circuit breaker as switch

[M. D.]

Here is a link to an older Westinghouse document that is still relevent today when it comes to breaker life.
I believe the UL test that are required that are shown on page 3 should impress most people.
Play close attention to the endurance tests at FL and NL.

http://www.eaton.com/ecm/idcplg?IdcService=GET_FILE&allowInterrupt=1&RevisionSelectionMethod=LatestReleased&Rendition=Primary&&dDocName=1016115402294

Would someone be so kind as to cut and paste the " good stuff " from page three ,.. this link would not open for me

 

[iwire]

I opened the link but it will not copy and paste, it's an image that would need a host.

But it appears to say that ULs position is that breakers used as motor controllers are OK.

Breakers are not HP rated as the tests to be a 'breaker' are far more then the test to be HP rated.

 

[M. D.]

Thanks Bob , and good morning

 

[iwire]

Good morning.

:smile:

 

[Dennis Alwon]

Would someone be so kind as to cut and paste the " good stuff " from page three ,.. this link would not open for me

I hope this is what you wanted-- I had to post in 2 post because of the length

Circuit breakers are not horsepower rated.
Unlike switches. circuit breakers are not horse*power rated because they are able to safely interrupt currents far in excess of the locked rotor value for any motor with which they may be applied. This ability is recognized in the N. E.C. as stated in paragraph 430-109, and is proven by the Underwriters' tests de~cribed in U/L Bulletin number 489, "Standard for Branch Circuit and Service Circuit Breakers".

 

[Dennis Alwon]

Part 2-- again I hope this is what you wanted

For example, a breaker must pass the U/L overload test consisting of breaking a current 600% of its ampere ratings. As motor branch circuit breaker ratings are usually 125% to 250% of motor full-load currents, this test establishes the ability of the breaker to more than interrppt locked rotor currents. Following the overload test and others, the breaker is called upon to successfully clear its rated short circuit current which is a minimum of 5000 amperes. This also is many times higher than motor locked rotor current. Because by defini*tion a circuit breaker is required to "open under abnormal conditionswithout injury to itself". the breaker must still be in operating condition after the test.

 

[iwire]

Dennis my copied as a image file, how did you get that to work?

 

[templdl]

I have reconfigure page 3 of a copt of the Westinghouse application data.
I hope this works better.

 

[M. D.]

Thank you guys ,.very interesting

 

[Dennis Alwon]

Dennis my copied as a image file, how did you get that to work?

I scanned it as a word document and then pasted it

 

[jetlag]

not good

not good

The starting current on that motor is over 100 amp that is way to much to put across the contacts on a 40 amp breaker . It needs to have a motor start button. If you are set on using a breaker it is ok to use an inverse time breaker up to 2.5 times the full load motor current or instant trip breaker of up to 8 times the full load current

 

[templdl]

Depending upon the breaker if it is a 100a frame breaker with a 40AT it most likely has been built with 100a contacts.
Also, the instantaneous trip of a 40at breaker is 10X the rating or 400a +-20%. As such the instantaneous trip could be as high as 480a amps where most breakers are probably calibrated anyway.
As an example the basic NEMA AB4 test for a 100a breaker is made at 300% of the rated continuous current of the breaker. If the breaker was a 100a frame with a 100at, 300% of 100 is 300a which should trip within 140sec at 300a. So the breaker will handle 300a for 140sec.
If a 40a breaker were built the same as the 100a then it would be able to handle 300a also except the thermal element would cause the breaker to trip.
A 40a breaker should trip in less than 80sec when 120a is applied.
When considering the starting current of a motor how long does it take to start a motor? Less than 80sec? And remember that the thermal trip curve is an inverse time current curve.
We're talking in generalities without any specifics. It would be simpler to look a breaker trip curve and compare it to the starting current of the motor and its duration.
The only time one could play with the contacts on a breaker is in the area of its withstand rating. The thermal magnetic time/current curve is designed to keep the breaker within its withstand rating tripping the breaker to prevent it from damage.
We have found however that on very rare occasions it is possible to damage the contacts of a breaker in an area just below the magnetic trip calibration so the breaker won't trip magnetically be not long enough in duration for the thermal element to trip. Such an occurrence can raise havoc with the contacts by teasing them.

 

[iwire]

The starting current on that motor is over 100 amp that is way to much to put across the contacts on a 40 amp breaker .

It is obvious you did not read the links. :smile:

A 40 amp breaker has to handle fault currents far exceeding the starting currents that the motor draws.

From the link.

they are able to safely interrupt currents far in excess of the locked rotor value for any motor with which they may be applied.

Personally I would rather use a motor starter but that does not mean it is necessary.