On Fri, 25 Oct 2019 05:38:10 -0700 (PDT), DManzaluni
wrote:

Well i have a test meter but I spoke to the building super and he said that the wiring behind all the CBs is the same. Which makes sense as the one which keeps tripping controls lots of different wall sockets all around the flat.

i wonder how much improvement I would see if I just change the offending CB from 15 amps to a 20 amp CB? i have no real reason to think there is anything wrong with the CB that keeps tripping, again, it ONLY trips when I leave that 1.5Kw microwave on. I have no real reason to think it isnt just doing its job. Properly.

What with what Clare said, i do accept that this microwave needs its own CB but should a 5 amp increase in capacity suffice to prevent overload?
The wiring is sized for 15 amps. Putting in a 20 amp would allow the
wiring to be overloaded - not a good idea.

I have another suggestion though. With everything else OFF turn on
the microwave, then tuen on everything else. DOes the breaker trip?
Might be high surge current when starting the microwave (or one of the
other devices - like a high power stereo amp) - and a "high magnetic"
breaker MAY solve the problem.
I had a problem with my central vac occaisionally tripping the
breaker. It was on a "dedicated" circuit (I DID plug my Wifi Router
and Cable Modem in to it). I switched the breaker out to a tandem
(120/240) breaker which is a "high magnetic" design and it has not
tripped since. The tandem breaker was a LOT easier to find (and
cheaper)than a single "high magnetic" breaker for my QO loadcenter.

from Schnieder website:

Standard QO115 and QO120 circuit breakers are manufactured to have a
magnetic trip point at approximately 8x to 10x the breaker rating.
There are some applications, however, in which a load has an inrush
current high enough to cause these standard circuit breakers to trip.
Examples of these loads include area lighting for athletic fields,
parking lots, or outdoor signs. To allow the high inrush current
without tripping the circuit breaker, a high magnetic breaker should
be used. These high magnetic breakers breakers have a magnetic trip
point set much higher than the standard circuit breakers. They can be
identified by the HM suffix on the catalog number (QO115HM). Current
production HM breakers are also identified with a "High Magnetic"
label on the side of the breaker (see photo).
NOTE: The above explanation comparing the standard breaker to the
high magnetic breaker applies to 1-pole, 15A and 20A, QO, QOB, QOU,
HOM and CHOM breakers only. Other ampere 1p breakers, and all 2-pole
and 3-pole versions are already high magnetic as standard. (Breakers
with suffix GFI, AFI, CAFI, or DF are not available as high magnetic).