# Fuse blew for no apparent reason **URL:** \ **Category:** Factual Questions\ **Created:** [January 27, 2005, 11:37pm UTC](https://boards.straightdope.com/t/fuse-blew-for-no-apparent-reason/287008 "2005-01-27T23:37:16Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![Roches](https://avatars.discourse-cdn.com/v4/letter/r/bc8723/32.png) [@Roches](https://boards.straightdope.com/u/Roches)\ **Post date:** [January 27, 2005, 11:37pm UTC](https://boards.straightdope.com/t/fuse-blew-for-no-apparent-reason/287008/1 "2005-01-27T23:37:16Z")
I blew a fuse today – 15 amps, Type “P” (probably a Canadian thing) – by plugging in too many appliances. My fusebox isn’t labeled and I couldn’t tell which fuse had blown, so I had to guess, and I ended up removing a couple fuses that hadn’t failed, then replacing the original fuse. Eventually I found the blown fuse and replaced it. I know why it blew, and I’m not concerned about it. What I am concerned about is that, later in the day, one of the fuses that I took out and replaced failed. It happened spontaneously. I didn’t plug anything in or turn anything on – it just blew. (One of the lights on the circuit flickered just before it happened.) It doesn’t look like the contact inside is broken, but there’s some black material on the contact at the bottom. (I’m not sure if it was there before.) Could this have happened because of something I did when I replaced the fuse (like not replacing it tightly, or getting some moisture on the contact), or is it a sign of a more serious problem? ---
**Author:** ![Fuji\_Kitakyusho](https://avatars.discourse-cdn.com/v4/letter/f/f0a364/32.png) [@Fuji\_Kitakyusho](https://boards.straightdope.com/u/Fuji_Kitakyusho)\ **Post date:** [January 27, 2005, 11:46pm UTC](https://boards.straightdope.com/t/fuse-blew-for-no-apparent-reason/287008/2 "2005-01-27T23:46:49Z")
It is entirely possible that the fuse was faulty. Also, when replacing fuses, it is prudent to reduce the load on that particular circuit as much as possible. Check the fuse rating as well, it might have been the wrong type - fuses vary not only by capacity, but also by response time. Some are intended to open immediately once a certain current threshold is exceeded, while others will sustain a small overload for a set period of time before opening. Have a look at the one you removed to check this. ---
**Author:** ![Roches](https://avatars.discourse-cdn.com/v4/letter/r/bc8723/32.png) [@Roches](https://boards.straightdope.com/u/Roches)\ **Post date:** [January 27, 2005, 11:48pm UTC](https://boards.straightdope.com/t/fuse-blew-for-no-apparent-reason/287008/3 "2005-01-27T23:48:20Z")
One other thing – when I plugged in the iron that made the first fuse blow, there was a burning smell, but it quickly went away. I’m not sure if it was coming from the fusebox or from the iron. I used the iron after the fuse blew with no problems. (There was a space heater on the circuit, so it’s pretty obvious that the blown fuse was caused by overloading.) ---
**Author:** ![danceswithcats](https://avatars.discourse-cdn.com/v4/letter/d/9e8a1a/32.png) [@danceswithcats](https://boards.straightdope.com/u/danceswithcats)\ **Post date:** [January 28, 2005, 2:08am UTC](https://boards.straightdope.com/t/fuse-blew-for-no-apparent-reason/287008/4 "2005-01-28T02:08:35Z")
A type P fuse is ordinary. Are there any appliances on that circuit, such as a refrigerator, or other motor type loads? If so, you might want to replace the type P with a time delay style of equal ampacity. ---
**Author:** ![Skywatcher](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/skywatcher/32/254_2.png) [@Skywatcher](https://boards.straightdope.com/u/Skywatcher)\ **Post date:** [January 28, 2005, 5:30am UTC](https://boards.straightdope.com/t/fuse-blew-for-no-apparent-reason/287008/5 "2005-01-28T05:30:47Z")
> [@Roches](#): > > What I am concerned about is that, later in the day, one of the fuses that I took out and replaced failed. It happened spontaneously. I didn’t plug anything in or turn anything on – it just blew. (One of the lights on the circuit flickered just before it happened.) It doesn’t look like the contact inside is broken, but there’s some black material on the contact at the bottom. (I’m not sure if it was there before.) Could this have happened because of something I did when I replaced the fuse (like not replacing it tightly, or getting some moisture on the contact), or is it a sign of a more serious problem? Could be something as simple as a massive discharge of static electricity. I’ve tripped circuit breakers this way. ---
**Author:** ![aliasalias1968](https://avatars.discourse-cdn.com/v4/letter/a/a8b319/32.png) [@aliasalias1968](https://boards.straightdope.com/u/aliasalias1968)\ **Post date:** [January 28, 2005, 6:01am UTC](https://boards.straightdope.com/t/fuse-blew-for-no-apparent-reason/287008/6 "2005-01-28T06:01:02Z")
Put a new fuse in. Sonds like a bad contact that overheated and carbonized. ---
**Author:** ![casdave](https://avatars.discourse-cdn.com/v4/letter/c/c6cbf5/32.png) [@casdave](https://boards.straightdope.com/u/casdave)\ **Post date:** [January 28, 2005, 2:17pm UTC](https://boards.straightdope.com/t/fuse-blew-for-no-apparent-reason/287008/7 "2005-01-28T14:17:58Z")
Fuses do tend to get tired and worn out, especially if they are carrying a load that will cause them to warm up. The wire, if its just a simple domestic type, still conducts, but becomes brittle with age and load use. Left alone the fuse could still work for years, but moving it can often critically weaken it, and this sounds like what has happened. You get the same effect with incandescant lamps, when they are brand new the element is somewhat flexible and ductile. When you use it, then it starts to become less flexible and brittle. Over time it does slowly weaken and break, but before that happens, its not unusual to use it elsewhere to replace another bulb that has blown, maybe in a more important place. The replacement bulb will often blow in very short order, as moving it has fatally weakend the element. ---
**Author:** ![Roches](https://avatars.discourse-cdn.com/v4/letter/r/bc8723/32.png) [@Roches](https://boards.straightdope.com/u/Roches)\ **Post date:** [January 28, 2005, 11:42pm UTC](https://boards.straightdope.com/t/fuse-blew-for-no-apparent-reason/287008/8 "2005-01-28T23:42:49Z")
There’s definitely a carbon deposit on the bottom of the fuse, and the link inside is still intact, so I think that’s what made it fail. It had been working fine for at least a year before I removed and replaced it, but there must have been something about how I replaced it that made it stop working about 6 hours later. Everything’s been OK so far, and I think I can rule out any electrical problems. ---
**Author:** ![lightingtool](https://avatars.discourse-cdn.com/v4/letter/l/57b2e6/32.png) [@lightingtool](https://boards.straightdope.com/u/lightingtool)\ **Post date:** [January 29, 2005, 12:06am UTC](https://boards.straightdope.com/t/fuse-blew-for-no-apparent-reason/287008/9 "2005-01-29T00:06:38Z")
To expand on **aliasalias1968** ’s post: I think the carbon deposit on the bottom is the best clue. I think the fuse wasn’t inserted properly, which led to poor contact and arcing at the base, which is what caused the carbon deposit. Once the carbon has built up, it blocked the flow of electricity, which also accounts for the flicker you experienced right before the lights went out. You can test this theory by carefully scraping the carbon off the base and using a tester to see if you have continuity through the fuse. If you do, it’s still good, although (to be safe) you should toss it anyway.