Breaker not tripping

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torint

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Went on a svc call yesterday...fixed the problem but also found this: Homeowner.. DIY'er installed a range ckt. He used 10/2 w/grd copper romex along with a 30A DBL P Square D Homeline breaker.
I turned the range on to full load and put an amp meter on it. I found: 35A-38A on each leg and the cdr was warm. The bkr did'nt trip after 10 min. I'm going to install the correct 40A ckt tommorow. (I understand the continuous load theory along with the cdr derateing theory). My question is: why did'nt the bkr trip? Was it because of +/- %tolerances?
 
Re: Breaker not tripping

Just not enough time for it to trip with such a small amount of overload.

If the load was say 32 amps it might take hours to never for a trip.

If the load was 60 amps the trip could be expected pretty quick.

Go to the Square D web site and you should be able to find the time / current info for that breaker. :)
 
Re: Breaker not tripping

Thanks Bob. so it is +/- %tolerances. I thought so but I was'nt too sure. I will check that site out. ;)
 
Re: Breaker not tripping

Originally posted by torint:
so it is +/- %tolerances.
Click to expand...
Well I am not sure I would call it that.

To me tolerances suggests an acceptable deviation from the design specifications.

I believe the correct term would be the Time-Current Characteristic Curve.

For a thermal magnetic breaker or fuses for that matter a small amount of overload is going to take a long time to either melt out the fuse or heat the thermal part of the breaker to the point of tripping.

This time is not random, it is planed and known by the manufacturer.

Within the Time-Current Characteristic Curves there are tolerances, each breaker is going to respond a little differently but in general will follow a predictable trip.

Now take an electronic breaker, this could be programed to literally trip immediately above its rating but that would lead to false tripping.

The homeline 2 pole 30 you had, if used for say an air conditioner unit would trip every time the unit started if it did not have the ability to stay closed with more current than the rating running threw it.
 
Re: Breaker not tripping

The trip curve for a QO 30 amp breaker shows that the trip time for 125% of rated current will be between 70 seconds and never. This is based on a 40?C ambient. The trip time may be longer at a lower temperature. I didn't find the time trip curve for the Homeline breaker. You can look at the QO curves here.
Don
 
Re: Breaker not tripping

Here is a Time-Current Characteristic Curve for a KTK fast acting fuse.

You can see that this fuse would carry about 150 amps for 1 second or 100 amps for 10 seconds.

KTK_R.1.jpg
 
Re: Breaker not tripping

Bob,
It looks that the 30 amp fuse will open in 200 seconds on a 30 amps load in that time/current chart.
Don
 
Re: Breaker not tripping

This is my concern: The cdr was pretty warm (not hot). I don't know the specs on the insulation melting so I am concerned that at a continuos load above the rated 30A spec for the 10/2 cdr, the cdr may cause a fire on the wood joist. This is a range ckt and may operate at a extended period of time. :eek:

[ November 22, 2003, 07:26 PM: Message edited by: torint ]
 
Re: Breaker not tripping

A #10 carrying a steady 25A would get noticibly warm.

Romex insulations will be OK for 140F/60C temps.
 
Re: Breaker not tripping

First off you should not have a situation where the connected load is larger than the circuit ampacity. :)
 
Re: Breaker not tripping

Don (and everyone else);

The image of the graph is rather difficult to interpret - most likely due to to .JPG interlacing / compression,
or scanning artifact removal not being done - but that's another story :eek:

Scott35
 
Re: Breaker not tripping

Scott,
You're correct, the 30 amp fuse curve crosses the 40 amp line at 200 seconds, not the 30 amp line like I thought it did at first look. I just didn't count the lines correctly when I made my first post.
Don
 
Re: Breaker not tripping

Scott, my friend from the "Left Coast". Thanks for your input and humor. My question was regarding the bkr not tripping when I thought that it should. The way we install Range ckts:
50A receptical, #8 awg cdr, 40A DBL P BKR.
So, yes I am going to correct the entire ckt. :cool:

[ November 23, 2003, 01:24 PM: Message edited by: torint ]
 
Re: Breaker not tripping

I think it could be a defective breaker.Maybe it came from the factory defective or has sustained damage due to this obvious improper installation.For example internal overheating.Also what is the history overall?How many times in it's lifetime has it been reset after tripping?I have seen electricity do some strange things.No wonder the manufacturers often issue a disclaimer recommending having a qualified electrician install their equipment.
 
Re: Breaker not tripping

Ranger12 Did you read Dons post at all?

The manufacturer of this breaker states flat out that the breaker loaded to 125% will trip in 70 seconds to NEVER.

This will be the case for all breakers, each will be a little different but they all have a time current curve.

The breaker in question here was a 30, so 30 + 125% = 37.5 amps.

You could have 37.5 amps passing through the breaker for 70 seconds to forever and that is with an ambient temp of 40C if it was colder it could pass even more current.

But before you say it will burn down the house look at 310.16 90C column for THHN which is what new NM is.

You will find 10 awg NM is rated for 40 amps. :)

Bob
 
Re: Breaker not tripping

Ranger
I think your comment about electricity sometimes doing some strange things is a very valid statement, and sometimes not brought up enough. The more time one spends in this industry, the more chance one will have to see the 'unexplainable' thing.
Scott
Thanks for the extra little time and explaination.

Pierre
 
Re: Breaker not tripping

torit:

The firm I work for test CB's the standard is to test at 300% (30 amp tested at 90 amps) of the circuit breakers rating typical molded case CB trips at 40 - 80 seconds.
We had a test spec that required 150% ( 30 amp CB at 45 amps) overcurrent testing, the CB ran and ran and ran.

Oh and the remember the curve supplied is for a KTK fuse.

Carefully making this statement. The test techniciaan I apprenticed with once told me. "Remember theses are molded case circuit breakers. Not fuses or relays it go no go then chuck 'em in the trash."
 
Re: Breaker not tripping

John I was hoping you would jump in here, I only used the KTK chart as it was the only one I could find that I could link to the forum.

I do not understand this statement.

Originally posted by brian john:
"Remember theses are molded case circuit breakers. Not fuses or relays it go no go then chuck 'em in the trash."
Click to expand...
What does that mean?

Bob

[ December 02, 2003, 03:41 AM: Message edited by: iwire ]
 
Re: Breaker not tripping

Brian,
It appears that your firm follows the testing procedure per NEMA AB4, Guidlines for inspection and preventive Mainenance of Molded Case Circuit Breakers Used in Commercial and Industrial Applications, with has two voltage testing ratings, OCPDs equal to or less than 250v and 251-600v. The test are to be conducted, in part, in an approximately 25degC ambient room temperature, with not less than 4' of rated cable
At 300% of the rated continuous current rating of the 250v breaker OCPDs from 0-30a must trip in less than 50 sec for 250, and less than 70 sec for OCPDs above 250v, 31-50a would be 80sec. and 100 sec, 51-100a is 140sec and 160 sec, 226-400a is 300 sec. and 350 sec. 80a-1000a is OCPDs above 250v in less than 600 sec. (that's less than 10 inutes would you believe.)
To do an actual UL calibration check requires a much more complex and elaborate setup which is not commonly available in the field.
Dave
 
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