Bad fuse

[Danw2481]

I don't know much about the inner workings of fuses... thought it was a very simple thing but I guess not. I found a 600amp dual element time delay fuse in a D.C. Disconnect not reading continuity, bet yet voltage still would pass through.. I replaced the fuse and it works now.. also once the fuse cooled down it was reading continuity... can someone explain what's up with the fuse... thanks !


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[Besoeker]

I don't know much about the inner workings of fuses... thought it was a very simple thing but I guess not. I found a 600amp dual element time delay fuse in a D.C. Disconnect not reading continuity, bet yet voltage still would pass through.. I replaced the fuse and it works now.. also once the fuse cooled down it was reading continuity... can someone explain what's up with the fuse... thanks !

Something doesn't make a lot of sense here.
Not sure what you mean by "voltage still would pass through".
Did you mean current rather than voltage?

 

[K8MHZ]

We used to call them 'hangars'.

The fuses will fail at the ends and open slightly when hot, creating a bunch of resistance. They would close up after they cooled off, and our meters would read them as OK.

The cause was usually a compromised connection in the fuse holder clips, like corrosion or a clip that's lost its tension.

 

[ActionDave]

I don't know much about the inner workings of fuses... thought it was a very simple thing but I guess not. I found a 600amp dual element time delay fuse in a D.C. Disconnect not reading continuity, bet yet voltage still would pass through.. I replaced the fuse and it works now.. also once the fuse cooled down it was reading continuity... can someone explain what's up with the fuse... thanks !


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A fuse that size usually can't be checked reliably with a multi meter, you need some kind of load through the fuse to check it. It must be the filler and fuse element that will allow a meter to read continuity even when it's blown.

 

[SG-1]

I don't know much about the inner workings of fuses... thought it was a very simple thing but I guess not. I found a 600amp dual element time delay fuse in a D.C. Disconnect not reading continuity, bet yet voltage still would pass through.. I replaced the fuse and it works now.. also once the fuse cooled down it was reading continuity... can someone explain what's up with the fuse... thanks !


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Do you mean it was reading nearly zero volts from end to end, good fuse, or
full voltage from end to end, bad fuse.

A 600 amp fuse would require a milli- or micro-ohmmeter to check. The meter would have to pass 50A or 100A through the fuse.
A normal DMM cannot provide a definitive test on this large of a fuse.

 

[Danw2481]

I was getting full voltage on the line side of the fuse, as well as full voltage on the load side.. no current because the inverter would not run


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[Danw2481]

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[jim dungar]

I was getting full voltage on the line side of the fuse, as well as full voltage on the load side.. no current because the inverter would not run

Voltage L-L or L-G?

Did you check voltage from Line to Load side?

 

[K8MHZ]

Replace the fuse with one that doesn't have brown ooze dripping out of it. If that works, it would be cool to cut open the strange behaving fuse to see what was going on in there. My bet is that the solder at the end has failed.

If the connection of the element to the end caps gets compromised, what you end up with is a very lackadaisical resistor.

If you do an autopsy, please post pics?

 

[K8MHZ]

When I worked at a foundry, I found so many fuses that would check out OK with a DVOM and yet not pass current I pretty much stopped using a meter to check them. If we pulled a fuse and put a new one in, the old ones always got thrown away. We were not allowed to use old fuses on the production line.

If I found an open fuse and all it took was a new fuse to get things running, there was usually visible evidence of internal failure, much like the brown ooze dripping from the collar and probably on the stab, too. In fact, the pic does show what looks like a discolored stab on the bottom and a pristine stab on the top.

 

[K8MHZ]

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Your meter is set on kohms and we can't really see what you are measuring. Nice pics, though!

 

[K8MHZ]

Voltage L-L or L-G?

Did you check voltage from Line to Load side?

Or did he check the voltage drop across the fuse?

 

[Saturn_Europa]

I was checking the fuses on a control transformer way in the back of an MCC bucket the other week. I was reading from line to ground.

I had voltage readings on both sides of the primary fuses because even though one of them was blown. This was because I was reading voltage back through the transformer. So basically the transformer was single phasing. Long story short I spent a whole day pulling a bucket to replace a transformer when it was only a blown fuse.

In my defense it was a bear of a place to take voltage readings or pull a fuse to ohm out. Lesson learned always take L-L and L-G

 

[grich]

If he's reading across the fuse, and it has a 56Kohm resistance, it is, to paraphrase John Cleese, an EX-FUSE.

Next question is why did it fail. I don't get around much, but it's been years since I replaced a fuse that failed due to a fault. Like K8MHZ, my usual fuse failure experience was heat from a jaw-related issue. One of the sites I care for was built with general-duty 100A SqD switches. The pivot point contacts would go bad and overheat. Even had one where a phase would stay closed when the handle was thrown to off. :rant: We got rid of all of those switches quick.

 

[AaronWood2008]

Are you sure you were not getting a voltage reading coming back from the recombiner?

 

[ptonsparky]

I had problems with a 240v 600 amp fuse a few years ago. I had never heard of hangers but the fuse would ok with a tester, but seem to open on load. Frustrated, I took the suspect fuse to a local Vet and asked him to xray it. He took me over to city hospital and we used their xray. One of the techs said "that sure doesnt look like a puppy to me." To much sand so it didn't give me the answer.