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Old 09-27-2020, 07:49 AM   #1
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How to "Read" a blown fuse

A fuse blows when there is too much current flowing through it. But there is more to it.

A fuse blows because of heat, which ultimately causes the element to melt and open the circuit.

Firstmost, fuses protect the wiring not the device / appliance.
An RV (tt, a,b,c,popup,whatever) has a *lot* of 14ga wiring in it. Nearly all of it is 14ga. This is why most fuses on fuse block are 15 amp! The fuse for your l.p./c.o. detector is 1 or 3 amp. Why? Because the wiring is much smaller at the device but still not device itself. TT usually are wired with 10ga from battery, and therefore require a 30Amp fuse. Slides / levellers / tongue and landing gear jacks come direct off battery efore 30A and 10ga wiring. Cable between converter & fuse block is 8 or 6ga. Converter has 2 or 3 fuses installed in parallel. NEC allows higher ratings for shorter cable runs. Keep that in mind.

Extreme overcurrent

If you find a blown fuse that not only has the element obliterated, but has a char mark then that fuse was likely subjected to a sudden direct short / high in rush current.

Anyone that has connected a battery backwards has seen this example on converter fuses.

Wrong Voltage
If you see a fuse, usually glass, that is blackened along the length, then it is probably a low voltage (32v) fuse being used in high voltage circuit. An arc was established, blackening the tube, and was eventually interupted. ALL my spares are rated for 250vac where I can. Some of my glass are low voltage just due to oddball physical size or capacity (older Norcold fridge dc fuse for example). Remember that voltage rating is to interupt the circuit and extinguish the arc. A 15amp fuse, high or low voltage, will still blow exposed to 20a at 12vdc. But a higher voltage fuse won't arc like a lower voltage will if misused in high voltage circuit.

Melted fuse / fuseholder
Can be found for glass, but common on blade fuses and especially mini fuses. What has happened is there was a less-than-optimal connection between the wire and fuse. To get an idea, 1 amp flowing through a resistance of 1 ohm yields 1 watt of heat. That is actually a lot of heat. The fuse holder can overheat, deform, but fuse won't have blown yet. Like every other electrical connection, these need to be connections need to be kept clean and dry. If you happen to see one of these on your main fuse block, make sure the break-a-way switch is not pulled out. If you see a blackened circuit board around where a glass fuse is held, usually too large or something else has happened.

The Slow Melt
A 15A fuse should carry 15A of current, right? Not always. If the element looks like it drooped (likely with balls) or fuse housing is distorted, it is a sign of a marginal or slow overload. Running 12A through a fuse that is located in hot area will show this. If the fuse / fuse holder is exposed to high ambient temperatures, i.e. Sun shining on it during the summer, you will be luck to see two thirds of its rating. There was an old hwh levelling system I once got to work on. Had the joysticks. By design, the circuit breaker was clamped to motor housing. High motor temperatures caused breaker to open, helping to protect motor. Same principle.

SlowBlo Explosion
Something really bad has happened. SlowBlo are intended, by element physical design, to absorb some heat and pass some current above its rating. Ceramics are a good example of these. When a SlowBlow is charred / blackened / cracked / obliterated... The first thing that comes to mind is power surge / lightning strike nearby. Remember the fuse in your $$$ OTR microwave / venthood combo? This is exactly. In this scenario, these fuses still protect very very well. But perform poorly on slow overloads of even 150% of rating.

Burnt wiring to/from fuse
We have all done it. Replace fuse without finding the cause. And used a larger one because didn't have a 15amp spare. This leads to melted wiring. What you may not realize that If the fuse is exposed to ambient winter air, it may not blow at all as the cold air is removing the Heat. Or it may just take longer for same reason. A slow overload will cause this as well if wiring is tightly bundled.

Always check GOOD fuses
I cannot tell you how many times I have looked at a good fuse, and proceeded to troubleshoot further down for broken wire. As soon as I put a meter or test light on it, bad fuse is confirmed. Element may have a hairline fracture (slight overload with large physical shock). Think lightbulb here. Lightbulb filament is easier to see because is unsupported, but a fuse is self supporting and you can't rattle a fuse. There may be a bad weld between element and terminal post. I have a 15amp fuse that acts good at current under 4 amps. Above that, it acts like a blinker. Thermal expansion / contraction not unlike auto-reset circuit breakers. At 10amps, an attached light becomes kinda strobish.

My favorite diagnostic tool
I use fuses, Nearly religiously, to identify and test unknown wiring. A 15amp fuse works great to make sure you indeed have the furnace signal wires. My meter is rated for 25Amps DC current. I test with 20Amp fuse inline. Fluke ceramic fuses are all kinds of crazy expensive.I have fused brake wiring on trailer axles temporarily to identify which magnet is shorted without pulling all 6 wheels and drums. Otherwise, I haven't seen the break-a-way switch ever fused. If that fuse blows (like shorted magnet) you lose all brakes, not just the 1. I have seen auto-reset circuit breakers but I don't care for those installed in that situation either.

I have also used to confirm a problem with a dimmable light controller coming on by itself briefly. Lights "off" put 1amp in place of light, and now I have proof and confirmation of problem.

