Short circuit question

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cppoly

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Can fuses be used to reduce available short circuit for fully rated system?

For instance, if 65 kA is available at point of connection of a panelboard, if a fuse is installed slightly upstream, can this reduce the 65 kA to 22 kA to allow for a lower AIC rating?

Or is this only allowed with series rated systems?
 
You're on the right track with needing series ratings in general. There's a problem with "dynamic impedance" messing up the clearing times of current limiting fuses upstream. Basically if there's a fault and your panelboad main CB starts to open up before the fuse clears, the upstream fuse won't see as much fault current so it won't clear as fast and therefor the current limiting aspect is hindered.
 
cppoly said:
Can fuses be used to reduce available short circuit for fully rated system?

For instance, if 65 kA is available at point of connection of a panelboard, if a fuse is installed slightly upstream, can this reduce the 65 kA to 22 kA to allow for a lower AIC rating?

Or is this only allowed with series rated systems?
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No.
I think by definition, if you are taking into account upstream and downstream protective devices in combination with each other, it is called a series system.

110.10 mentions using circuit impedance and device rating but does not explicitly mention device performance.
 
I am not sure cppoly's question is really answered? Would not a 65kA fuse up stream of a 22ka Disconnect be a series system and be acceptable? I have a similar situation with the Service Power Co. saying their AIC is 44kA at the 167 KVA transformer (14.4KV to 240v 1ph). The power goes 12 ft to a distributon secondary box then 12 ft to six 200 amp disconnects rated at 22kA each. I am considering an upstream fuse solution. Any help or suggestions?
 
Episto said:
I am not sure cppoly's question is really answered? Would not a 65kA fuse up stream of a 22ka Disconnect be a series system and be acceptable?
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His question was answered, and for yours: it depends. Is the disconnect fused? Is there more than 22kA available at the disconnect? If fused, is the fused disconnect and upstream fuse series rated?

Just putting two devices in series is a series circuit by definition, yes. But that's not what we're saying. You need a tested "series rated" combination. This comes from the manufacturer. They need to say that these two specific overcurrent devices connected in series will work properly to safely limit the fault current and allows you to use the higher rating.

If you do a search on "series combination ratings" you'll see what we mean.
 
bwat, thanks. May share more specifics of my problem. The plan is to work from a to be installed 167 KVA pad mounted transformer to a 1 KA secondary distribution pad mounted cabinet to 6 200 Amp 22kA disconnects. Power company expects by analysis of the power lines from the substation to the construction service to have a 44kA AIC. Since there are 6 disconnects running parallel, would that have any influence on the AIC? Secondly, if the transformer was fitted with a fuse 45 kA rated, would that still need be a series rated situation? The disconnects breakers and cabinets are both rated for 22 kA and are not upgradeable and I do not seem to have a reasonable solution.
 
We probably know what you mean, but terms can be important for this topic. AIC is Ampere Interrupting Capacity and is a rating of an ocpd like a fuse or breaker, and always needs to be greater than the available short circuit current at the point where the ocpd is installed (sometimes called SCCA or many other things... “available” is key), but this SCCA is not to be confused with AIC which is a rating of an overcurrent device or SCCR (short circuit current rating) which is similar to AIC in that it is a rating, but this usually applies to an assembly of equipment or just a single piece of equipment that doesn’t have an interrupting capability.

So the 44 kA AIC sounds more like available SCC from the utility, but where is this point located? That’s important. On the secondary of your 167 kVA transformer? And what voltages are we talking about?

You may or may not have an issue based on where the calculations are showing you have what current. Transformer impedances and cable lengths can reduce the SCCA without you having to do anything. So a sketch of your setup may go a long way here.
 
Episto said:
Since there are 6 disconnects running parallel, would that have any influence on the AIC?
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No. Each disconnect would still need to be rated for the available fault current of 44KA - your conductor impedance (or be a tested series combination). Have you calculated what the fault current will be at the equipment terminals?

One other thing, an often overlooked aspect, since this is single phase, remember L-N fault current can be higher than L-L so you should use about 1.5 times the L-L.
 
Bwat and electrofelon New Data: I checked with the power company and they did a more complete Fault Analysis that considers all the transmission lines from the substation to the street to the 167KVA transformer and then calculates it to the 15 ft 4/0 UTA cables to the 6 Disconnects finding 27.197kA Ph-N and 18.899kA Ph-Ph. Presently, the six disconnects have 22kA AIC. The Disconnects feed via 150 ft. of underground service cables to indoor panels of 200 Amp 25kA AIC in individual buildings. The disconnect are centrally located for fire fighting personnel. So the problem is the 22kA breaker and the panel does not appear to be rated for more than that. I considered Fused disconnect but I am not sure that it would be "series-rated" with the rest of the system and upstream seems to be frowned on. If switches only seem to only have a 10kA value and I am not sure the underground cable would offer enough reduction in AIC for the 25kA end panels. Not sure where to go?
 
Episto said:
Bwat and electrofelon New Data: I checked with the power company and they did a more complete Fault Analysis that considers all the transmission lines from the substation to the street to the 167KVA transformer and then calculates it to the 15 ft 4/0 UTA cables to the 6 Disconnects finding 27.197kA Ph-N and 18.899kA Ph-Ph. Presently, the six disconnects have 22kA AIC. The Disconnects feed via 150 ft. of underground service cables to indoor panels of 200 Amp 25kA AIC in individual buildings. The disconnect are centrally located for fire fighting personnel. So the problem is the 22kA breaker and the panel does not appear to be rated for more than that. I considered Fused disconnect but I am not sure that it would be "series-rated" with the rest of the system and upstream seems to be frowned on. If switches only seem to only have a 10kA value and I am not sure the underground cable would offer enough reduction in AIC for the 25kA end panels. Not sure where to go?
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You do seem to be in a pickle with the pesky Available fault current. The indoor panels will be fine even without any series rating due to the impedance of the 150 feet of conductors, but the disconnects need to be rated above 27k or an OCPD added ahead of them to get a series rating. IT is very common for manufacturers to have breaker series ratings with fuses.
 
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