Interrupting rating on Breakers

Status
Not open for further replies.

Rsteenson

Member
Can anybody give me a good explanation on why interrupting ratings on breakers will vary from one application to the next? For example on a 800 amp MB Disconnect the trip rating is 50K @ 480v. On a 150 amp feeder breaker the rating is at 35K, and on a motor control bucket the rating is 65KA (instantaneous). (These are all specs from an Allen Bradley MCC) What do these ratings mean and how do you go about deciding which application is right for your particular task?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Those ratings are not "fixed" you can buy any of those items with any of those ratings as well as some ratings you did not list. In general the higher the rating the more expensive the breaker.
As far as what rating to use, you are required, by 110.9. to use a device with a rating that equals or exceeds the available fault current at the point in the circuit where the breaker is installed.
 

renosteinke

Senior Member
Location
NE Arkansas
You've hit on the reason I'm so puzzled by all the confusion / panic when someone is asked to do an 'arc flash calculations.' This is the same calculation we - and especially EE's- are supposed to have been doing all along. That is, calculate the available fault current.

The short version is that the OCPD's need to be able to open under the highest available fault current. Switchgear needs to be designed to function under those currents. "Selective coordination" isn't a new thing, either.

The 'fly in the ointment' is that at some point you need to know the specifics - especially the impedence - of the PoCo transformer, and your distance from it.

Let's imagine a factory. At the service disconnect, you calculate 65K available fault current. This means your service disconnect needs to be rated for 65K AIC.

Now, let's go a hundred feet or so downstream. Because of the influence the distance has on the calculations, these panels might need to be rated to only 35K. By the time you reach the motor starters, the required AIC might very well be under the 'industry norm' of 16K.

So what does "AIC" mean? It means that the breaker or fuse will open, or interrupt, that much current without endangering folks around it at least once. The device might never function again, but it didn't blow up.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Different size devices will have different 'standard' AIC ratings, as you can see from your 800A and 150A examples. As don pointed out, it is possible to purchase a 150A breaker that exceeds the 35,000 AIC rating.

The starter bucket is slightly different, in that it is using a listed series-combination (bucket plug-on stabs + breaker + wiring to starter+ starter). The interaction of all of these impedances, during a fault, effectively allow each part to help the other ones, resulting in a higher AIC rating.

There are no true 'standard' AIC ratings based on sizes of devices, however marketing considerations and UL test levels have driven devices to have similar values.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
You've hit on the reason I'm so puzzled by all the confusion / panic when someone is asked to do an 'arc flash calculations.' This is the same calculation we - and especially EE's- are supposed to have been doing all along. That is, calculate the available fault current. ...
Not really. When you just ask for the available short circuit current, the utility will most often give you the infinite bus value and not the real world actual value, which is a moving target for the utiltiy system. The infinite bus current is fine for picking equipment, but can be an issue for arc flash calculations.
A lower actual available fault current can, in some cases, result in a greater arc flash hazard. The arc flash hazard is based on both the current and time to trip. With a lower current the time to trip goes up and this can make the arc flash hazard much greater. The engineer doing the calculations has to make some "reasonable" assumptions as to the "real" fault current when doing the arc flash calcs. Often they will use a number of "reasonable" currents and use the worst case for the lables. I say "reasonable" currents because if you go really low you can make almost any system show as over 40 cal/cm?.
 
Status
Not open for further replies.
Top