# Fuses---BEWARE



## jwjrw (Jan 14, 2010)

RIVETER said:


> I have worked with a lot,lot of fuses in my career and lately have noticed something that all you OLD SALTS probably already know, but you NEWBIES need to pay attention to ,as well.
> While mitigating the arc flash requirements I started to notice that the larger fuses,blade type, did not always read continuity from the cylindrical shell to the blades. Blade to blade-OK...blade to shell--not ok,continuity wise. "A" phase may read continuity, blade to shell..."B" phase would not. The reason for this post is the obvious; Always double check. If you are locked out, you're ok.


 
Ive never noticed that. Ill be checking next time I have any circuits with fuses though.


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## MDShunk (Jan 7, 2007)

Funny fuse story I want to share, at my own expense. 

I was working on a Blodgett oven in a restaurant a good many years back. The control panel had a good many fuses. From the symptoms, I knew that one of the fuses must be blown. To simplify the procedure, I just yanked the group of three (three phase) that I figured contained the blown fuse. I laid them all on a table that was next to the oven, and ohmed them out. They all tested good! I started puzzling on the schematic, when it came to me. DUH! I had them laying on a stainless steel table when I was ringing them out. :laughing:


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## RIVETER (Sep 26, 2009)

Okay, I've done that too. Also, a couple of months ago I removed a 400 amp current limiting fuse because it tested blown. When I got it out, it tested good.I sawed it in two to see why. It was an RK5 type and the interior was a mass of individual copper fuse links...very fragile. When I tampered with it, I apparently jostled a couple of links back together. The fuse was still bad.


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## Mike_586 (Mar 24, 2009)

MDShunk said:


> Funny fuse story I want to share, at my own expense.
> 
> I was working on a Blodgett oven in a restaurant a good many years back. The control panel had a good many fuses. From the symptoms, I knew that one of the fuses must be blown. To simplify the procedure, I just yanked the group of three (three phase) that I figured contained the blown fuse. I laid them all on a table that was next to the oven, and ohmed them out. They all tested good! I started puzzling on the schematic, when it came to me. DUH! I had them laying on a stainless steel table when I was ringing them out. :laughing:


:laughing:

I can see that happening to anyone and throwing them for a loop if they aren't thinking of the table and what its made of.


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## Zog (Apr 15, 2009)

RIVETER said:


> I have worked with a lot,lot of fuses in my career and lately have noticed something that all you OLD SALTS probably already know, but you NEWBIES need to pay attention to ,as well.
> While mitigating the arc flash requirements I started to notice that the larger fuses,blade type, did not always read continuity from the cylindrical shell to the blades. Blade to blade-OK...blade to shell--not ok,continuity wise. "A" phase may read continuity, blade to shell..."B" phase would not. The reason for this post is the obvious; Always double check. If you are locked out, you're ok.


Contrary to popular belief, fuse manufacturers do not generally design their knife-blade fuses to have electrically energized fuse caps during normal fuse operation. ​ 
Electrical inclusion of the caps into the circuit occurs as a result of the coincidental mechanical contact between the fuse cap and terminal extending through it. In most brands of knife blade fuses, this mechanical contact is not guaranteed; therefore, electrical contact is not guaranteed. Thus, a resistance reading taken across the fuse caps is not indicative of whether or not the fuse is open.​


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## RIVETER (Sep 26, 2009)

Zog said:


> Contrary to popular belief, fuse manufacturers do not generally design their knife-blade fuses to have electrically energized fuse caps during normal fuse operation. ​
> Electrical inclusion of the caps into the circuit occurs as a result of the coincidental mechanical contact between the fuse cap and terminal extending through it. In most brands of knife blade fuses, this mechanical contact is not guaranteed; therefore, electrical contact is not guaranteed. Thus, a resistance reading taken across the fuse caps is not indicative of whether or not the fuse is open.​


That is true but in an instance of a person's inattention they may use the shell as a test point and it COULD show HOT, and when checking the next fuse it may NOT show HOT. That's what I' saying.


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