MEDIUM VOLTAGE FUSIBLE SWITCHGEAR AND CABLE DAMAGE CURVE

[PE (always learning)]

Hey,

Hope everyone is doing well. I have a project where I am doing a power systems study for a building that is bringing in 13,800V to utility switchgear that utilizes S&C SM-5 400E fuses and then goes on to feed a primary enclosure with 350 kcmil copper MV EPR in 5" conduit in underground ductbank. The utility wants to know if their fuses are properly sized to protect our cable. I have modeled this in skm power tools and it seems to me that our cables are protected by the fuse in the short circuit region, but not in the overload region where the ampacity is shown to be 362.7A. Also another part where I'm confused is the long time region of the fuse curve where it seems to me that the fuse is protecting for overloads of 1000 amps and not 400 amps. I would like to answer the utilities question and let them know whether or not their fuse is protecting our primary cable. I have attached the oneline information and the SKM power tools TCC to this thread. I normally don't work with medium voltage, but the utility is asking for my help and I'd appreciate if anyone could shed some insight on this. Also, is there a way to load an image without it becoming resized and distorted? Whenever I load them into this thread they end up getting resized a lot smaller than what I have uploaded.

Best Regards,
Engineer in training

 

[Ingenieur]

400 is not the amp rating per se
it trips at ~220% of that
not for toc use

read this
http://www.cooperindustries.com/con...al_library/BUS_Ele_Tech_Lib_Fuses_E_Rated.pdf

http://www.sandc.com/globalassets/s...ocuments---all-documents/tcc-number-153-4.pdf

not really designed for toc
APPLICATION—Like all high-voltage fuses, these refill units are intended to accommodate overloads, not to interrupt them. Accord- ingly, they feature fusible elements which are designed with a min- imum melting current of 200% of the refill-unit ampere rating (for refill units rated 100 amperes or less) or 220% of the refill-unit ampere rating (for refill units rated over 100 amperes). As a result, these refill units have considerable peak-load capabilities; however, they should never be exposed to loading in excess of the peak-load capabilities listed in S&C Data Bulletin 240-190.

 

[pv_n00b]

Generally, HV system protection is geared toward fault protection and not preventing conductor overload, unlike LV systems where the OCPD protects from both. Preventing overload in HV systems is done by design.

 

[topgone]

Hey,

Hope everyone is doing well. I have a project where I am doing a power systems study for a building that is bringing in 13,800V to utility switchgear that utilizes S&C SM-5 400E fuses and then goes on to feed a primary enclosure with 350 kcmil copper MV EPR in 5" conduit in underground ductbank. The utility wants to know if their fuses are properly sized to protect our cable. I have modeled this in skm power tools and it seems to me that our cables are protected by the fuse in the short circuit region, but not in the overload region where the ampacity is shown to be 362.7A. Also another part where I'm confused is the long time region of the fuse curve where it seems to me that the fuse is protecting for overloads of 1000 amps and not 400 amps. I would like to answer the utilities question and let them know whether or not their fuse is protecting our primary cable. I have attached the oneline information and the SKM power tools TCC to this thread. I normally don't work with medium voltage, but the utility is asking for my help and I'd appreciate if anyone could shed some insight on this. Also, is there a way to load an image without it becoming resized and distorted? Whenever I load them into this thread they end up getting resized a lot smaller than what I have uploaded.

Best Regards,
Engineer in training

Your 350MCM, copper cable is rated 310A (75 deg.). You could use a 200E fuse which will have a total clearing time of 592 seconds (about 10 minutes) when the current is about 485A (156% of the cable rating). The choice of a bigger fuse rating will allow the downstream protections to pickup and clear the fault before the fuse blows. Take note that most fuses will vary in actual clearing times and could be less than what they publish.

 

[mayanees]

Generally, HV system protection is geared toward fault protection and not preventing conductor overload, unlike LV systems where the OCPD protects from both. Preventing overload in HV systems is done by design.

.... what he said.

NEC 240.100 is your basis for analysis, specifically (C) Conductor Protection and 240.101(A).
The fuse can be rated at 3 times the ampacity of the conductor, but the design of the system must be such that overload is prevented.

 

[Ingenieur]

Your 350MCM, copper cable is rated 310A (75 deg.). You could use a 200E fuse which will have a total clearing time of 592 seconds (about 10 minutes) when the current is about 485A (156% of the cable rating). The choice of a bigger fuse rating will allow the downstream protections to pickup and clear the fault before the fuse blows. Take note that most fuses will vary in actual clearing times and could be less than what they publish.

using a 200 may cause issues
it is rated for a continuous fla of 200

the fuse may be damaged if run at 300 continuously

 

[beanland]

Overloads vs Faults

Overloads vs Faults

Inegnieur (sp?) is correct; MV fusing is meant to coordinate for faults and permit overloading. MV fusing is designed to coordinate properly for faults; not protect cable from overloads. In fact, as a rule of thumb, we select fusing to allow 150% loading of transformers so that we do not have unintended fuse operation. Cable is sized for the load; fuses are selected to clear faults.

 

[smoothops10]

To properly awnser this question you should also be looking at the minimum available LG fault current. Would the fuse ever see a LG fault with the minimum available LG fault current as provided by the utility? Tell the utility you would need this value to answer their question.

 

[topgone]

using a 200 may cause issues
it is rated for a continuous fla of 200

the fuse may be damaged if run at 300 continuously

I suggest you plot the damage curve of a 350MCM vs a 200E fuse. The fuse clears the 156% overload in 592 sec (9.87 minutes). Your claim of running 200% overload does not happen because the fuse melts at 156%.:happysad: How much more when running at 300% without melting the fuses?

 

[Ingenieur]

I suggest you plot the damage curve of a 350MCM vs a 200E fuse. The fuse clears the 156% overload in 592 sec (9.87 minutes). Your claim of running 200% overload does not happen because the fuse melts at 156%.:happysad: How much more when running at 300% without melting the fuses?

it is only rated for 200 A continuous
his load is 300
although the fuse is rated to carry ol's, it is only for limited duration

this fuse type is not rated for toc protection
per the ul and mfg stds

if he want toc protection he needs another fuse type or a vcb w/relaying

 

[Ingenieur]

that fuse is rated 225
nml conditions can carry 270 for 30 min
beyond the values on the chart performance is not guaranteed
no more than 234 for 8 hrs
https://www.sandc.com/globalassets/...nts---all-documents/data-bulletin-240-190.pdf