Class J, Time Delay fuse showing 1000 ohms of resistance?

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sw_ross

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I was sent to look at an Air Handling Unit that had a motor changed out by the heating guys, they couldn't get the motor to run. When I arrived the motor starter (a newer Allen Bradley, fused starter) showed proper voltage at L1 - L2 (208v), but it wasn't showing voltage at the leads feeding the motor.
When I pulled the fuses out to check them for continuity, one was showing 1000 ohms of resistance, and the other was showing no continuity at all. These were newer fuses that have the indicator window in them, which didn't indicate that they were burnt up.

I'm perplexed by this, why would a fuse show resistance, and why would the other fuse show a lack of continuity in it and the indicator window make it look like the fuse is okay?

Do I need a new meter? Or a new brain?
Thanks,
Sky
 
I suppose that mechanical damage could cause that effect.
Is the indicator supposed to look different between a short or a slow overload?
 
Well, if your meter is good then the fuse is no good. I have used these fuses and they usually do indicate when blown. I am assuming these were the original fuses.
Did the original motor take them out? Anyhow, maybe they made a bad batch. A good fuse shouldn't show 1000 ohms.
Why only 208V, most 2 leg circuits are 220V. Just curious.
 
Quite possible that durring the event of blowing the fuse it created a carbon trace within the fuse. This may-be why your getting a reading as carbon is semi conductive. Unless you happened to be touching the ends of fuse with your fingers at the same time testing.
 
A number of years ago we got a box of fuses that about half were that way. Fuse manufacturer said they were probably returns. People apparently send back dead fuses sometimes.
 
sw_ross said:
...I'm perplexed by this, why would a fuse show resistance, and why would the other fuse show a lack of continuity in it and the indicator window make it look like the fuse is okay?

Do I need a new meter? Or a new brain?
Thanks,
Sky
Click to expand...
Your meter is probably fine. Your brain may or may not be depending on how you spent your youth.

The kind of fuses you are dealing with can show continuity with a meter when blown, but will not carry any load and the indicator window has to be examined very closely; if it there is even a hint of anything other than lilly white it is blown.

The best way I know to check them is in place with power on, line side of one fuse to load side of the other.
 
petersonra said:
A number of years ago we got a box of fuses that about half were that way. Fuse manufacturer said they were probably returns. People apparently send back dead fuses sometimes.
Click to expand...
What? Distributors scamming a fuse manufacturer because they didn't want to have to absorb the cost of a mistake they made and had to cover the customer's cost by giving them free rlacement fuses? Shocking! :jawdrop:
 
Thanks for the info. This is at a school, and this part of the old building has 208Y/120. We changed out the motor starter last year as the old Federal Pacific motor starter had an overload relay that had gone bad and wouldn't reset. At that time the motor in the AHU (which was original, probably 30 years old) seemed to be working ok and they didn't want to change that.

Since then the other AHU in the same area (old gym/stage area) was acting up so they got a quote from a heating company to change out both motors. The motor was changed out in the unit that had the new motor starter,but they couldn't get it to run. When I got there I found what I described above.

I wouldn't be surprised if the old motor had taken out the fuses, and the maintenance guy wasn't even aware of it? These fuses were the original fuses that we're supplied with the motor starter.
I'll know more on Monday when I put new fuses in an see what's up at that time.

Thanks for any additional feedback.
 
The heating guys?

The heating guys?

If those are supposed HVAC guys, and cannot troubleshoot power they should not be changing out motors.
 
sw_ross said:
I was sent to look at an Air Handling Unit that had a motor changed out by the heating guys, they couldn't get the motor to run. When I arrived the motor starter (a newer Allen Bradley, fused starter) showed proper voltage at L1 - L2 (208v), but it wasn't showing voltage at the leads feeding the motor.
When I pulled the fuses out to check them for continuity, one was showing 1000 ohms of resistance, and the other was showing no continuity at all. These were newer fuses that have the indicator window in them, which didn't indicate that they were burnt up.

I'm perplexed by this, why would a fuse show resistance, and why would the other fuse show a lack of continuity in it and the indicator window make it look like the fuse is okay?

Do I need a new meter? Or a new brain?
Thanks,
Sky
Click to expand...
Stop fretting. Just replace the fuses.
 
petersonra said:
A number of years ago we got a box of fuses that about half were that way. Fuse manufacturer said they were probably returns. People apparently send back dead fuses sometimes.
Click to expand...

Jraef said:
What? Distributors scamming a fuse manufacturer because they didn't want to have to absorb the cost of a mistake they made and had to cover the customer's cost by giving them free rlacement fuses? Shocking! :jawdrop:
Click to expand...
Sounds more like something a big box store would do then a manufacturer to me. Customer service knows nothing about what is good or bad on an item like this and will give return credit for about anything - then the bad item gets placed back on the shelf for some poor guy that really needs the product now - only to find it defective when he tries to use it.:(

I recall getting a 3 pack of P&S GFCI receptacles at such a store once, don't know how long they sat on my shelf, but when I finally opened the box at a job I find one P&S receptacle in the box and two cheap GFCI's with no name on them, they were lucky I had time to cool down before I returned them.

StarCat said:
If those are supposed HVAC guys, and cannot troubleshoot power they should not be changing out motors.
Click to expand...
They still end up calling an electrician, I like it when I look like I know what I am doing and the HVAC guy makes it obvious he is not an electrician:happyyes:
 
kwired said:
Sounds more like something a big box store would do then a manufacturer to me.
Click to expand...
When I worked at a distributor we had to inspect all of the contractor returned material. We regularly received boxes that contained floor sweepings and other construction detritus. Evidently the conscientious contractors used the new packing for the jobsite junk, and the shop then returned it as excess.
 
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