I have a pair of electric clothes dryers, one for our use and one for the tenant downstairs. These are on fuse panel electrical systems. The dryers are different makes/models and each is on its own fuse panel. About once every few months, one of the two fuses will blow on one of the two dryers. I know it’s happened because the dryer will still spin but there’s no heat.
It’s my understanding that the heating elements are 240 V and use both fuses, but the drum motor is 120 V and only uses one of the two legs. The fuses are all 30A.
My question is: If the fuse is going to blow, why is it always the one that does not affect the motor? It seems like that one would be more heavily loaded.
Also, why does the fuse blow in the first place? 30A should be plenty for an electric dryer, right?
Thanks for your help with this little riddle.
Alec
Replies
All fuses and most circuit breakers are basically heaters calibrated to disconnect a circuit when a thermal set-point is reached. The ides is that all the current flowing out to the appliance goes through the fuse and that that is current causes an element to heat, 'I' square 'R' in direct proportion to the current flowing though them.
First thought is that the fuse clips on the side that keeps blowing is overheating slightly. This is the first thing I would check. A loose or corroded connection near the fuse holder or the clip being corroded itself can cause this. The way to check this is to run the dryer for a time and check the heat of the fuse and the connecting clips or clamps.
Easiest to do with a non-contact IR thermometer. Compare the readings of the clamps, connections and the fuses themselves. There is not a correct reading, outside an extremely high one. It is a matter of comparison. A difference of more than around 10C should trigger more investigation. These IR units are handy. They have come down in price to around $80 for a good one. Once you have one you spend a month checking the temperatures of everything in sight. Neat, and useful, tool and toy.
OK, you don't have a IR thermometer. Plan 'B': Have the fuse covers removed and a voltage checker, multimeter or my favorite a 'Wiggy', at hand. Run the machine for a good time. A load should do it. With it still running and you standing by have the DW turn off the power at the panel feeding the fuse block. If this will kill the lights have a flashlight handy. One power is off quickly double check it at the fuse block to make sure everything is dead. Use your fingers to get a rough estimate of relative temperatures. Usually anything that it too hot to comfortably touch is running hot and deserves investigation.
If you find a problem it is usually a matter of relanding a connection, cleaning contact areas, polishing and tightening the clips, tightening a screw or, worse case, replacing the fuse block. Common sense and basic safe practices will serve you well.
Lets say that the fuse blocks check out clean. Everything running cool. Everything clean and tight. As a rule fuses are more reliable than circuit breakers. They are a pain to keep a supply on hand and replace but they have no moving parts and operate on a basic law of physics.
Due to this I would have to assume that the fuse is doing its job and is reacting to a real overload. The problem is south of the fuse block. Check the plug, the cord and ultimately the machine itself. I would use a clamp ampmeter to check the current in each conductor at the fuse block as the drier is running and the coils operating. Tracking this over current down should be fairly simple. The drier receptacle can be checked in a fashion similar to the fuse block. The cord should be checked by feel and looking at it with a bright light.
When you get into the appliance itself don't forget most major appliances have a circuit diagram on an inside cover. Having this can greatly simplify troubleshooting.
Alec,
I'd look at the fuse holders of the fuses that always fail, and the wire connections to the fuse holders. A loose clip or wire termination, or oxidation on a clip or terminal, can cause localized heating at the fuse. Increased resistance at a point can cause hot spots, big time. This will heat the fuse and lead to it fusing (melting) under normal load.
I assume that the fuse(s) open by melting, instead of blowing (as in a short circuit). A fuse that's melted from overload often looks normal; one that has blown from a short can look burnt or even have a small hole in the side. Do you recall what the opened fuses looked like? A fuse that's blown can have more of a burnt smell than one that melted due to overload.
If you have a good digital multimeter and know how to safely use it on energized circuits, you could measure voltage drop across the fuse and terminals; that would allow you to track down a point of resistance heating. And it's a lot cheaper than thermography.
