ECN Forum Forums General Topics of the Trade General Discussion Area What makes a breaker trip?

Scott35
Scott, that looks like a UL1077 supplementary protector type breaker where the magnetic structure is a bit more peculiar than the common UL489 device.
In any event, to answer the question of how a breaker trips.
It's commonly know that the breaker is made up of a line and load end, the line end can either plug onto a bus and be constructed as such to attach a cable of wire to.
The line end is attached to the stationary contacts. The moving contacts are held against the stationary contacts with contact springs. A trip bar instantaneously releases the contacts to the open position. As the contacts travel to the open position they travel through what is called an arc chute. The arc chute consists of a series of metal plates that spilt up and cool the arcs as it is pulled buy the moving contacts. The combination of the speed at which the contacts open and the arc chute extinguish the arc.
The trip bar is activated by either a thermal element of magnetic element. When either responds to trip it instantaneously hits the trip bar, the trip bar releases the contacts causing them to open.
The current passes from the moving contact though a flexible shunt to the thermal/magnetic elements.
The thermal element commonly consists of a bimetallic element. This strip of two different metals that are bonded together will bend when heated. You may identify this bimetallic element in the picture as supplied by Scott35. Just locate the line end terminal at the top right of the breaker, follow the shiny conductor to the left just below that Phillips head crew. Just to the left of that screw you can see the factory thermal calibration screw adjustment. Then the bimetallic thermal element slopes down to the right. When that element flexes at the far right end it will release the moving contacts. When properly constructed and calibrated it will respond to a given overload current based upon an I2t time current curve. Less over current=longer time to trip. More current=shorter trip time, etc.
The current also passes through a magnetic trip element that responds to short circuit currents for which there is no time delay. This element is commonly made up of a simple piece of steel that is pulled magnetically by the magnetic field created by fault current. Like the thermal element, it to will hit that trip bar releasing the contacts.
Magnetic elements often are calibrated to trip at 6-7x the rating of a residential breaker and 10x for the commercial/industrial breaker with a calibration allowance of +-20%.
The, the current path completes it way to the load end connections.
The thermal element of this breaker not only will respond to the heating affect of overcurrent but it is also influence by the ambient temperature that the breaker is operating. As the ambient temperature increases this thermal element will bend even without any load. When this happens, the breaker can not carry as much current causing it to trip. This is call derating that is the breaker automatically derates when the ambient temperature increases.
On the subject of the interrupting rating or kaic, it must be remembered that the supply source will dump a give amount of current into a short circuit or faulted conductor the magnitude of which is base largely upon what the source is able to supply called available fault current.
The breaker must be able to open its contacts clearing short circuit currents that can be as high as 10,000a in residential locations to many times that in commercial and industrial faculties.

To give you an idea of the power that must be address if a breaker did have to interrupt 10kaic at 120v that would be 10,000a x 120v= 1,200,000watts in a fraction of a second as the contacts open. That is why breakers that have multiple voltage ratings, that the KAIC rating is lower at the higher voltages.
It is extremely important that a circuit breaker be applied within its rating.

Scott35, thanks for all of your brilliant explanations of all things electrical. I often read the Technical Area section (and Theory), and I would like to say thank you.

What does AIC stand for?

Alternating Interupt Current?

[This message has been edited by ShockMe77 (edited 07-15-2006).]

[This message has been edited by ShockMe77 (edited 07-15-2006).]

Amps interrupting capacity AIC and 'k' for kilo(AIC/1000).
Alternating interrupt current? I'm not familiar with the context that you have found this term used in. As far as alternating current in concerned there may be both symetrical an asymetrical interrupting ratings that may be provided with some devices.
Dave T

ShockMe77,

Thank you for the kind words, and for checking out the stuff posted here!
Glad to be of help!!!


Amperes Interrupting Capacity.

AIC is typically rated in "kilo-Amperes", or "X * 1000" Amperes (10 KAIC = 10,000 AIC).

There is another abbreviated term:

A.I.R.

This is Amperes Interrupting Rating

"A.I.R." (Interrupting Rating)is actually the marked rating on the Circuit Breaker, as established by testing.
"A.I.R." is also more related to the Bus Short Circuit rating and bracing than the term "A.I.C."

"A.I.C." (Interrupting Capacity) is the _Actual Test Current_ + Laws, which the Circuit Breaker "sees" during testing, for standard circuit breaker applications (as paraphrased by IEEE!).

Something to consider when figuring SCA at various points along a system:

Motors, along with Reactive Loads (like Ballasts, Discharge Lighting, Inductive Heating, Arc Welders, etc.), are major contributors of Fault Current.
These items need to be figured into the available Fault Level from the Power Transformer, for a complete SCA value.

The Reactive contributions on projects having less than 10 KVA of Reactive type Loads (Motors + XL / XC devices), are normally not significant enough to impact things - mainly because the Power Transformer may only be able to supply 4KA fault current, and all the Reactive loads combined would only contribute another 1KA maximum fault current - if even that much!

For instance, this example customer might be fed from a 208Y/120V 3 Phase 4 Wire pole mounted Transformer array, with a capacity of 50 KVA - and an "overall" Impedance of 1.6% Z.
The available SCA at the Transformer(s) is somewhere around 9KA.

Let's say the customer has an overhead service drop, with a distance of maybe 100 feet between the Transformer(s) and the actual point of demarc for the Utility company (this would be where the utility's feeders terminate to the Service Entrance feeders).

The distance + the small size of the overhead feeder conductors on the utility side, reduce the available fault level down to maybe 5KA maximum, at the point of demarc.

Additional Impedance of the Service Entrance conductors, plus the Service Disconnect device, brings the available level down to maybe 4.5KA maximum.

Add the possible 1KA fault level of the Reactive items to the possible 5KA at the Service, and the available SCA is less than 10KA.
If the AIR of the Service Equipment + Devices is at least 10K, the system is compliant.

This is what would be found on individual small commercial type offices - like upto 2,000 sq, ft. and only HVAC Motor loads.

When feeders sizes are larger, &/or in multiple, Transformers are larger, closer, low % Z, Service Capacity is large (800 Amps and up), Motor loads are > 10 KVA; the available fault level is much greater.

That's all for now!
Let me know if you would like additional information.

Scott35