Partially failed fuse

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MrFixit1599

Member
Location
Lomira, WI
Today I was working on a piece of machinery with a small part of the circuitry being 24 VAC. Every time I would try to use the machine controller to pull in the necessary contactor, it would not pull it in. I removed the contactor and connected it directly to the transformer. Pulled in just fine. When the relay that controls the contactor would energize, my 24 VAC would drop from 24 VAC to 0 VAC. Eventually tracked the problem down to a slow blow fuse that did not blow completely. Turned into a 33K ohm resistor. So I could see voltage just fine, but it wouldn't carry any current.

Has anyone else seen something like this? Been doing this a long time and that was a new one on me.

Yes I know, you always check the fuses first, but when I had voltage, one makes assumptions. Wasted 2 hours today on it. Lesson learned I guess.
 

ActionDave

Chief Moderator
Staff member
Location
Durango, CO, 10 h 20 min from the winged horses.
Occupation
Licensed Electrician
...Has anyone else seen something like this? Been doing this a long time and that was a new one on me.
Yep. You, me and a lot of others.
Yes I know, you always check the fuses first, but when I had voltage, one makes assumptions. Wasted 2 hours today on it. Lesson learned I guess.
Don't beat yourself up. When you are troubleshooting finding the problem is the tough part. The solution is always easy.
 

meternerd

Senior Member
Location
Athol, ID
Occupation
retired water & electric utility electrician, meter/relay tech
Most fuses have a voltage rating. Using a 250V fuse, for example, on a 24V circuit could cause what you describe because the amount of energy needed to fully blow the fuse may not be available. Ditto for DC versus AC. I know how tempting it is to just use the closest amperage fuse you have in the fuse drawer, but there are different fuses for a reason. Characteristics, interrupting capacity, slow blow, fast acting....the list goes on. Just a quick story.... "OH NO, NOT ANOTHER STORY!!!"

I had a Fluke digital multimeter that I used when checking voltage on some power transformers. Two were side by side. Both were 7200/12,470 Primary. One was 480V secondary. PPE was just a pair of secondary rubber gloves. Voltage read 279/485 on all 3 phases. Good. The next one over read zero phase to ground, but I could hear the transformer humming. Great, said I....meter just quit. On the way to the truck to get my other one, the "light in my brain turned on". The second transformer was 7200/12,470 Primary and 2400/4160 secondary. STUPID!!!

But the point of the story is....it was a Fluke with the proper fuse in it, and the fuse blew. Nothing else. The very next day, we had a safety meeting that included a video of an accident where one electrician died and one received 3rd degree burns over 50% of his body. You can probably guess how...reading 4160 Primary, thinking it was the 480 secondary. Meter exploded. Why? Because the correct fuse had been replaced with an off the shelf time delay fuse. I checked every meter in the shop for proper fuses. Fortunately, all were fine. Made sure we had boxes of proper fuses with BIG labels saying DMM use only! I credit the Fluke with saving me from a very bad day, and I still never use any other kind. OK...I'm done.
 
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qcroanoke

Sometimes I don't know if I'm the boxer or the bag
Location
Roanoke, VA.
Occupation
Sorta retired........
Has anyone else seen something like this? Been doing this a long time and that was a new one on me.

Just had a post light photocell do that to me.
It would open and close like it should. No voltage, voltage. But when you applied a load , nothing.
 

Fulthrotl

~Autocorrect is My Worst Enema.~
"OH NO, NOT ANOTHER STORY!!!"

I had a Fluke digital multimeter that I used when checking voltage on some power transformers. Two were side by side. Both were 7200/12,470 Primary. One was 480V secondary. PPE was just a pair of secondary rubber gloves. Voltage read 279/485 on all 3 phases. Good. The next one over read zero phase to ground, but I could hear the transformer humming. Great, said I....meter just quit. On the way to the truck to get my other one, the "light in my brain turned on". The second transformer was 7200/12,470 Primary and 2400/4160 secondary. STUPID!!!

But the point of the story is....it was a Fluke with the proper fuse in it, and the fuse blew. Nothing else. The very next day, we had a safety meeting that included a video of an accident where one electrician died and one received 3rd degree burns over 50% of his body. You can probably guess how...reading 4160 Primary, thinking it was the 480 secondary. Meter exploded. Why? Because the correct fuse had been replaced with an off the shelf time delay fuse. I checked every meter in the shop for proper fuses. Fortunately, all were fine. Made sure we had boxes of proper fuses with BIG labels saying DMM use only! I credit the Fluke with saving me from a very bad day, and I still never use any other kind. OK...I'm done.

stories are ok.

and about ten years ago, when i worked at a public utility, the topic was a
line patrolman who tested phase to phase on 5kv with a DMM, and the experience
was his last. it was an in house death. not sure of maker of his DMM, they said there
wasn't much left of much of anything. closed casket.

congratulations on winning the lottery.
 
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