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
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
RE: Fuse NOT opening
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
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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".
But that second sentence is such a mouthful that they find it simpler to just blame the fuse
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: Fuse NOT opening
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
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
At least they used copper pipe...
"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
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If we learn from our mistakes I'm getting a great education!
RE: Fuse NOT opening
"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg