Fuses

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Dennis Alwon

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Which fuses are more common in a fused safety switch. Timedelay or fast acting? Thanks.


It depends but I would have to say time delay as most safety switches are used for some motor or a/c units. If the equipment is not a motor of any kind then you will see non time delay. At least that has been my experience. We hardly ever see fuses anymore.
 

jim dungar

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In general, there is almost no need to use non-time delay fuses in the <600A range.
Equipment designers will usually tell you when you need fast acting ones.
After all, circuit breakers are time delay.

When I do arc flash studies, a common recommendation is to get rid of the inexpensive fuses.
 

jim dungar

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After all, circuit breakers are time delay.
I didn't mean this the way it sounds to me now. Molded Case Breakers operate without an intentional time delay once they enter their Inst range.

The OP may want to clarify what size fuses brought up this question.
 

kwired

Electron manager
Location
NE Nebraska
If supplying a motor, transformer, capacitor, or anything else that has an "inrush" upon starting, the use of time delay fuses generally will allow for lower overcurrent level selection. This can mean the difference between a 400 amp frame sized fuse for time delay fuses vs a 600 or even 800 amp frame size in non time delay fuses to supply same equipment to prevent failure during starting operations.

There are also times when current limiting fuses may be specified.
 

mstrlucky74

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Location
NJ
It depends but I would have to say time delay as most safety switches are used for some motor or a/c units. If the equipment is not a motor of any kind then you will see non time delay. At least that has been my experience. We hardly ever see fuses anymore.

Most switches you install nowadays are non fused?
 

big john

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Location
Portland, ME
If proper overcurrent protection is already provided upstream - it is not necessary to provide it again at a local disconnecting means whether the disconnect is required or not.
Agreed, but in my experience the vast majority of stock disconnects are gonna be fused.

When I did plant maintenance I spec'd non-fused because it was one less part to troubleshoot, and they were often special-order.
 

Dennis Alwon

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Agreed, but in my experience the vast majority of stock disconnects are gonna be fused.

When I did plant maintenance I spec'd non-fused because it was one less part to troubleshoot, and they were often special-order.

That is probably true in commercial work but we never see a heat pump or an a/c unit in residential that demands a fuse. I can't remember the last time we used a fused disconnect.
 

kwired

Electron manager
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NE Nebraska
That is probably true in commercial work but we never see a heat pump or an a/c unit in residential that demands a fuse. I can't remember the last time we used a fused disconnect.
And especially if you are in a competitive situation you are going to use the $12 - $15 pull out type unfused disconnect that is popular on AC units.
 

iwire

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Massachusetts
When I did plant maintenance I spec'd non-fused because it was one less part to troubleshoot, and they were often special-order.

That is probably true in commercial work but we never see a heat pump or an a/c unit in residential that demands a fuse.

Keep in mind regardless of the units requirements non fused switches are limited to 10,000 amps fault current, likely not an issue in a dwelling unit but it is something to keep in mind in both commercial and non commercial work

And especially if you are in a competitive situation you are going to use the $12 - $15 pull out type unfused disconnect that is popular on AC units.

A lot of those are only rated 60c so that cost savings has the potential to require a larger conductor.
 

kwired

Electron manager
Location
NE Nebraska
A lot of those are only rated 60c so that cost savings has the potential to require a larger conductor.
No problem if in an application where 60c NM cable is being used anyway.

Now you are going to make me check the temp rating of the ones I generally use, I can't recall if I have ever checked their rating or not, I have never had an install rejected for this issue though.:ashamed1:
 

cadpoint

Senior Member
Location
Durham, NC
This Link is a little off the direct question.
My only problem with the link is the author didn't state exact NEC Code references. I found it to be in most parts a good over-all review of what goes into selecting or applying fuses. Yes it suito turns into
a type of fuse sales pitch.
My thoughts are that most (like me) apply and meet the requirements set by NEC but are lacking a complete understanding of what all involved when fusing, at times... :)

If proper over current protection is already provided upstream - it is not necessary to provide it again at a local disconnecting means whether the disconnect is required or not.

I fully understand..

I like and I don't like the statement, I've frankly never thought as a motor not having some sort of a disconnect however the requirement is met and all always thought that a fused motor disconnect is there to protect and give working aspects to the motor.

I learned it as the branch panel breaker is there for the wire protection, the disconnect is for the motors protection. Yes a branch circuit can cover and meet motor requirements. Underlying is my thought and practice that in the end address all the needs of the electrical equipment your supplying power to. This is all stated on the motor name plate, work from there with the NEC.

Overcurrent Protective Device, Branch Circuit
Overcurrent Protective Device. Supplementary both from Article 100.
 

big john

Senior Member
Location
Portland, ME
Keep in mind regardless of the units requirements non fused switches are limited to 10,000 amps fault current...
You sure on that? Some of the literature on the Siemens switches I used to use says 100,000 ASCCR. Not sure if they have an interrupt rating as they are not OCPDs.
 

under8ed

Senior Member
In general, there is almost no need to use non-time delay fuses in the <600A range....When I do arc flash studies, a common recommendation is to get rid of the inexpensive fuses.

Both hold true in our facility, instantaneous are rarely used; and since arc flash studies have been done, any RK5 fuses are to be replaced with the RK1 class whenever the opportunity arises.
 

don_resqcapt19

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Location
Illinois
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retired electrician
You sure on that? Some of the literature on the Siemens switches I used to use says 100,000 ASCCR. Not sure if they have an interrupt rating as they are not OCPDs.
I would bet the fine print says when used with upstream protection using a specific fuse or breaker.
 

jim dungar

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Wisconsin
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Both hold true in our facility, instantaneous are rarely used; and since arc flash studies have been done, any RK5 fuses are to be replaced with the RK1 class whenever the opportunity arises.
Why?
Below 60A there is almost no difference in performance.
From 100-400A fuses might even be slower than breakers.
Above 800A there is rarely enough fault current to get past the time delay and into current limiting range, so fast acting are often preferred.
 
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