Why is this?

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Oakey

Oakey

Senior Member
Location
New Jersey
In another post I inquired about wiring a detached gaeage on a single circuit. I have a question about an after effect more in the lines of breakers so I'm writing in a new post,hope it's ok.
After I was done with the install today I turned the power on. I ran an outlet outside controlled from a Gfi inside the garage. I reset the Gfi for the first time and "poof" one of the bubble covers screws hit the hot lead on the outside device (my dumb) a quick fix and all good.
When I went to reset the gfi again I noticed there was no power in the garage but the single 20 in the small sub in the garage did not trip. The single breaker in the main panel in the house did instead. Why is this?
 
Oakey said:
When I went to reset the gfi again I noticed there was no power in the garage but the single 20 in the small sub in the garage did not trip. The single breaker in the main panel in the house did instead. Why is this?
Click to expand...
Because even 'identical' breakers trip slightly differently due to mere manufacturing tolerances; the same experiment might even trip the other one next time.
 
Breakers tripping downstream

Breakers tripping downstream

Yes sometimes a small short can cause the main or a smaller panel main to trip upstream of the short This is one of the many reasons why it is not a good practice to flash out breakers for identification. If the smaller breaker doesnt react fast enough it could have enough current to take out a main or trip the gfi protection at the main . Many times the gfis are not set properly
 
I've noticed that too...

I've noticed that too...

Some years ago, while working with Uncle Sam's Misguided Children, I was in Kuwait, and all I had for tools was my trusty Swiss Army MacGyver...

Took out a switch, and while removing a wire, I accidently touched the metal box... so, in addition to getting shocked, having the MacGyver fly out of my hand, all the blades fling out at ME, I tripped a breaker...

Problem is, I didn't trip the 20A breaker... I didn't trip the sub-panel main breaker, I didn't even trip the main breaker... I tripped a breaker in some electrical warehouse across the street... you know, the old kinds that have to be wound up and then turned on... I think it was a 2000A Main...

All for 1 switch!

Go figure...

That's when I learned about trip settings!
 
The breaker?s manufacturer can give you a graph that shows the time it takes the breaker to trip, as a function of the amount of current you push through it. With higher fault currents, the breaker will trip faster. But the graph would also show you that the trip time is not just one number, but rather a range of numbers. For example, if the fault current is 500 amps, the breaker might trip in anywhere from 0.001 seconds to one half second (I made those numbers up ? don?t have a copy of a graph with me).

When you put two breakers in series with each other, each breaker will have a range of tripping times. If there is any overlap, it becomes a random guess as to which one will trip first. This really has nothing to do with one being (for example) a 20 amp branch circuit breaker, and the upstream one being a 100 amp main breaker. If one has a trip time of 1-3 seconds, and the other has a trip time of 2-4 seconds, you cannot be certain which will trip first.

Somewhere in the rules for hospitals, I believe, there is a requirement that the breakers be coordinated. That means someone (i.e., an engineer or perhaps the switchboard vendor) must perform a study to verify that the breaker nearest to the fault point will trip before any breaker further upstream. This is called (among other names) ?selective coordination.?
 
greg;

If this was system was installed to US codes, and was 480/277, the most likely reason the 2000 amp main triped is GFP. If the GFP is not properly set, it is not unusual for a 20 amp CB to trip this device. Minimum settings are typically 100 amps through a maximum of 1200 amps with delay from Instantaneous-1.0 seconds.
 
brian john said:
greg;

If this was system was installed to US codes, and was 480/277, the most likely reason the 2000 amp main triped is GFP. If the GFP is not properly set, it is not unusual for a 20 amp CB to trip this device. Minimum settings are typically 100 amps through a maximum of 1200 amps with delay from Instantaneous-1.0 seconds.
Click to expand...

i've had this happen as well....
the customer isn't usually happy with the explanation, however...
i got a trouble call for a 20a 3ph 480v water heater that faulted,
and it went upstream 4 breakers, and took out a 3000a. main,
leaving everything behind it untripped... shut down a whole food
packing plant, and it wouldn't reset, and nothing else downstream
would trip to indicate where the problem was..... it was a busy day.
had to open everything, and find the subfeed that was faulting,
and do it again to find the next breaker in the downstream path..... etc.

fulthrotl
 
Fulthrotl said:
... shut down a whole food
packing plant, and it wouldn't reset, and nothing else downstream
would trip to indicate where the problem was.....
Click to expand...
Now that's just plain scary, right there.

You mean you tried to reset it, perhaps multiple times, without doing proper testing? You must have nine lives.
 
From experience

From experience

Square D is famous for sending all of its large switchgear out at the lowest settings when I have talked to the factory about this they say that it is sent out at the lowest setting for safety. Prersonally I think it is to send a tech out to set up the gear at probably 2 grand a day plus expences. I cant tell you how many times I have been able to resolve this problem on the job. I also make sure that I get in writing the owners desired settings for the breaker because I dont need to take liability for the settings causing a problem for me. I do however feel that Square D is one of the best switchgear on the market. I guess you get what you pay for.
 
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