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Fuse NOT opening

Fuse NOT opening

Fuse NOT opening

(OP)
I am not directly involved with motor controls, as I deal more with control systems. We have an application where we are using a 0-40a/4-20ma current xmitter to monitor a 15 hp 480/3ph pump motor. The motor is fed from an MCC and is controlled via an across the line starter and protected by FRS-R-30a time delay fuses. Apparently the motor controller failed to disconnect all legs of the motor when called upon to do so, and single phased. When operations heard the motor "chattering" and approx. 14 hrs. after the current xmitter "pegged out", the electrician clamped the motor and found a leg pulling 102a. Subsequent checks found all fuses good. The time vs. currnt curve shows this fuse should have cleared in a minute or less. I was getting some of this info second hand. The problem has been resolved and I wasn't in on the troubleshooting, heater sizing etc, but it made me wonder how often fuses fail to open when conditions exist to do so, and what could have caused this fuse (fuses) not to open.     

RE: Fuse NOT opening

The more typical issue with fuses is that they sometimes melt when they shouldn't.  It's hard to say - you have a lot of second-hand info.  Fuse could be mis-labeled, or you could have bad information on the current level, duration of current, or the actual fuse size.  

Sad to say, but usually the first step is personally verify everything - trust no one and never assume anything.

I'd definitely be replacing all three fuses, regardless.
 

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

RE: Fuse NOT opening

(OP)
Thanks for thr response- I filled the original post with fluff- I was just curious how often if ever, others had experienced a fuse that didnt clear when expected. In the original case, I believe that a 30a fuse is the highest amp. value that will fit in the holder. I believe that a 35a is larger. Also, based on the current xmitter pegging out at 40a, I tend to believe that the current value was substantially greater than 40a. (The fuse was a 25a fuse, I stated 30a, as the chart doesnt show a curve for 25a.        

RE: Fuse NOT opening

Thanks for advising your question was basically BS - that will help with future responses.  

When current is just slightly above the fuse minimum melt curve, the fuse response is difficult to predict with great accuracy.  You'd have to be at about 200% of the fuse rating before you can have much confidence in the fuse curve.   40 A continuous through a 30 A fuse may never cause the fuse to melt.

 

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

RE: Fuse NOT opening

Ambient temperature can have a significant effect, especially at low multiples of rated current.
  

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If we learn from our mistakes I'm getting a great education!
 

RE: Fuse NOT opening

My experience has been that when someone tells me "The fuse didn't open", it usually means:

"The fuse DID open, several times in fact, but we got tired of replacing fuses so we cut a piece of pipe to fit in the clips, then when the motor fried we quickly removed the pipe and put in a brand new fuse for you to see".

But that second sentence is such a mouthful that they find it simpler to just blame the fuse wink


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: Fuse NOT opening

Jeff,

This might be a useful reference: Fuse Replacement Guide
 

 

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If we learn from our mistakes I'm getting a great education!
 

RE: Fuse NOT opening

"The fuse DID open, several times in fact, but we got tired of replacing fuses so we cut a piece of pipe to fit in the clips, then when the motor fried we quickly removed the pipe and put in a brand new fuse for you to see".

Another cute trick was to replace the links for renewable fuses with two or three links in parallel.  Then you could trace the fault by following the wire with the insulation dripping off it.

old field guy

RE: Fuse NOT opening

Yes, look at these nice shiny new "fuses"!


At least they used copper pipe... wink


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: Fuse NOT opening

You have to be kidding... what's wrong with polishing the pipe??
  

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If we learn from our mistakes I'm getting a great education!
 

RE: Fuse NOT opening

Polishing shows someone taking pride in their work...   

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

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