Help with AIC ratings for this MechE..

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dreyday

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Mechanical Engineer
Hi All,

Frequently end up on this forum when i have electrical questions and someone's already answered them (thank you sparkies btw), but right now I'm a little stumped and hope someone can help. When MCCBs have different kAIC ratings, what physical change if any enables the higher ratings? For instance if chose to use a 65kAIC 3-pole breaker versus a 10kAIC, with amperage and voltage being identical, what design characteristics allow for the higher rating? I hope I'm making sense.
 
In MCCBs, short circuit ratings are determined by the breaker's ability to create an arc internal to the breaker and then extinguish it. About 40 years ago there was a revolution in breaker internal design that led to a huge increase in short circuit ratings. "Blow-apart" or "reverse-current" contacts rely on opposing magnetic forces to force breaker contacts under high current to open before the breaker trip mechanism even trips, speeding up clearing times and increasing interrupting ratings. If you look at a time-current curve for the instantaneous trip of a modern MCCB, it shows the clearing time going DOWN as the current increases. This is pretty remarkable. This reduces the mechanical forces on the breaker under high current. Other improvements to arc chutes and breaker design also led to increases. This resulted in a near step change from 22 kA to 65 kA.
 
In addition, the mechanical forces taking place in that process are increased exponentially as the fault current increases, so the physical structure, the “molded case”, is beefed up substantially.
 
Jraef said:
In addition, the mechanical forces taking place in that process are increased exponentially as the fault current increases, so the physical structure, the “molded case”, is beefed up substantially.
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And most of this beefing up is not visible from the outside. For moderate increases the changes may be as simple as gluing the molded case pieces together and changing any rivet material.
 
electrofelon said:
What do you think of the theory that sometimes there is no actual difference besides the part number and the price? Or is that just a conspiracy theory?
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By conspiracy theory, do mean the manufactured have a standard design that is good for 65kAIC but they sell it for less money when they put a 10kAIC label in it? Yes I am sure that is done for the special size/options where the normal sales volume doesn't justify multiple manufacturing.

If you mean any cheaply built breaker can interrupt 65kA so they just jack up the price, then you don't understand what happens internal to a breaker during an arc and the cost of getting each part number through the UL489 testing.
 
jim dungar said:
By conspiracy theory, do mean the manufactured have a standard design that is good for 65kAIC but they sell it for less money when they put a 10kAIC label in it? Yes I am sure that is done for the special size/options where the normal sales volume doesn't justify multiple manufacturing.

If you mean any cheaply built breaker can interrupt 65kA so they just jack up the price, then you don't understand what happens internal to a breaker during an arc and the cost of getting each part number through the UL489 testing.
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Yes i was referring to the former (although your two scenarios seem to be equivalent, just with different connotations in the sentence structure).
 
jim dungar said:
By conspiracy theory, do mean the manufactured have a standard design that is good for 65kAIC but they sell it for less money when they put a 10kAIC label in it? Yes I am sure that is done for the special size/options where the normal sales volume doesn't justify multiple manufacturing.

If you mean any cheaply built breaker can interrupt 65kA so they just jack up the price, then you don't understand what happens internal to a breaker during an arc and the cost of getting each part number through the UL489 testing.
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Having worked for a breaker mfr., I can absolutely confirm this is the case, although not typically from 65kA down to 10kA, it’s more often that there is no difference between the 30kA and the 65kA versions, other than the sticker. I knew this from the internal engineering standpoint, but I had a huge argument with a marketing guy from one of the manufacturers in later years when someone was insisting that I pay more for 65kA breakers that were going into products I was designing that needed 65kA SCCRs. I called BS on them insisting the breakers were different, they came back with some story about the arc chutes etc etc. I asked their engineering people for the repair parts list, the parts were all 100% identical, including the cases themselves. They ONLY manufactured a 65kA breaker, but if you only needed a 30kA label, they just sold it to you cheaper because the volume was a lot higher.
 
Jraef said:
Having worked for a breaker mfr., I can absolutely confirm this is the case, although not typically from 65kA down to 10kA, it’s more often that there is no difference between the 30kA and the 65kA versions, other than the sticker. I knew this from the internal engineering standpoint, but I had a huge argument with a marketing guy from one of the manufacturers in later years when someone was insisting that I pay more for 65kA breakers that were going into products I was designing that needed 65kA SCCRs. I called BS on them insisting the breakers were different, they came back with some story about the arc chutes etc etc. I asked their engineering people for the repair parts list, the parts were all 100% identical, including the cases themselves. They ONLY manufactured a 65kA breaker, but if you only needed a 30kA label, they just sold it to you cheaper because the volume was a lot higher.
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Sounds like tesla
 
I also discovered this while rewiring our 40-foot sloop. Normal electrical products where double in price, if the word "Marine" was put on the item.
 
