Humming Fuse

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Flex

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Opened up the main disconnect at a school the other day. Sounded to me like the fuse on "B" phase was humming quite loud. 450 amp fuses all seated tightly. No loose connections that i could find. I checked the load no more than 80 amps on any phase. Would a fuse do this before it went bad, or is something else likely the problem?
 
I might do an FOP test on that phase, just to see if that shows anything. I can't really say that I'd have opened the disco, myself, just because it was humming. It must have been humming remarkably for you to open it, eh? Don't suppose you tried moving the fuses around to see if the problem followed the fuse?
 
Whats an FOP test? And yea it was humming like a transformer that hasnt had the rubber feet loosened. Couldnt move the fuses cause the teachers were still there.
 
Flex said:
Opened up the main disconnect at a school the other day. Sounded to me like the fuse on "B" phase was humming quite loud.
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Why did you open the main? Scheduled or due to other issues?
 
Flex said:
Whats an FOP test?
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With the disconnect closed, set your meter to millivolts. Check from the conductor material on the line side to the conductor material on the load side. See what millivolt reading you get. Compare the humming phase to the two good phases. The higher the millivolt reading, the more "resistive" the connection is from line to load. If you can stab the actual conductor material, before the lug, you'll be checking the line and load side conductor termination, the fuse blad termination, and the fuse internals in one swoop.

This test might not show anything, but then again, it might. If (for instance) you get 3 millivolts on A phase, 5 millivolts on C phase, and 78 millivolts on B phase, you'll know you've got at least one problem. It might not be the cause of the hum, but getting the B phase tightened up might solve it, and it will bring the FOP reading down.
 
mdshunk said:
With the disconnect closed, set your meter to millivolts. Check from the conductor material on the line side to the conductor material on the load side. See what millivolt reading you get. Compare the humming phase to the two good phases. The higher the millivolt reading, the more "resistive" the connection is from line to load. If you can stab the actual conductor material, before the lug, you'll be checking the line and load side conductor termination, the fuse blad termination, and the fuse internals in one swoop.

This test might not show anything, but then again, it might. If (for instance) you get 3 millivolts on A phase, 5 millivolts on C phase, and 78 millivolts on B phase, you'll know you've got at least one problem. It might not be the cause of the hum, but getting the B phase tightened up might solve it, and it will bring the FOP reading down.
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FOP or Fall of Potential as described in this article at Biddle Megger has to do with ground(soil) resistance testing. Not sure why you use FOP to describe which is just a check for a voltage drop across a component, in this case a fuse?
 
I think of electrical as, the first by-product is heat the second is noise, I like Marc's call on moving the fuses, A paper towel roll, to your ear can isolate the sound.
 
wptski said:
Not sure why you use FOP to describe which is just a check for a voltage drop across a component, in this case a fuse?
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What is the difference? They literally have the same meaning. Fall = drop.
 
I like Marc's call on moving the fuses, A paper towel roll, to your ear can isolate the sound.
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You are not suggesting putting your ear on a paper towel roll and putting it on a live 450A fuse are you? I would be hesitant even to open the cover.

I've heard that noise can turn into arc flash pretty quick. It's like "What's that noi......"
 
wptski said:
Not sure why you use FOP to describe which is just a check for a voltage drop across a component, in this case a fuse?
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...says the guy who has no clue what an FOP test is, or what value it can have.
 
mdshunk said:
...says the guy who has no clue what an FOP test is, or what value it can have.
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Here we go again, typical attack mode!:rolleyes: Yes, I do have a clue what it is and what it isn't also.

Do a Google search of "Fall of Potential Test" and let us know what you find.
 
I dont see anything wrong with Marc terminology (And I am mad at him too), it is correct. For measuring the soi resistance you do a 3 point (Or 4 point) FOP test, what marc is describing is a 2 point FOP test and is symatically correct.
 
zog said:
I dont see anything wrong with Marc terminology (And I am mad at him too), it is correct. For measuring the soi resistance you do a 3 point (Or 4 point) FOP test, what marc is describing is a 2 point FOP test and is symatically correct.
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Yep, you said it, resistance! Here, you are check for the voltage drop created by the resistance. There are numerous on-line "Voltage Drop Calculators" but try to find one called a "Fall of Potential Drop Calculator".
 
wptski said:
Yep, you said it, resistance! Here, you are check for the voltage drop created by the resistance. There are numerous on-line "Voltage Drop Calculators" but try to find one called a "Fall of Potential Drop Calculator".
Click to expand...

You are comparing apples to oranges. You want a FOP calculator? Try Ohms Law.
 
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