Protecting equipment with current-limiting fuses

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kamomeno68

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Here is my situation: NEC2014, Service conductors feed a meter, and then the meter feeds a typical panelboard. The meter housing and panelboard both have a 10kAIC rating. Let's assume the available fault current is 13kAIC at the input to the meter housing, and is 12.5kAIC at the input to the panelboard. Our local utility allows current-limiting fuses, which would dampen the available fault current below 10kAIC, to be installed on the line-side of the meter housing to provide protection from fault currents. The local AHJ is fine with this protection of the meter housing, but will not allow these current-limiting fuses to protect the panelboard. Even with the current-limiting fuses in place, the breakers in the panelboard are required to have kAIC ratings above 13kAIC. Can someone help me understand why? Are there any code references which support this paradigm? Thank you,
J
 
kamomeno68 said:
Here is my situation: NEC2014, Service conductors feed a meter, and then the meter feeds a typical panelboard. The meter housing and panelboard both have a 10kAIC rating. Let's assume the available fault current is 13kAIC at the input to the meter housing, and is 12.5kAIC at the input to the panelboard. Our local utility allows current-limiting fuses, which would dampen the available fault current below 10kAIC, to be installed on the line-side of the meter housing to provide protection from fault currents. The local AHJ is fine with this protection of the meter housing, but will not allow these current-limiting fuses to protect the panelboard. Even with the current-limiting fuses in place, the breakers in the panelboard are required to have kAIC ratings above 13kAIC. Can someone help me understand why? Are there any code references which support this paradigm? Thank you,
J
Click to expand...

NEC 110.9 says that protective devices must be rated for the fault current at their line side.
One of the ways to accomplish this is by putting something 'in series' with the downstream breakers, as allowed by NEC 240.86. There are 2 acceptable methods for determining the effective 'series rating'. The most popular method, and the only one for new installations, is via actual testing by the breaker manufacturer.

Your inspector should approve the installation if you can produce some manufacturer's documentation showing the proposed combination of devices. Also, don't overlook that you will need to follow NEC110.22 in regards to labeling the equipment.
 
kamomeno68 said:
Here is my situation: NEC2014, Service conductors feed a meter, and then the meter feeds a typical panelboard. The meter housing and panelboard both have a 10kAIC rating. Let's assume the available fault current is 13kAIC at the input to the meter housing, and is 12.5kAIC at the input to the panelboard. Our local utility allows current-limiting fuses, which would dampen the available fault current below 10kAIC, to be installed on the line-side of the meter housing to provide protection from fault currents. The local AHJ is fine with this protection of the meter housing, but will not allow these current-limiting fuses to protect the panelboard. Even with the current-limiting fuses in place, the breakers in the panelboard are required to have kAIC ratings above 13kAIC. Can someone help me understand why? Are there any code references which support this paradigm? Thank you,
J
Click to expand...

My understanding is that, at least in general, the idea of using current limiting fuses to protect devices other than overcurrent protection devices is pretty sound.

The use of current limiting fuses to protect other OCPD some times works and some times does not, and the only way to tell if a particular combination will work is to test it. If the panelboard manufacturer has tested his main CB and branch CB with that current limiting fuses and found that it works, you could do this. However, there is not a lot of incentive for a PB manufacturer to do so and not many such combinations have been tested.
 
I can think of two possibilities:

One is he's concerned that the current-limiting fuses have not been tested to be series-rated with your breakers. If you have an engineer calculate the series-rating, you can still get away with it. See 240.86

The other is maybe he's misapplying the AIC rating: The current-limiting fuses themselves would have to be rated to handle the available fault current, but not necessarily the downstream equipment. 13kA is low, and most limiting fuses I've seen have AICs of 100kA, so that shouldn't be a concern.

But if you satisfy both those conditions there's no reason the downstream gear should need an AIC higher than the SCCR of the panel it is in. Because if fault current exists to justify the higher AIC then you couldn't legally install that panelboard, either.
 
Thank you.

Thank you.

Thank you for the input. It looks to me like the key in my case is that the meter housing is not intended to interupt faults, so can be protected by current-limiting fuses without triggering any series rating concerns. And the panelboard in my case is not an "existing installation", so having a PE sign-off as described in 240.86(A) would not meet the requirement either. And meeting the testing requirement in 240.86(B) wouldn't be worth it. Thank you all again.
 
petersonra said:
The use of current limiting fuses to protect other OCPD some times works and some times does not, and the only way to tell if a particular combination will work is to test it. If the panelboard manufacturer has tested his main CB and branch CB with that current limiting fuses and found that it works, you could do this. However, there is not a lot of incentive for a PB manufacturer to do so and not many such combinations have been tested.
Click to expand...

To my knowledge all breaker manufacturers that have tested breakers in series, have also tested fuses in front of breakers.
I have used tables of fuse-breaker combinations from Square D, Eaton, Siemens, and GE.
 
big john said:
...
One is he's concerned that the current-limiting fuses have not been tested to be series-rated with your breakers. If you have an engineer calculate the series-rating, you can still get away with it. See 240.86. ...
Click to expand...
It is somewhat unlikely that you could get a PE to sign off on that.
 
jim dungar said:
To my knowledge all breaker manufacturers that have tested breakers in series, have also tested fuses in front of breakers.
I have used tables of fuse-breaker combinations from Square D, Eaton, Siemens, and GE.
Click to expand...

for single pole 10kAIC CBs? I have not been able to find such tables.
 
petersonra said:
for single pole 10kAIC CBs? I have not been able to find such tables.
Click to expand...
You will find few that use standard class R fuses, most combinations seem to use Class J.


Try:
http://www.digestplus-us.schneider-electric.com/
Panelboards Section 9
Table 9-1 (moved to page 9-43)
Class J fuses protect QO breakers.

and

www.eaton.com/ecm/idcplg?IdcService=GET_FILE&dID=451564
Page 10 Table 7
 
Pierre C Belarge said:
It is my understanding that current limiters on conductors are to protect each individual conductor during fault conditions to reduce the amount of damage to individual conductors. Not to protect equipment per se.
Click to expand...

Current limiters do not protect conductors as much as they remove faulted 'paralleled' conductors from service, thereby minimizing damage to other conductors and raceways.
 
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