If the fuses actually blow, let me know and we'll go from there.
Cliff
The other day, I touched the top of my dryer when it was running and couldn't believe how hot it was. On further inspection, the new exit hood I had installed to the exterior had a wide anti-rodent mesh that was collecting lint.
>>If the fuse is going to blow, why is it always the one that does not affect the motor? It seems like that one would be more heavily loaded.
I don't think this has been addressed in this thread.
This an an area of electricity which is counter-intuitive.
Actually, the heating element is much more likely to draw more current. Motors only draw heavy current on start up and the heat buildup while running is minimal.
In fact, as a (very loose) general rule, you can "under fuse" a motor circuit by one "step" - e.g. a 20 amp breaker and #12 CU lead will generally serve a 30 amp air conditioner compressor motor.
I emphasize that this is only a general rule and each motor circuit has to be individually accessed.
"It is as hard for the good to suspect evil, as it is for the bad to suspect good."
-- Marcus Tullius Cicero, statesman, orator, writer (106-43 BCE)
"Actually, the heating element is much more likely to draw more current. Motors only draw heavy current on start up and the heat buildup while running is minimal."
On a dryer the heating element is on the 240. That is BOTH hot legs.
The motor, light, and timer are usually 120. That will be ONE leg and the neutral ALONG WITH ONE SIDE OF THE HEATER.
Alecs is right. If this was just a case of too small a fusing then the most likely failure is on the side with the motor.
"In fact, as a (very loose) general rule, you can "under fuse" a motor circuit by one "step" - e.g. a 20 amp breaker and #12 CU lead will generally serve a 30 amp air conditioner compressor motor."
Wrong, you have it backwards.
The wire is sized by the nameplate load. That is the continus operating current. If the nameplate was for 30 amps, you would need a minimum of #8 (after derating to 80% continous load).
And since the motor load is protected by either internal protection or it's controller and this is a dedicated circuits so that the load is know the CB does not have to protect against overload of the circuit, but rather against faults. Because of this the CB can be oversized, I *think* upto 140%, but I have to read the details. Lots of specialized requirements in motor circuits. This is allowed specifically to prevent CB tripping on hard to start motors.
Thanks for all the suggestions. I do have a good DMM and will check voltage drop and current on both legs at the fuse panel. I guess what is even more curious to me is that both of my dryers do the same thing, and it's always the fuse that does not run the motor (120V) side.
One dryer is on cartridge fuses and the other on screw-in fuses. Obviously can't tell on the cartridge fuse what it looks like inside. But on the screw-in fuse that I looked at last night, the metal element inside did not seem to be broken/burnt out, but there is a piece of paper behind the element that says "OK" on it. This paper was discolored to the point of not really being able to read the letters. From the sound of it, that would mean a general overload, not a short circuit, right?
The wiring from the fuse panels to the dryers is 10/3, about 30 feet run, and 30 A fuses. Anything sound wrong with that picture? I guess I'll check the terminals and make sure the connections are good, clean, and tight at the panel.
Alec
Let's get you description straight. From the posts, Both (either) dryers occasionally blow a fuse, not just one of them. One dryer is on cartridge fuses, the other on screw in, and either can fail, but the motor keeps running.
So:
1. why do you still have fuses?
2. the prev. comments on loose fuse holders is valid for the cartridge fuse, valid only for the screw in if you don't screw it in tight.
3. An option to look at if only one of the dryers blows a fuse would be to suspect an intermittent short of the heating element to ground, probably near the middle but nearer the non-motor terminal, however, the chances of both dryers having that problem are low.
4. Is the fuse panel right next to a heavy door that often gets slammed? see if there is any correlation with dryer blowing fuse and tenants having a door slamming fight? Vibration is also a fuse killer.
Yes, your description is correct. Both dryers occasionally blow a fuse. Motor always continues to run. Replacement circuit breaker panels are sitting on the floor, waiting to be installed. Haven't gotten there just yet, but soon.
