Yesterday my wife told me the AC in the bedroom of our brand new condo was not working,

I go to the breaker box and nothing is tripped.

I switch everything on and off anyway.

No change.

Have maintenance guys come up and I hear "Oooh".

I take a look and this is what is pointed out to me.

I could not see this because the cover was on and I don't mess with such things.

They swapped breakers as they didn't have a replacement nor were the electrical people around to properly fix the thing. So now my office (2nd BR) has no AC but that is fine till tomorrow morning when they come back.

Any idea what might have caused this? The maintenance people guessed that someone didn't tighten something well enough.

Me, I have no clue. Guessing I'd say that the breaker was of insufficient amperage rating for the load.

Zoomed in view of the fried breaker in case that is helpful.

Pound to a penny that was a loose connection that's been fizzing away for a while although a faulty breaker is also a possibility. Did you not notice a 'burning electrics' smell?

20A is ample for an average aircon, even if it was overloaded all that should happen is that it trips, that's what it's there for.

I'll assume the penny trick was in jest. Otherwise bring over some satangs and I'll stand back and watch your demo on how to install them like a pro.

We've also had some odd smells here yes, which include sewer gas, so identifying all these smells has been tricky.

The smell in the cabinet the box is housed in has a smell that I DID smell earlier, so there was a warning sign but it did not smell like something burning or electrical as we might expect therefore went unchecked.

The fried breaker is 20amp? I see a "C20" printed on the breaker but that means nothing to me. Googling brought up I dunno what but I think this is the closest I got here: http://www.schneider-electric.com/products/th/en/4200-power-circuit-breakers-switches/

FYI we've only been living here a week or so.

Yup, the designation "C20" is a 20A C-curve breaker.

The 'curve' defines the current required to cause an 'instantaneous' trip, and also determines how long the breaker takes to open in the event of small but longer term overloads. C is usually used for motor loads with high inrush currents (like aircon) but seems to be used just about everywhere here.

http://en.wikipedia.org/wiki/Circuit_breaker