Maintained KAIC Rating Downstream

[Designer69]

If my main service equipment has a 65KAIC rating but the downstream smaller panels or loadcenters have a 10 or 22KAIC rating then wouldn't they be the weakest link? How is the initial 65KAIC maintained throughout?

Thank You

 

[ron]

The 65kAIC rating doesn't have to be maintained throughout.

The rating of the panels just need to be equal to or higher than the available calculated fault current at each individual panel considering the impedance and generated fault currents from all sources.

 

[dkidd]

The 65kAIC rating doesn't have to be maintained throughout.

The rating of the panels just need to be equal to or higher than the available calculated fault current at each individual panel considering the impedance and generated fault currents from all sources.

Series rating could also be used.

 

[Designer69]

Series rating could also be used.

Thanks, could you expand on this just a little, I'm not sure how a series rating is used.

Thanks

 

[drktmplr12]

Series rating could also be used.

i was not aware that you could get a series rating with a piece of gear rated 65k and a downstream panelboard rated 22k. I thought series rating referred to components such as fuses in line with drives, or breakers in line with power distribution blocks.

Either way, requiring series rating will constrain allowable replacement parts in the future.

 

[petersonra]

Thanks, could you expand on this just a little, I'm not sure how a series rating is used.

Thanks

Manufacturers of OCPD pay UL or another NRTL to test a specific lower AIC rated OCPD in series with a higher AIC rated device. if the lower rated device does not explode it passes the test and is said to be series rated. That way you can use it downstream of the higher rated device and use a higher rating (usually the AIC rating of the higher rated device).

Say you had 22KAIC and 10 KAIC breakers in series. If UL applied 22 KA through the series circuit and one or the other opened up with no damage to the 10 KAIC device, the 10 KAIC device would be said to be series rated and can assume the 22KAIC rating when used this way. However, the rating ONLY applies with the two specific device part numbers that were tested together in this way.

 

[Jraef]

It's worth noting here that IF there is a possible Series Rating of main and sub panels, it will ONLY be with those from the SAME manufacturer, because that testing that Bob mentioned is very expensive. So a mfr will only go to that expense with their own products.

 

[GoldDigger]

...
Say you had 22KAIC and 10 KAIC breakers in series. If UL applied 22 KA through the series circuit and one or the other opened up with no damage to the 10 KAIC device, the 10 KAIC device would be said to be series rated and can assume the 22KAIC rating when used this way. However, the rating ONLY applies with the two specific device part numbers that were tested together in this way.

Note also that in the case of breakers, the panelboards that they are installed in are part of the series testing process, and so you may not be able to use the same tested combination of breakers if the lower AIC breaker is in a lower AIC panelboard that was not also explicitly tested.

 

[dmsv10]

It is typical to have the Mains higher, regular load distribution breakers for power, 120/240 and 277 circuits at the 22k then critical distribution breakers inside let's say a PDU or UPS distributions panel boards, are typically 10k. The primary reason is that you don't want any higher rated breakers on your critical path because it could end up shorting out the entire panel board and or sending a higher spike back to the UPS or other critical systems. Did you consider a ground coordination study?

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[don_resqcapt19]

It is typical to have the Mains higher, regular load distribution breakers for power, 120/240 and 277 circuits at the 22k then critical distribution breakers inside let's say a PDU or UPS distributions panel boards, are typically 10k. The primary reason is that you don't want any higher rated breakers on your critical path because it could end up shorting out the entire panel board and or sending a higher spike back to the UPS or other critical systems. Did you consider a ground coordination study?

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I don't understand the part of your comment that I have shown in bold. The interrupting rating of an OCPD has nothing to do with the let through current in the event of a fault. That number is just the maximum amount of fault current that the device can safely interrupt. The interrupting rating of the device must be greater than the available fault current at that point in the system.

 

[jim dungar]

It is typical to have the Mains higher, regular load distribution breakers for power, 120/240 and 277 circuits at the 22k then critical distribution breakers inside let's say a PDU or UPS distributions panel boards, are typically 10k. The primary reason is that you don't want any higher rated breakers on your critical path because it could end up shorting out the entire panel board and or sending a higher spike back to the UPS or other critical systems. Did you consider a ground coordination study?

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How would the AIC rating of a breaker 'end up shorting' something out?

The AIC ratings of breakers has nothing at all to do with sending spikes to critical systems.
The AIC ratings of breakers, of the same type or frame size, typically has no impact on the coordination of a system.

The Amps, Interrupting Capacity (AIC) of a protective device is simply about its ability to not be damaged while clearing a fault.

 

[dmsv10]

When it's on a panel that's servicing critical load with a main and sub main, all PDUs have 10AIC,

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[jim dungar]

When it's on a panel that's servicing critical load with a main and sub main, all PDUs have 10AIC,

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I still don't understand what point you are trying to make.

The AIC rating of a breaker has no effect on loads or equipment. Do not confuse the amount of available fault current in the circuit with the interrupting capacity of the protective device.
PDUs larger than 150kVA are likely to need devices with AIC ratings above 10kA. Selective coordination is also possible to achieve with series rated equipment.

 

[ggunn]

The Amps, Interrupting Capacity (AIC) of a protective device is simply about its ability to not be damaged while clearing a fault.

Maybe this is the same thing, but I was under the impression that it just meant that fault current greater than the AIC would arc through the device and not be interrupted.

 

[jim dungar]

Maybe this is the same thing, but I was under the impression that it just meant that fault current greater than the AIC would arc through the device and not be interrupted.

No, fault current does not 'arc through' a device. However, it is possible that during an arcing event the amount of current being interrupted may cause the contacts to weld together preventing the fault from being cleared. But, for many molded case breakers simply going up from 10kAIC to 22kAIC can be accomplished simply by using glue to hold the bereaker halves together along with stronger rivets. Going up to 65kAIC often even involves internal changes to the breaker.

 

[GoldDigger]

No, fault current does not 'arc through' a device. However, it is possible that during an arcing event the amount of current being interrupted may cause the contacts to weld together preventing the fault from being cleared. But, for many molded case breakers simply going up from 10kAIC to 22kAIC can be accomplished simply by using glue to hold the bereaker halves together along with stronger rivets. Going up to 65kAIC often even involves internal changes to the breaker.

So there will be at least two possible failure mechanisms with high available current:
1. Mechanical damage from magnetic forces, rapid heating, and arc blast (requiring structural improvement)
2. Damage to contacts and failure to quench the contact-opening arc with the high fault current (possibly requiring contact and gap geometry and spring force (speed of separation) changes)
3. Any other candidates?

 

[kwired]

Thanks, could you expand on this just a little, I'm not sure how a series rating is used.

Thanks

Series rating is something you have practiced without even realizing it at times.

Purchase a typical 200 amp main breaker load center -the main breaker has probably 22 or 25 kAic rating.

You have been installing 10kAic branch breakers in those panels without giving much thought - they are series rated for use with the 22kA main.

Now use a main lug only panel and supply it from some other manufacturers breaker from distribution equipment - now those branch breaers probably aren't series rated with another manufacturer's breaker - mostly only because they haven't done the testing. However the length of the feeder very likely reduces the amount of available fault current to less then 10kA anyway - so they are still good to go. This is usually only a problem when the feeder is pretty short distance, over 25 feet or so is rarely a problem.