In search of a fuse that limits arc flash incident energy

Hello everyone,

I have a project where a 208V, 3 phase, 400 amp busway is being fed by a 400 amp fusible disconnect on the secondary side of a 150 KVA transformer. I'm working on the power systems study for this project and I am trying to find a fuse that will allow fast fault clearing and also provide a little bit of time delay for the downstream equipment on the busway (I know this is kind of a contradiction because fast fault clearing and time delay are two separate things). Currently they have chosen 400A TR Class RK5 time delay fuses and this does not allow mitigation of high arc flash hazard on the busway. The available fault current range from 6000 amps at the beginning of the bus duct to 3000 amps at the end. Just want to know if anyone has a good fuse type that they know of for reducing arc flash energy.

Best Regards
 

Hv&Lv

Senior Member
Location
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Occupation
Engineer/Technician
Hello everyone,

I have a project where a 208V, 3 phase, 400 amp busway is being fed by a 400 amp fusible disconnect on the secondary side of a 150 KVA transformer. I'm working on the power systems study for this project and I am trying to find a fuse that will allow fast fault clearing and also provide a little bit of time delay for the downstream equipment on the busway (I know this is kind of a contradiction because fast fault clearing and time delay are two separate things). Currently they have chosen 400A TR Class RK5 time delay fuses and this does not allow mitigation of high arc flash hazard on the busway. The available fault current range from 6000 amps at the beginning of the bus duct to 3000 amps at the end. Just want to know if anyone has a good fuse type that they know of for reducing arc flash energy.

Best Regards
Are you looking for a fast curve fuse or you want to reduce arc fault incidence?
personnel protection or equipment protection?

https://ep-us.mersen.com/services-documents/arc-flash-info-center/reducing-arc-energies-fuses
 

ron

Senior Member
Even though there is 6000A available bolted fault current at the beginning of the busway, you need to determine the available arcing fault current, then plot that on the 400A fuse curve and you want it to clear way down on the Y axis (time).
 
Even though there is 6000A available bolted fault current at the beginning of the busway, you need to determine the available arcing fault current, then plot that on the 400A fuse curve and you want it to clear way down on the Y axis (time).
That’s what I did, I looked at the worst case arcing fault current (around 2000A at the beginning bus and 1000A at the end) and the fuses that are there don’t clear the fault current fast enough to get a low incident energy. I think that the issue is I’m needing two different things at this location, time delay, and fast fault clearing which I can’t have both. I don’t even want them working on this busway live to be honest.
 
Last edited:

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
That’s what I did, I looked at the worst case arcing fault current (around 2000A at the beginning bus and 1000A at the end) and the fuses that are there don’t clear the fault current fast enough to get a low incident energy. I think that the issue is I’m needing two different things at this location, time delay, and fast fault clearing which I can’t have both. I don’t even want them working on this busway live to be honest.
That was why I was asking my question about personal or equipment protection. I don’t think you can do both effectively.
For our HLT all delays are taken off and we use a 50 element with a very minimum value for pickup. I don’t care about what’s downline, I want personnel protection over anything else.
The relays I see with a HLT function do little more than block reclosing and use the fastest curve available.
With the experiments I have done with a relay tester, not using the factory logic and using the 50 element, I can reduce the maximum amps by ~600-800 amps. That’s a lot to the guy doing the work. Sometimes it’s actually me on a hot bus, so I prefer these changes.
 

ron

Senior Member

paulengr

Senior Member
If you just take a blown fuse and plug it in, the incident energy is reduced to zero. Works every time, no calculations needed.
 

pv_n00b

Senior Member
Location
CA, USA
This is more than an academic exercise. Starting on 1/1/2020 NEC 240.67 kicks in and we need arc energy reduction in circuits with fuses 1,200A and above. Right now there is no fuse that can provide this, but I am hoping that an electronic fuse shows up that can do it. Otherwise it's going to require expensive arc flash detection circuits and additional relays.
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
This is more than an academic exercise. Starting on 1/1/2020 NEC 240.67 kicks in and we need arc energy reduction in circuits with fuses 1,200A and above. Right now there is no fuse that can provide this, but I am hoping that an electronic fuse shows up that can do it. Otherwise it's going to require expensive arc flash detection circuits and additional relays.
I don’t think these arc flash relays are too high in cost at all. We aren’t talking about a lot of money for what they can do and the multi functionality of them.
 

paulengr

Senior Member
This is strictly for higher voltages but if I remember correctly Mersen has a “bring your own relay” system and S&C has their own relays where they supply a medium voltage fuse and a capacitor mechanism that stores enough energy to blow the fuse (high current low voltage). When the relay triggers it the fuse blows. This gives you breaker/relay control at fuse speeds.

Also ABB has two versions of their arc flash relays. The first version uses “point” light sensors. The idea is Mount one sensor per cubicle. The other version uses a single unclad fiber to collect light that can be quite long. So busway light detection is certainly not impractical.

Realistically though when the fault current approaches operating current there is just not enough headroom for fuses to discriminate effectively. Then again neither does anything else. Breakers do better up to a point.
 

pv_n00b

Senior Member
Location
CA, USA

paulengr

Senior Member
Since arc currents are typically much lower than the available fault current and not in the current limiting range of current limiting fuses, the fuses won't clear in the required 0.07 seconds.
That’s a lot of assumptions. Arcing currents are around 50-100% of available fault current. The current limiting range is a separate issue. It is linked to the fuse rating but neither one is related to the available fault current. It’s not uncommon to see both high enough that the fuse easily current limits and trips on both arcing and bolted faults, particularly when they are fairly small.


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