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#35020 - 03/02/04 09:57 PM Breaker Trip Curves  
drillman  Offline
Joined: Mar 2003
Posts: 97
Somewhere in Texas
Is there a resource to find the above on the net or even in a catalog?

We have a large electric grill that was badly underwired. It is 3 phase 208 with 4 elements. It pulls 37 amps with #10 wire on a 30 amp breaker. The breaker does not trip. I noticed that it warms up within 10 minutes then the tstat cycles the elements and the current goes from 12 to 25 depending on how many elements are running. In fact they all turn off after 20 minutes or so.

Just to add to the story, theres two feeds to the grill both 3 phase, instead of using 2 - 3 phase breakers, they used 3 - 2 phase breakers. Parts are ordered to fix that.

The reason this whole thing came to my attention was that it did not work right, one phase had low voltage, turns out that the feed is run through a contacter to shut it down during a fire alarm. One of the contactors contacts had melted due to a bad connection.

The breaker is a GE TEB if that means anything.

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#35021 - 03/02/04 10:24 PM Re: Breaker Trip Curves  
Ron  Offline
Joined: Mar 2002
Posts: 582
White Plains, NY
Here's the curve. I don't think it will help you much. @ about 1000 Seconds of continuous full load, it will trip at its trip rating.


#35022 - 03/03/04 10:57 AM Re: Breaker Trip Curves  
C-H  Offline
Joined: Sep 2002
Posts: 1,497
Stockholm, Sweden
If I'm reading this right, it would take 1.1-1.5 times the rated current for it to trip when the ambient is 25°C? (1000 seconds or more) That is 33-45A, and 37A is within this interval.

I noticed another thing: The current needed for a magnetic trip seems to be 12-50 times the rated current. Is this normal?

[This message has been edited by C-H (edited 03-03-2004).]

#35023 - 03/03/04 11:51 AM Re: Breaker Trip Curves  
Ron  Offline
Joined: Mar 2002
Posts: 582
White Plains, NY
For a typical breaker, especially a thermal mag breaker, it is typical for the left side of the curve in the long time pickup region to reach the "trip" value at about 1000 seconds with an ambient of 40 deg C.. It is also typical for the left side of the curve in the instantaneous region to be at 10-12 times the "trip" value. The only aspect of the curve that get affected by different ambient conditions is generally the long time pickup/delay portion of the curve (approx above 10 seconds).
The thickness of the curve (left to right above .02 seconds) is manufacturers tolerances permitted. So depending on which batch you have, the current to trip the breaker at 1000 seconds, may be as much as 1.4 x the "trip" value.
Again generally, instantaneous trip is for short circuits, and long time trip characteristics is for overloads.
For solid state breakers, there is also another characteristic between long time and instantaneous, which is short time pickup/delay.


#35024 - 03/03/04 02:22 PM Re: Breaker Trip Curves  
resqcapt19  Offline
Joined: Nov 2000
Posts: 2,148
The trip curve that I looked at from another manufacturer shows that the trip time for a load equal to 125% of the breaker rating will be between 1000 seconds and never. The GE trip curve in the link does not show the maximum trip time for a 125% load. The time in that table stops at 1000 seconds.


#35025 - 03/03/04 05:38 PM Re: Breaker Trip Curves  
Ron  Offline
Joined: Mar 2002
Posts: 582
White Plains, NY
The presentation of the curve traditionally stops at 1000 seconds. It doesn't mean the the characteristic goes straight north from 1000sec. I means that you have to get more information from the manufacturer.
For example, the presentation of a medium voltage fuse curve does not show when it reaches its "trip" value, as it is way north of 1000 seconds.


#35026 - 03/03/04 06:28 PM Re: Breaker Trip Curves  
Joined: Jan 2003
Posts: 73
I thought that the KA rating had something to do with the time curve, "the point a which it will trip".

The higher the KA rating the faster it will trip?

Regarding the breakers (using 3 2-pole instead of 2 3-pole) the breakers don't see what the other phases are doing?

Re. the 37 Amps. Is it possible one of the elements is bad?
Do you know the configuration of how they are connected?

#35027 - 03/03/04 08:11 PM Re: Breaker Trip Curves  
Ron  Offline
Joined: Mar 2002
Posts: 582
White Plains, NY
The AIC rating of a circuit breaker will not be reflected in the characteristic curves of a breaker.
Any current limmit characteristic of a device is reflected in the let through curve, which is unreleated to coordination related settings.


#35028 - 03/03/04 10:52 PM Re: Breaker Trip Curves  
drillman  Offline
Joined: Mar 2003
Posts: 97
Somewhere in Texas
No wonder the breaker did not trip. 1000 seconds is 15 minutes and change. The elements start cycling off after 10 minutes.

To answer CDN, no the elements are good. I pretty much took this whole thing apart to figure it out. This was a replacement grill that was bigger than the first one and nobody bothered to upsize the feed.

As for the 3 breakers vs 2 breakers, its not an overcurrent problem as much as a disconnect problem. If one element shorts it will only disconnect 2 of the 3 phases. Or someone unaware of this is working on the grill may not know to disconnect all of them. Somewhere in the code it states that all 3 phases must be disconnected at the same time. I should add that the breakers are the disconnecting means.

I guess UL has decided that having #10 wire at 37 amps for 15 minutes will not hurt it, other wise why would they list a breaker that could do that?

More breaker stories:

Had a problem with a square D breaker tripping. It was a 20 A single pole, turns out that there was a lift station pump on the light circuit and the combined amps was 25 (12 plus 13) and it would trip if the pump ran for more than 5 minutes.

Another time we had 6 - 20 amp lighting breakers pulling 18 to 19 amps. They were grouped together. Would start tripping at 10:00. Two hours after the place opened.

Yet another time had a kitchen exhaust fan on a 3 phase 20 amp breaker trip after 55 to 60 seconds every time it was started. Problem was shorted winding to ground and it pulled 40 amps. This was GE brand.

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