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Ask Renton-Sparky Your Own Question
Renton-Sparky, Electrical Technician
Category: Electrical
Satisfied Customers: 13
Experience:  I have been in the electrical industry now for over 25 years. From Commercial, to residential, and industrial applications.
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Why are the littlefuse 100 amp fuses blowing in SES before

Customer Question

why are the littlefuse 100 amp fuses blowing in SES before the breaker can trip
Submitted: 9 months ago.
Category: Electrical
Expert:  Mike G. replied 9 months ago.

Hi, I'm Mike and I'll be assisting you. Please stand by while I review your question.

Expert:  Mike G. replied 9 months ago.

Breakers are an inverse time over current device, fuse trip when the amperage exceeds the rating immediately, unless they are a dual element time delay fuse.

Customer: replied 9 months ago.
I know that. what i am asking is if the design were proper would these not go in order? what is purpose of of having overcurrent protection on a 100 amp rated wire blow before the breaker trips protecting a 20 amp rated wire
is this just doing the bare minimum requirement to protect the wire regardless of practicality?
Expert:  Mike G. replied 9 months ago.

There is also an SUV rating of over current devices. The rating may be lower on the breaker than on the fuse.

Customer: replied 9 months ago.
that still doesn't answer the question about is this a fixable problem or simply finish the remodel and replace all the fuses that are blown we are talking about a standard 600 amp and SES uses pull outs as the only means of disconnect between it and the panels there is no Main at the panels throughout the building there are six and all are fused exactly the same way. The panels are a modern challenger commercial breaker (snap in)
Expert:  Mike G. replied 9 months ago.

The 100A breaker must have have multiple branch circuits on it. The combined loads with a short in one of them may have been sufficient to trip the breaker.

Customer: replied 9 months ago.
sorry, my phone died I had to get plugged in we are not quite on the same page here we have a 600 amp SPF outside the building it has six three phase 100 amp fuse block with i littlefuse 100 amp JLLN fuses. From there the conductors go to a main lug only 100 amp three phase panel where the branch circuits are protected by the individual breakers unfortunately when a 20 amp breaker trips do to a ground fault , it blows the fuse for that phase every time as well. is this a design flaw or could it be the result of something else, say a voltage variation? I don't know why it would, but I just throwing it out there for Instance.
Customer: replied 9 months ago.
Expert:  Mike G. replied 9 months ago.

You're correct, we're not on the same page. I'll opt out and put your question back on the board. Please stand by for another expert.

Expert:  Renton-Sparky replied 9 months ago.

As a fault occurs, it travels very quickly.

a 20 amp breaker versus a 100 amp fuse, both will open the circuit under a fault.

Chance are good , the fuse will open quicker than a breaker.

if you get a time delay fuse you can get it to act a little slower.

Coordination can be achieved, just will take a little homework.


Customer: replied 8 months ago.
that sort of answers my question, but, if you read my earlier remark to the previous tech, you'll see I've already said that I can put a Time delay fuse on it. The fact is I can't find a little fuse that is 100 amp time delay enough to let the breaker react. what I was trying to chase down is how to get these designed right as I find this problem a lot given that I am mostly a service at remodel contractor.
Customer: replied 8 months ago.
is there a code covering this if so I can't find it but I do remember being taught in school that the design should be such that the overcurrent device is react in order starting with any protection survived for the individual utilization device or machine/motor, branch circuit then typically ahead of the distribution panel from there at the SES and back to the transformer from power company.

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