Commercial/Residential Current Limiting Fuse

Status
Not open for further replies.
E

Electriman

Senior Member
Location
TX
I am trying to pick a current limiting fuse for residential/light commercial applications. I welcome any suggestion. My service 120/240 single phase 150-200A. The calculated short circuit current is 24kA and I want to reduce it to 10 kA.

Thanks
 
You cant reduce fault current that way. I know that is what they seem to be for, but under the NEC no can do. Check the series ratings of your 10kA equipment to see what combinations are available.
 
I agree with Ethan. I don't fully understand the physics, so I can't explain the situation completely. The basic issue is that the behavior of the upstream "current-limiting fuse" (CLF) will be influenced by the behavior of any downstream device (breaker or fuse) that will itself react to high currents by attempting to terminate the event. So if the thing you are trying to protect at 10KA is, for example, a controller for an elevator or an air conditioning system, then that controller's internal fuses will change the way the CLF will function during a fault. The result is that the CLF may wind up passing more than the desired limit of 10KA down to the controller.

A solution that I have used in the recent past is to insert a series reactor upstream of the controller you are trying to protect.
 
electrofelon said:
You cant reduce fault current that way. I know that is what they seem to be for, but under the NEC no can do. Check the series ratings of your 10kA equipment to see what combinations are available.
Click to expand...

Where in NEC states that CLF can't function as it is designed or it can't reduce the fault level?
 
charlie b said:
I agree with Ethan. I don't fully understand the physics, so I can't explain the situation completely. The basic issue is that the behavior of the upstream "current-limiting fuse" (CLF) will be influenced by the behavior of any downstream device (breaker or fuse) that will itself react to high currents by attempting to terminate the event. So if the thing you are trying to protect at 10KA is, for example, a controller for an elevator or an air conditioning system, then that controller's internal fuses will change the way the CLF will function during a fault. The result is that the CLF may wind up passing more than the desired limit of 10KA down to the controller.

A solution that I have used in the recent past is to insert a series reactor upstream of the controller you are trying to protect.
Click to expand...

I would like an explanation too. I've never been really clear in this. I thought a CLF DOES indeed limit fault current by itself, but for some reason the NEC doesn't allow their use? Maybe I'm off base though.
 
I'll stand by for the correct answer as well. I feel I must add the disclaimer that I'm not well versed in code to all of my posts.

That said my understanding is that while fuses certainly limit fault current, to be code compliant for what the OP wishes, they must be listed/tested/accepted by the breaker manufacturer for a series arrangement.

For example older GE tri-break CB's had internal fuses to achieve a 100K rating.
 
Russs57 said:
I'll stand by for the correct answer as well. I feel I must add the disclaimer that I'm not well versed in code to all of my posts.

That said my understanding is that while fuses certainly limit fault current, to be code compliant for what the OP wishes, they must be listed/tested/accepted by the breaker manufacturer for a series arrangement.

For example older GE tri-break CB's had internal fuses to achieve a 100K rating.
Click to expand...

Correct. Look at 240.86. For series rating it allows engineering supervision to select breakers, or tested combinations.
No such language exists in Part VI of 240 for fuses.
 
packersparky said:
Correct. Look at 240.86. For series rating it allows engineering supervision to select breakers, or tested combinations.
No such language exists in Part VI of 240 for fuses.
Click to expand...

to elaborate on packer's reply: 240.86 governs when devices are used in a fault-current environment that's higher than what they are rated for and there are two choices: (A) Engineering supervision for EXISTING Installations; or (B) Tested Combinations. So you could engage a PE to make an analysis that shows that devices proposed work together on a Time Current Characteristic curve to interrupt the fault current safely, which requires the upstream device to trip before the downstream device which can be tricky/impossible because the downstream device tries to open and becomes a dynamic impedance; or take the simpler and more direct route and find devices that are listed together in the published series combo rating charts.
 
This is a quick and dirty explanation: breakers are not as slow and fuses are not as fast as a many people think.

Molded case breakers often begin to open before a fuse begins to limit current. Once the breaker contacts begin to open the resulting arc can greatly impact the amount of current flowing through the fuse, further preventing it from entering its current limiting region, especially for relatively large fuses and low fault currents.

The NEC requirement that a breaker and another protective device be 'series rated' by actual testing, has been around for some 30 years now.
 
electrofelon said:
So I understand what the NEC says, but what is a current limiting fuse for then?
Click to expand...
Its purpose is exactly what its name implies. The issue is that you can't automatically install one anywhere and under any configuration, and expect it to function the way you want it to. Here is what I (imperfectly) remember: it's all about "dynamic impedance."

When a breaker begins to open or a fuse begins to melt, there is, for a brief moment, an air gap between the upstream and downstream sides of the OCPD. An arc will be drawn, again briefly, across that gap. The arc will be extinguished when the breaker fully opens or the fuse completely melts. Before the fault occurred, the impedance of the OCPD was nearly zero. During the time that the arc is bridging the air gap, the impedance of the arc is not zero. The total impedance of the circuit changes during that brief time. Put another way, the impedance is "dynamic." The fault current will be reduced as a result of the added impedance, and can drop below the current-limiting fuse's design current limit. That will prevent the CLF from doing its intended job.

The bottom line is that you can install a CLF and it will prevent the fault current from exceeding a given value, but not if there is an OCPD downstream of the CLF. That includes any fuses that are part of the elevator controller or other equipment's controller.

 
Status
Not open for further replies.
Top