Fault Current

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brian john

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Kilmarnock, Va
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Retired after 52 years in the trade.
I received a call today regarding a site where the contractor furnished and install all the distribution equipment. Problem was no coordination study was performed. Long story short they had some tripping issues and decided to perform a coordination study. As part of the study it was found all Switchgear exceeds available fault current, but the 480 VAC distribution panels rated 25,000 AIC do not meet the available fault current of 40,000 A.

This contractor that I do work for is searching for answers. My answer was new panels. He was told by another contractor that installing current limiting fuses would do the trick. And?

Any thoughts would be appreciated.
 
Current limiting fuses could work, but only if the fuse-breaker combination has been tested and series rated. Manufacturers publish data on combinations. See the cutler hammer document, 1C96944H02 for example
 
dkidd said:
Current limiting fuses could work, but only if the fuse-breaker combination has been tested and series rated. Manufacturers publish data on combinations.
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I agree with this statement and have had same verified by two separate PE's. One was from a large breaker manufacturer and the other from a large fuse manufacturer.

Pete
 
Current limiting fuses or breaker and series protection are two different ways of protecting the equipment. A current limiting fuse or breaker in an approved panel would protect everything down stream limiting the inrush current to the design limits of the fuse or breaker. Series protection might be feasable if your equipment upstream and at the distribution panel are of the same make. If so contact your local dealer rep and they will help engineer a series protected system if it is possible with the gear presently in place. Either way I would contact the local rep as you seem unfamiliar with what is required and they could be very helpful.
 
RICK NAPIER said:
Current limiting fuses or breaker and series protection are two different ways of protecting the equipment. A current limiting fuse or breaker in an approved panel would protect everything down stream limiting the inrush current to the design limits of the fuse or breaker. Series protection might be feasable if your equipment upstream and at the distribution panel are of the same make. If so contact your local dealer rep and they will help engineer a series protected system if it is possible with the gear presently in place. Either way I would contact the local rep as you seem unfamiliar with what is required and they could be very helpful.
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I don't agree. Current limiting fuses or breakers cannot be used to protect downstream OCPDs. They can be used to protect down stream equipment that does not contain OCPDs, but you need a listed and tested series combination when the downstream equipment includes OCPDs.
 
dkidd said:
Current limiting breakers cannot necessarily protect modern breakers that have dynamic impedence.
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These 'modern' circuit breakers were first introduced at least 60 years ago.
If the breaker has an instantaneouse (magnetic) trip then it needs to be part of listed series combination.

RICK NAPIER said:
... A current limiting fuse or breaker in an approved panel would protect everything down stream limiting the inrush current to the design limits of the fuse or breaker.
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Current limiting protective devices do not limit inrush current. They can only limit when they are 'opening' therefore they limit fault current.
 
jim dungar said:
These 'modern' circuit breakers were first introduced at least 60 years ago.
If the breaker has an instantaneouse (magnetic) trip then it needs to be part of listed series combination.
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Agreed. As the author's response in article reference above briefly mentions, there are sophisticated electronic trip OCPDs on the market that can be engineered to allow coordination in a range of difficult applications without requiring testing.

jim dungar said:
Current limiting protective devices do not limit inrush current. They can only limit when they are 'opening' therefore they limit fault current.
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I'm a bit confused about the terminology. Maybe it's just my limited experience in this neck of the woods, but I most often associate inrush current with transformers and rotating machines. what we're talking about here is limited the most fast acting part of the fault current which I don't usually associate with that terminology. Can you elaborate on what's meant by the "inrush current".
 
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skeshesh said:
Agreed. As the author's response in article reference above briefly mentions, there are sophisticated electronic trip OCPDs on the market that can be engineered to allow coordination in a range of difficult applications without requiring testing.
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That may be true for selective coordination, but not for all series-combination ratings for high fault current and low device AIC ratings. Breakers need to have withstand ratings not just AIC ratings, in order to contemplate using the NEC allowance for 'engineered ratings'

I'm a bit confused about the terminology. Maybe it's just my limited experience in this neck of the woods, but I most often associate inrush current with transformers and rotating machines. what we're talking about here is limited the most fast acting part of the fault current which I don't usually associate with that terminology. Can you elaborate on what's meant by the "inrush current".
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You are correct, my response was to a posting by Rick Napier.
 
