<...When is the perfect indication to replace a contactor?...>

Before it looks like that↑.

not a preventive maintenance you

indications amigo

<...amigo...>

How abstruse.

No contact!

UI → University of Iowa

Ith → A ridge in Germany's central uplands.

no joke amigo, no joke

No joke at all. From the information you gave us I gave you a valid set of answers. If these answers don't satisfy you then you are holding back information from us. At the very least you should say why you are not satisfied with these answers.

There is no perfect indicator.

Ui=User index.

ith - Wiktionary

It is boring to verbally joust with the unarmed.

What context? I have no idea if "Ui" and "Ith" are hand written on the chassis wall near a contactor, embossed into the wires connecting to the contactor, silk screened identifications on terminals of the contactor, part of the data sheet for a contactor or are part of a sub-assembly that contains a contactor. I also have no idea if the object you do not understand is even a contactor because you've not provided us a model or even a manufacturer for any of us to look up and translate the manufacturer's data into something you can understand.

What 'context'?

Amigo. A bit testy, what?

There is no "perfect" indication to replace contactor. I'd go with an imperfect indication, like failure to close properly. Or chatter.

Now, run along and play.

You need to perform an analysis based on operating conditions (current, power factor), number of operations, ambient temperature, MTBF, etc and then reference it back to the manufacturer's data in an attempt to predict the probability of failure.

This is the theoretical way, and it is rather complex and subjective even if you have all of the data to analyse, but will give you an idea of when to swap out the contactor during yearly/etc scheduled maintenance before failure occurs.

For your application, we have no idea as we don't know what brand or operating conditions the contactor is being subjected to.

If it were me I would perform a basic analysis and inspect the contactor and its contacts during periodic routine maintenance to see if there is appreciable wear and if so replace the contactor. This will also give you a better starting point in future for when to actually replace the contactor relevant to your particular application.

_{Jack -Not perfect, but then nothing in this world ever is.}

Yes, now, never mind.

Dum de dum de doooo......

De do do doooo.....

Di di di deeeee.....

Do be do be do.....

La la la la laaaaa....

De doo doo doo.....

De doo doo doo.....

Tra la la laaaa....

Triddly-diddly dee.....

Dum de dum de doooo.....

Oh, and one for Stinky Pete (WTH - it's Friday):

Amigo,

Hoy en dia la frase es la siguiente: Usted puede educar mudos pero no puede reparar estupido

_{Disclaimer - Not a personal attack, merely a fact}

Ui = This

Ith - This

Now you tell us and we will go from there...........¿Si?

Perfection is unattainable, good indications are.

1-Burnt points (contacts)

2-Cracked housing (plastic frame)

3-Discoloured or open holding coil (some coils can be replaced independently)

4-Chatter (excessive noise when energized)

5-Over heating (check connections, and tighten first)

Ui, possible UL rating, with dirty or damaged imprint. Ith, haven't got a clue.

GA to JRaef. Great explanation of U for voltage in German.

I deal with Siemens instrumentation reps and they are clueless as to the German terminology. In fact, I think I'll pass this on to them.