contacts and 'normally closed' (N/C) contacts. 'Normally' relates to its de-energised state.
So take the circuit mentioned above, by using a set of N/C contacts, when the contactor de-energises (loss of power to the freezer) the N/C contacts will close and complete a circuit to the warning light. The N/C contacts could even be in the 'pull in' circuit for a seperate contactor which will feed the lights.
and so on, and so on
You can use the same principle to bring in a stand-by generator, for, say, a hospital, when the mains fails.
And imagine connecting a circuit to a generator that was off. the genny would be at zero hertz and volts. By the time it got to 50Hz and 230/400V the circuits connected to it would be fried. So we put in a contactor which wont connect the output until its at correct V and Hz. And the same contactor will 'drop out' if it drops to say, 45Hz (when the diesel runs out and its stopping), for example
PLC's are just banks of 'electronic contactors', and without them, most industrial machinery would be buggered.
Many many uses.
hope this helps