Wierd Fuse

[Mr_Fixit_eh]

I’m stumped. The wife says last night at bed time, “The dryer’s not working. I have to put a load through 3 times for it to get dry.” Now that makes for a restful night’s sleep, hunh!

I checked the dryer with a load running. First time it felt warm, but not really hot. 10 minutes later it felt really hot, then after another 10 minutes it felt lukewarm again.

I have an older home with a fuse panel. I pulled out the fuse block with 2 x 30A fuses, and checked them with my meter. The one fuse initially showed 15 ohms resistance, then settled within 10 seconds to 0.2 ohms. The other fuse was a different story—the meter reading continued to fluctuate widely from 1 ohmn to infinity for as long as I watched it. It would seem to read between 0.2 and 30 ohms for awhile, then between 80 and infinity for awhile, then back to 0.2 – 30. Weird.

I put in a new fuse, and it seems to be heating up better.

I’m assuming the fuse failed, but I would have expected to see no continuity. Why would I be seeing fluctuating resistance readings?

PS. My meter is a Sperry DSA-700 Digisnap.

Mr Fixit eh!


Mr Fixit eh!

[HayZee518]

Depends on the fuse' construction. Slo-blo fuses have many fine strands of silver material packed in a refractory sand mix within the insulating tube. Some of these stranded would melt and "short" out with each other ( lack of words to properly describe this action ) Others are just a notched piece of eutechtic alloy fastened at both shell ends. A "fusetron" may have a spring at the midpoint to enhance the opening ability of the fuse in instances of dead shorts.
Analog meters will always show a dead short with little od no resistance. Digital meters are a little more sophisticated and read the resistance at each end connection withing the fuse cap.

Normally they should read continuity or a dead short.

[Rewired]

Hey EH!

I have seen that sort of thing as well regarding fuses. I was told long ago by an electrician that there is "tracking" going on inside the tube of the fuse. Usually fuses are filled with some sort of sand as HayZee518 has said, and thats supposed to quench the arc during the operation of the fuse.. SOMETIMES the fuses do not contain any sort of filler ( "one time" and "renewable" fuses.. ones you could replace the actual fuse link inside the tube.. popular years ago, but found to be dangerous due to some of them actually exploding!!.. not popular any more.) These fuses could / will arc a lot inside during operation, especially during a severe overload or dead short.. the little piece of metal fuselink would vapourize and get deposited on the inside of the tube of the fuse... The fuse has in effect done its job and cleared the fault ( that is if it wasn't so severe the fuse EXPLODES), BUT, with the metal deposited on the inside of the fuse's tube, it may show continuity or a high resistance when you put a digital meter across it.

A.D

[LazyPup]

I see another possibility that could account for a erratic reading. It is rather difficult to hold a fuse and both meter probes at the same time. Often people make the mistake of holding the probes in place with their fingers. The result is that the meter will actually read the resistance through your body rather than through the device in question.

Another common mistake is laying the fuse down to measure and not taking into consideration that the surface the fuse is laid on may have some electrical continuity which would again result in a false reading. (Often people lay fuses on the floor while testing, contrary to public opinion, concrete is not a suitable insulator, especially if the concrete happens to be damp)

Whenever i measure a fuse i make a point of laying it down on a known non-conductive surface and make sure i gripe the probes by the insulated plastic grips to insure no possibility of false readings.

[Mr_Fixit_eh]

Thanks for your help guys...the wife is happy now. There was indeed a large build-up of lint in the blower motor/filter/ductwork--about 4 cups full!!! I bought the dryer about 4 years ago and I've been procrastinating on cleaning out the lint for a couple of years now.

The wierd fuse....I was just not making firm enough contact with the fuse. The fuses are not cartridge type, but 2 standard 30A fuses that screw into one fuse block that clips into the panel. You have to remove the fuseblock before you can remove either of the fuses. All contact points are clean.

I tested the continuity of the heater element at 10 ohms and there is no continuity to the chassis. There is 240volts between the heater element leads with the dryer turned on. Connected meter leads with the dryer cord UNPLUGGED, of course. It would appear that there is no electrical failure after all.

She's hot now (the dryer, that is)!

Thanks for your help.

Mr Fixit eh!

[LazyPup]

Technically speaking the heating element is a drier is air cooled. That sounds silly, but drier elements require a good air flow to prevent it from overheating.

There is a special thermostat called the "High Limit" switch that will turn the heating element off if the airflow is obstructed to prevent overheating or potentially igniting the lint.

Kinked drier hoses, lint build up or a stuck flapper on the vent are common causes of drier failure.