On Monday, July 10, 2017 at 12:14:36 PM UTC-4, Lenny Jacobs wrote:
On 10/07/2017 10:46, wrote:
On Mon, 10 Jul 2017 07:58:16 +0630, Lenny Jacobs
wrote:

When a breaker is tripped, it's always the main breaker, not the
individual breaker. Shouldn't the individual breaker be tripped first?

This is troublesome because I then have to turn off all individual
breakers with the main breaker on and flip each breaker to see which one
causes the trouble.
If you have a full panel - say even 21 breakers - all 15 amp, all
loaded to 80%, you have a 252 amp draw - no branch breakers are
overloaded, yet you are 25% overloaded on a 200 amp main breker.


Theoretically possible but that's not the case here. Every year, when
the raining season comes and the main breaker is tripped, I can always
trace to the same sub-breakers that cause the problem and turn them
down. They are not tripped all at the same time but they are always the
same four breakers that have the problem.


How do you identify which one caused it? Does it immediately trip the
main again when you put the offending breaker on, ie the fault is
still there? If so, if it's not intermittent, I would think you'd
immediately further track down what the source of the fault is.



I studied the breaker box and found that it's not like what I usually
saw in the US. It has a 75 A main breaker labelled Ground-First FL Main
(this is a two-story building). There are two 63 A breakers labelled
Ground FL Main and another two 63 A breakers labelled First FL main.
Looks like this is a three-tier system. It is the same 63A breaker that
is tripped during raining season.

Do you really mean tripping or that it's something on that 63A breaker that
causes the main to trip? You said the main was tripping, not the
other breaker. Also this picture is different than what I think most
of us were envisioning, which was a typical main panel with individual
branch circuits, not a main that feeds what appears to be two subpanels,
That shouldn't really matter though.

What do you believe the total load normal load on it is, just before
it trips? What's going on at the subpanels when this happens,
those 63A breakers must feed subpanels of some kind.
Anything tripped there? As to why the 75A trips first, it makes
more sense now. Breakers have curves as to how fast they trip
versus the size of the fault. Different breakers will have different
curves and they are not exact. What you have is a 63A breaker and
a 75A both exposed to an overload. Additionally, you don't tell us
what else is on that main panel besides the two 63A breakers, but
if there are other circuits, then the 75A is seeing whatever the 63A
one is, plus those other loads. With other loads, no surprise it
trips first. Even without other loads, if you present a major over
load to both, the 75A might trip first if it's curve is slightly
different.

I think most of us were envisioning that you had a big main breaker,
eg 150A and then smaller breakers, eg 20A that feed those other
circuits.





Usually, there are only no more than 10 lights on, two full-height and a
half-height refrigerators constantly on (doesn't mean the compressors
are constantly running), and a ceiling fan, a water heater. That's it.
Seems to me there is absolutely no reason a breaker would trip with a
load like this. That's why I suspect the culprit is moisture or
underground cable getting short circuited due to rain. Now, two breakers
are turned down. During dry season, all breakers are up.

By the way, this is 220 V.

Where are you? We were assuming this is US.