No doors near the fuse panels. Problem spans more than one tenant, so I don't think it's related to them.
hmmm. a mystery indeed
Why does the motor keep running when one fuse blows? Well, if it blows the fuse feeding the heater only, then the remaining fuse feeds the motor and the now cold element. However, if it blows the fuse that feeds the motor and heating element, now the fuse feeding the OTHER side of the element runs the motor current through the element to the motor. With only the 3 or 4 amps of motor current, the heating element only drops 20 or 30 volts, and the motor continues to run on 90 volts.
Checking the fuse temperature is a good idea - and I remember a noncontact infrared sensor at Radio Shack for $30 or so! (It might have been a closeout, though.)
Jim,
Thanks for the interesting reply. I think I will have to look at the circuit diagram to convince myself that your explanation is correct, but I like it.
Alec the Mech Engr
Jim is right.
It is one of those cases where there was a red herring and everyone kept looking at the wrong leg.
Now, if it is the leg with the motor, that is a little more logical as that leg will have more current.
BUT, that in it'self does not explain WHY.
Are these standard residential driers?
What is the name plate data on each of them?
I agree, Jim is right.
I checked out http://fixitnow.com/ which in and of itself is a pretty funny website.
There are circuit diagrams for dryers that proved that it does not matter which fuse is blown, the motor will still run.
Anyway, to answer Bill Hartmann's latest question - these are both standard kenmore/whirlpool dryers. One is a little older (avocado color) than the other.
I'll have to check the nameplate and see what the rating is, but I always thought dryers went on 30A circuits.
Alec
BTW, now we don't know which leg the blow fuse is in and it can be in either one.
One test is to measure between the 2 legs with the motor running.
If the blow fuse is in the motor leg then you will measure a fair voltage difference between the 2 legs. I am guess about 15-30 volts.
If it was in other leg the difference would be zero.
It will be interesting to see what happens when I replace the fuse panels with C/Bs.
At least with a C/B, both phases will interrupt at the same time!
I wrote that very unclearly.
My understanding is that, as a general rule, if a motor requires a 30 amp fuse, the conductor can be # 12cu. See. e.g., chart ET-7 and the related examples in the Electrical "Code Check" book.
Is that still wrong?
The example they give:
3 HP motor @ 230v = FLC 17 Amp x 125% for sizing overload protection =21.25 A = #12 CU Wirw
Nameplate 15 AMP FLA x 125% thermal protection rated 19 A
Breaker 250%x 17A FLC = 42.5A breaker
"It is as hard for the good to suspect evil, as it is for the bad to suspect good."
-- Marcus Tullius Cicero, statesman, orator, writer (106-43 BCE)
"My understanding is that, as a general rule, if a motor requires a 30 amp fuse, the conductor can be # 12cu. See. e.g., chart ET-7 and the related examples in the Electrical "Code Check" book."
I would not say that.
Again you are working it backwards.
As the example shows if starts with the nomimal full load current from the tables. That determines the MINIMUM wire size and the MAXIMUM breaker size. And fuses have a different multiplier for maximum allowed fusing.
It is not any simple relation between wire size and breaker size after you figure in things like derating and long wire runs.
Backwards? I'm not sure what you mean.
I mainly see A/C units as the motor circuits in residences - so maybe I'm talking about a distinct subset. The manufacturer plates almost always call for a conductor size "1 step" smaller than would usually be required based on the breaker size required for the unit, and my understanding was that those requirements were becasue of it's considered a motor circuit.
(Whereas, my understanding was that although a elec dryer circuit runs a motor, it is not considered a motor circuit because the main current draw is for the heater elements.)
>>It is not any simple relation between wire size and breaker size after you figure in things like derating and long wire runs.
That I understand - now, if I can just make sure I understand the relationship between breaker size and conductor size ....!
"It is as hard for the good to suspect evil, as it is for the bad to suspect good."
-- Marcus Tullius Cicero, statesman, orator, writer (106-43 BCE)