Circuit breaker threshhold value.

[hisham1986]

Dear All,

Can anyone tell me please at what current a normal thermo-magnetic single pole circuit breaker labeled 30 Amp would trip? is it 30 exact or more than 30?

Thank you and best regards

 

[charlie b]

More than 30, by a lot. The higher the current, the faster the trip. As an example, a GE type TED 30 amp breaker can hold 45 amps for about 70 seconds before tripping, and can hold 35 amps for perhaps 5 minutes before tripping. With a current level of 150 amps, the breaker will still not trip for at least 2 seconds. Here is a link to the Time Current Curve (TCC) for that particular breaker:
http://www.geindustrial.com/publibrary/checkout/GES-6119D?TNR=Time Current Curves|GES-6119D|generic

 

[don_resqcapt19]

The UL standard says that the 30 amp breaker can feed a 40 amp load forever without tripping. It also would require the 30 amp breaker to trip in 60 minutes or less with a 40.5 amp load. Most devices will trip sooner than that. You need to look at the trip curve for the device.

 

[templdl]

These are the values from the NEMA AB4 "GUIDELINES FOR INSPECTION AND PREVENTIVE MAINTENANCE OF MOLDED CASE CIRCUIT BREAKERS USED IN COMMERCIAL AND INDUSTRIAL APPLICATIONS"

NEMA AB4 includes an accepted procedure for field testing brealkers.

 

[JoeStillman]

Thermal-magnetic breakers have two separate trip mechanisms. At currents near the rated amps, the thermal mechanism operates. For currents approaching the short-circuit rating, the magnetic mechanism operates.

The thermal trip current is temperature dependant. In a hotter ambient temperature, the breaker can trip at a lower current. I believe breaker trip curves are all calculated at a standard ambient temperature. I have never seen published data for the temperature-dependance of the trip mechanism. You would need to stipulate the ambient temperature to say with any certainty what current will trip the breaker.

 

[templdl]

Please note that the thermal element calibration is +-10% and is usually toward the +10% side and the instantaneous +-20% which is also commonly toward the 20% side.
If the field testing procedures of NEMA AB4 are followed. The table from AB4 that I have posted would provide a fairly good idea that the breaker is within calibration.
But attempting to test the magnetic calibration is basically useless as you can't ramp the voltage and measure the voltage peak in the field. I have run a factory calibration check in a warranty return lab and it pulses current at different magnitudes to arrive at the breaker's tripping value.
With a TM breaker at least you have the advantage of having a thermal element to prevent damaging the breaker if you are so inclined to attempt to check the breaker's magnetic element.
With magnetic breakers up to 150a the breaker they will most certainly be destroyed as you will have fried the magnetic element. I had to advise Cape Kennedy that their attempt to test their MCPs actually destroyed them. All you had to do is take a whiff of them and removing the cover revealed thermal element that had been completely destroyed.

 

[zog]

Thermal-magnetic breakers have two separate trip mechanisms. At currents near the rated amps, the thermal mechanism operates. For currents approaching the short-circuit rating, the magnetic mechanism operates.

The thermal trip current is temperature dependant. In a hotter ambient temperature, the breaker can trip at a lower current. I believe breaker trip curves are all calculated at a standard ambient temperature. I have never seen published data for the temperature-dependance of the trip mechanism. You would need to stipulate the ambient temperature to say with any certainty what current will trip the breaker.

NEMA states the tests should be performed at 77 degrees ambient. There are no correction factors(However there are some general guidelines for expected variance from EPRI studies) Additionally the cables to and from the test set have to be the proper size and at least 4 feet in length as they function as a heat sink during testing, shorter cables would reduce the trip time.

 

[zog]

These are the values from the NEMA AB4 "GUIDELINES FOR INSPECTION AND PREVENTIVE MAINTENANCE OF MOLDED CASE CIRCUIT BREAKERS USED IN COMMERCIAL AND INDUSTRIAL APPLICATIONS"

NEMA AB4 includes an accepted procedure for field testing brealkers.

Note that these tollerances are for the thermal element of thermal magnetic trip devices only, electronic trip units should be tested to the TCC for that trip unit.

 

[zog]

Please note that the thermal element calibration is +-10% and is usually toward the +10% side and the instantaneous +-20% which is also commonly toward the 20% side.
If the field testing procedures of NEMA AB4 are followed. The table from AB4 that I have posted would provide a fairly good idea that the breaker is within calibration.
But attempting to test the magnetic calibration is basically useless as you can't ramp the voltage and measure the voltage peak in the field. I have run a factory calibration check in a warranty return lab and it pulses current at different magnitudes to arrive at the breaker's tripping value..

Where do you get that 20% from? AB-4 has tollerances for magnetic elements and they are +40% -30% (for lowest setting, -25% for all other settings on adjustable). And if you are using the pulse method you are supposed to subtract another 5% off the low end.

 

[templdl]

Where do you get that 20% from? AB-4 has tollerances for magnetic elements and they are +40% -30% (for lowest setting, -25% for all other settings on adjustable). And if you are using the pulse method you are supposed to subtract another 5% off the low end.

As a person who is in the field whatever it is really is of no consequence when you a faced with an instantaneous rise in current it will trip the breaker no matter.
With the smallest breakers the calibration range is more likely in the area that you suggest which is an amazingly great range and with the commercial/industrial breakers they could be -20% to +10%.
The fact remains that there is a magnetic element that trips the breaker when it's subjected to a fault, which the max calibration is set to protect the breaker such that it is below the breaker's SCCR such that the breaker is not damaged.

The point being that the magnetic trip values is really mute point. The breaker must protect itself and with adjustable trips you can try to coordinate breakers by setting one lower than an upstream breaker. But there is no guarantee in doing that because often time a short circuit can be of a high enough magnitude that both breakers will see it and it will be a race to see which one trips first. How many time to you have a fault in a branch circuit protected by a 20a breaker and you trip the 100a main?

And with the mag. only MCP its intent is to detect motor failure by trip should there be an arcing fault within the motor. The objective is to limit motor damage as a motor begins to fail. As such the MCP had a different purpose that the standard TM breaker where the mag element is looking to keep the breaker from being destroyed.

 

[george89]

Any industrial circuit breaker comes with a specified tolerance and within those tolerance limits, it only breaks the circuit when it gets heated up. As for current above 30A, the magnetic mechanism will come in play and it?ll break the circuit. More the value of current, more quickly will the magnetic coil in the circuit breaker get magnetized and break the circuit.