But my hands down favorite use of a fuse? To confirm a leaking check valve on water pump. Turn water pump on, let charge lines and stop, then replace fuse with a 1amp. If pump doesnt turn on fuse will still be good. Otherwise, time for another check valve. Similar scenario when fridge may have briefly gone to "dc travel" cooling using 12v heaters when it wasn't supposed to. Confirmed with a 1amp fuse blown during propane operation. Norcold still scratching their heads, but they changed the eyebrow.
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Old 09-27-2020, 08:24 AM   #2
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Interesting stuff - thanks. Iím always looking for ways to improve electrical trouble shooting.
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Old 09-27-2020, 10:26 AM   #3
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Nice write up.
Good info/

I understand your theory about water pump....but pump check valve is NOT only thing that can cause pump to short cycle
City water inlet check valve not closing...drips
Water Heater T&P Relief Valve weeping
Toilet inlet valve leaking....numerous fittings that could be source of leak
FILL valve leaking thru
Pump Short Cycling is due to a leak......from any number of possible sources not just pump check valve
Is it time for YOUR Medication or Mine?
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Old 09-27-2020, 10:32 AM   #4
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Thanks, I put that in a Word Doc and saved it in my book, errrr folder of tricks.
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Old 09-27-2020, 11:20 AM   #5
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Always check GOOD fuses

AKA "The Intermittent Fuse" and "Nobody would believe me"

Your write up is the most complete I have ever seen. Really enjoyed it. My favorite part is the hairline crack and "resettable" fuse. Hard to explain that one unless you have experienced it.
Thanks for the effort.
Phil & Sonia
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RV caught on fire! But it`s OK.... sandstorm came along and put it out!
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Old 09-27-2020, 11:40 AM   #6
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Proper sized fuses weren't always available when having a need back in the 50's and 60's.......so a 1/4" bolt or a wrap of cigarette pack tin foil was often used....
2010 Rockwood Sig Ultra lite 5er 8280......'99 Ram 2500, Cummins. 4x4.....Pullrite Superglide.........
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Old 09-27-2020, 11:49 AM   #7
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Always check new fuses before you need them! And if there is any way that the symptoms could be caused by an open fuses then always check the "looks good" fuses.

Back in the 1970s I spent 4 years working at NASA's Jet Propulsion Lab in Pasadena CA. Despite the name of "Jet Propulsion" we ran all of the unmanned space missions, Houston took care of the manned missions. And if you are ever plan on being in Pasadena in the March-April period then it's worth calling the Lab and asking when the annual Open House is.... and then juggling your schedule... especially if you have kids with you. Call 818-354-4321 and ask for the Public Information Office. The JPL web site is very impressive.

Back then my position was one of 26 techs that took care of all the spacecraft data processing systems... 24x7x366. The JPL designed equipment all used 3AG glass barrel fuses and we learned fast to test EVERY fuse with an ohmmeter before installing... and then to put the new fuse in clip leads and rap one end on a tabletop while watching the ohmmeter needle.

I can't tell you how many new fuses we found that LOOKED perfectly good but were open or intermittent inside.

And we learned that fuses can fail internally and still look perfectly good through the glass barrel.

Personally, I had at least three 5-packs of 1/2a, 1a, 2a, 5a, 10a, 15a and 20a fuses in my roll-around tool cart. When I used up the last one from a 5-pack I'd get another from the stockroom and before it went into the tool cart I'd spend a few minutes testing all 5 fuses.

Looking back, I'd guess that our shop would have a 1/2% to 1% failure rate on new fuses.

And these were USA made Littlefuse or Buss brand units.

Semi-retired technogeek...electronics / computer network / 2-way / ham radio... WA6ILQ
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Old 09-27-2020, 08:12 PM   #8
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A couple of comments from my automotive background.

A fuse can carry 110% of it rating for many seconds (minutes ?) before it blows. It can even carry 200% of its load for up to 5 seconds (which is why fuses in inverter are a joke; transistor fail much faster).

Personally, I hate glass or any kind of small, fragile "cartridge" fuse. To easy to break.

The best way to test a fuse is with a 12V incandescent (not LED) test light. It should be left in the circuit when you believe there is current flowing through. Ground the test light (validate that you have a good ground by testing the light on a known 12V source). ATO "blade" fuses have each side of the blade exposed so that you can test both side.
Retired. 31 year of automotive engineering for one of the Detroit 3, specializing in Powertrain Control Systems.
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Old 09-28-2020, 07:25 AM   #9
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Originally Posted by Old-Biscuit View Post
Nice write up.
Good info/

I understand your theory about water pump....but pump check valve is NOT only thing that can cause pump to short cycle
City water inlet check valve not closing...drips
Water Heater T&P Relief Valve weeping
Toilet inlet valve leaking....numerous fittings that could be source of leak
FILL valve leaking thru
Pump Short Cycling is due to a leak......from any number of possible sources not just pump check valve

You are correct. I didn't go into it, but the problem was city water filling fresh tank up and discharging out the vent. Fill tank, turn pump on, add fuse, and wait. Diagnosing a particular check valve using this method does require eliminating other possible influences.
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Old 10-05-2020, 08:28 AM   #10
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Back when I was an ASE mechanic I learned the hard way to NOT test fuses with a VOM. Some defective fuses will show 0 ohms but will not pass voltage. ALWAYS use a voltage appropriate test light to test fuses.
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Old 10-06-2020, 07:30 AM   #11
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Very good read, should be a Sticky.

Thank you OP
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Old 10-06-2020, 07:51 AM   #12
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i especially liked the part that says test all fuses, even the ones that look good.
and your methods of testing. im printing your post, and keeping it in my book also.
i cant tell you how many ive found that looked perfect, but were defective.
open fuse holders that have a cap, but isnt into place, is my pet peeve. hwh uses a 40 amp fuse with a holder that has a cap. its never replaced, and as it corrodes, the fuse is still good, but the holder is defective. and of course, hwh is at fault.
owners, or techs that fail to reinstall the cap on that fuse holder is a major source of problems.
i really like your post. im keeping it.
paul maddox
i work full time for hwh corp
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