May or may not be practical but lengthening the supply conductors will result in less available fault current at the load end of those conductors, you just need to figure out how much more length is needed, if it will still satisfy voltage drop, and if it is practical or cost effective to do so.
 
kwired said:
May or may not be practical but lengthening the supply conductors will result in less available fault current at the load end of those conductors, you just need to figure out how much more length is needed, if it will still satisfy voltage drop, and if it is practical or cost effective to do so.
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This one worked for me in a lot of instances! Try your calcs with longer cables, and maybe it might.
 
brian john said:
I... it was found all Switchgear exceeds available fault current, but the 480 VAC distribution panels rated 25,000 AIC do not meet the available fault current of 40,000 A.
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Were the length and size of the feeder conductors (along with just about everything else in line from the transformers to the panels) taken into consideration when it was determined that the 40KA was available at the panels?
 
LarryFine said:
Were the length and size of the feeder conductors (along with just about everything else in line from the transformers to the panels) taken into consideration when it was determined that the 40KA was available at the panels?
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They're talking about extending the length of the conductor to increase the line impedance to reduce the available fault. While this works in some cases, it is only a good solution when the fault exceeds the rating somewhat marginally. If the fault exceed the rating greatly then a much longer cable is required to accomplish this method and voltage drop, etc. come into play.
 
Good suggestions so far.
I've also been able to research the innards of the panel to determine that their withstand is higher than the installed breakers and that they are listed to accept higher rated breakers. So we replaced the branch/feeder breakers as required to get the higher rating.
Note: The panel must be listed with those new breakers.
 
ron said:
Good suggestions so far.
I've also been able to research the innards of the panel to determine that their withstand is higher than the installed breakers and that they are listed to accept higher rated breakers. So we replaced the branch/feeder breakers as required to get the higher rating.
Note: The panel must be listed with those new breakers.
Click to expand...

There is nothing mechanical going to fail on a bus except maybe bolted on sections. A breaker has many components that may fail (possibly rather violently) if subject to more current than it was designed for, same with fuses or any current interrupting type device. A bus is just a conductor designed to accept the connection to the breakers. Conductors for general wiring do not have an AIC rating because they do not interrupt current - they just carry it, same with the bus. A panel should not have an AIC rating because it does not interrupt current - the breakers do. It may have a withstand rating - that will be much higher than the AIC rating of any breaker attached to it.
 
kwired said:
There is nothing mechanical going to fail on a bus except maybe bolted on sections. A breaker has many components that may fail (possibly rather violently) if subject to more current than it was designed for, same with fuses or any current interrupting type device. A bus is just a conductor designed to accept the connection to the breakers. Conductors for general wiring do not have an AIC rating because they do not interrupt current - they just carry it, same with the bus. A panel should not have an AIC rating because it does not interrupt current - the breakers do. It may have a withstand rating - that will be much higher than the AIC rating of any breaker attached to it.
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This is a good point to keep in mind - that busses do have withstand ratings related to the strength of the magentic forces involved and how strong the supports are holding the bus in place. As kwired indicates, bus ratings are usually quite a bit higher than most people are interested in and the breakers are the limiting factor. I just researched this issue on medium voltage 5 kV class switchegear and found out the bus in my 250 MVA class gear is braced for 58 kA - almost double the 31 kA interrupting rating of the breakers.
 
kwired said:
There is nothing mechanical going to fail on a bus except maybe bolted on sections. A breaker has many components that may fail (possibly rather violently) if subject to more current than it was designed for, same with fuses or any current interrupting type device. A bus is just a conductor designed to accept the connection to the breakers. Conductors for general wiring do not have an AIC rating because they do not interrupt current - they just carry it, same with the bus. A panel should not have an AIC rating because it does not interrupt current - the breakers do. It may have a withstand rating - that will be much higher than the AIC rating of any breaker attached to it.
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I just worked on a GE SWBD where the withstand was 65kA and the breakers installed were 100kAIC. The available was 90kA.
As you could see, there are many flavors of situations, and in this case, someone tried to put a higher rated breaker than the withstand of the SWBD bus in order to satisfy the available fault current. It didn't work. The SWBD had to be replaced.
 
ron said:
I just worked on a GE SWBD where the withstand was 65kA and the breakers installed were 100kAIC. The available was 90kA.
As you could see, there are many flavors of situations, and in this case, someone tried to put a higher rated breaker than the withstand of the SWBD bus in order to satisfy the available fault current. It didn't work. The SWBD had to be replaced.
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Unless they had their own warehouse of parts to build this gear you would think somewhere along the line that the manufacturer/distributor would have noticed this problem. You would think that would be covered by warranty that they send out a unit that is listed.

Don't send the order in late in the week or right before a holiday. The assembly people probably just assemble what is on the order, someone else did not verify that the order was correct.
 
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