Thanks for your input everyone. Have a follow-up question: would short circuit rating be affected if the vents on the line / load side were blocked or restricted in any way? Or would performance in the XYZ sequence in UL489 be degraded?
 
dreyday said:
Thanks for your input everyone. Have a follow-up question: would short circuit rating be affected if the vents on the line / load side were blocked or restricted in any way? Or would performance in the XYZ sequence in UL489 be degraded?
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Venting is important so that the gases escape with sufficient 'speed' to relieve internal pressures and exit far enough away from the breaker to prevent a restrike when they come in contact with other energized parts such as bus bars and terminals.
 
jim dungar said:
Venting is important so that the gases escape with sufficient 'speed' to relieve internal pressures and exit far enough away from the breaker to prevent a restrike when they come in contact with other energized parts such as bus bars and terminals.
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Quick follow-up, when you mean re-strike, what exactly is re-striking? The contacts inside the breaker?
 
dreyday said:
Quick follow-up, when you mean re-strike, what exactly is re-striking? The contacts inside the breaker?
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If I may, I think he means the arc re-striking. The gasses coming out of the breaker when opening under a fault will be ionized and if not immediately dissipated and dispersed widely enough, could cause another arc to strike from other nearby energized components, i.e. the line side terminals and bus bars that would still be live.

Watch this starting at time mark :40, it shows a slow-mo arc clearing inside of a small breaker and the arc re-striking due to the continued presence of the plasma inside. On larger breakers, a similar thing can take place outside due to the ionized gasses that escape the breaker.
 
Testing procedures where cganged ehen a manufacturer discovered that a panel that passed a 65kA trsted failed at about 38kA.
 
jim dungar said:
By conspiracy theory, do mean the manufactured have a standard design that is good for 65kAIC but they sell it for less money when they put a 10kAIC label in it? Yes I am sure that is done for the special size/options where the normal sales volume doesn't justify multiple manufacturing.

If you mean any cheaply built breaker can interrupt 65kA so they just jack up the price, then you don't understand what happens internal to a breaker during an arc and the cost of getting each part number through the UL489 testing.
Click to expand...
And I have no problem with this. It is capitalism at work. When you buy a new car and they offer special coatings on the undercarriage, there was a story going around that he coating was actually there whether you paid for it or not. I believe that was confirmed, but you were paying for the warranty that came with it.
 
electrofelon said:
What do you think of the theory that sometimes there is no actual difference besides the part number and the price? Or is that just a conspiracy theory?
Click to expand...
I had it on pretty good authority that a fair number of the old AB line of breakers were internally the same but were just labeled with lower AIC ratings because most people wanted to pay less for lower AIC ratings.

How good the information is I can't tell but the guy that it came from was well plugged in to AB.

I have also been told by people who might (or might not) know that many older style breakers were the same internally except the higher AIC rated ones had heavier duty springs. This would be before these more serious breakers were replaced with IEC breakers that managed to barely pass the UL489 tests.

I also heard once that UL does not require short circuit testing for breakers rated at 10 kA. No idea if that is true or not.
 
petersonra said:
I had it on pretty good authority that a fair number of the old AB line of breakers were internally the same but were just labeled with lower AIC ratings because most people wanted to pay less for lower AIC ratings.

How good the information is I can't tell but the guy that it came from was well plugged in to AB.

I have also been told by people who might (or might not) know that many older style breakers were the same internally except the higher AIC rated ones had heavier duty springs. This would be before these more serious breakers were replaced with IEC breakers that managed to barely pass the UL489 tests.

I also heard once that UL does not require short circuit testing for breakers rated at 10 kA. No idea if that is true or not.
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It is relatively easy to go from 10kA to 22kA using different fastening materials.
65kA and higher usually requires a different current path to help blow the contacts apart as springs are not enough.
 
petersonra said:
I had it on pretty good authority that a fair number of the old AB line of breakers were internally the same but were just labeled with lower AIC ratings because most people wanted to pay less for lower AIC ratings.
Click to expand...
Yes, and the old line AB breakers were brand labeled Westinghouse, then by virtue of the buyout, Eaton. It was Eaton that I had the argument with on AIC ratings when I was with an OEM. AB not longer brand labels the Eaton breakers though.
 
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