The Point of Fuses

[mena661]

Fuses are to protect wiring, correct? If that is really so then I have a question posed as an example. Suppose I have a 55 amp converter. It is connected to a bank of batteries with 4 ft of #4 wire. ~180 amps is the max rating for 105C #4 wiring. What would be the point of fusing this wire if a catastrophic failure of the unit only sends a max of 55 amps down the wire?

[MEXICOWANDERER]

"Oh, sixty different ways to say the same thing ;)"

[Bob_Landry]

A wire can get chaffed or pinched anywhere between the source and the load. That's the purpose of having protection as close to the source as possible. Fuse size also becomes critical because if you over fuse a given size wire and there is a problem, you want the fuse to blow before the wire has time to overheat to the point of burning insulation and possible causing a fire.

[Alan_Hepburn]

wildtoad wrote:
To my way of thinking fuses/circuit breakers have two functions. One is to protect the device at the end of the wire from getting too much current and frying.

Not really - current doesn't get pushed down a wire; it gets pulled by the load. And if the load is getting too much current it is because there is a problem in the load, and that problem is one of the things that the fuse/circuit breaker is there for.

[mena661]

Thank you much Bob!!! Makes total sense to me. I just wasn't getting it but that's pretty simple to understand.

[Bob_Landry]

The purpose of a fuse or circuit breaker is always to protect the wire, and it needs to be a close as possible to the electrical source. In the marine trade, ABYC is very specific about this required distance for both shore power and DC power sources and it varies with whether or not the wire is in a protective sheath or cover. The fact that so may inches of wire are visible or you think it is sufficiently protected is irrelevant. If there is a short on a battery cable, even the largest cable will turn white hot in seconds and will take a long time to burn through. A lot of heat can be generated in that short time, easily enough to cause a fire. The fact that you may be using heavy gauge cable is even more reason to be fuse or C/B protected because it will take longer to burn through. You might want to do a little reading on reserve capacity. it states how long a battery will put out power at a given amperage and it's usually very high amperage for a pretty long period of time. Anyone who thinks it's OK to not use fuse protection is either very uninformed or very lucky.

BTW, the small fuse mentioned on the back of your stereo does protect the device from a large electrical jolt, but it does nothing to protect the wire between the battery and the radio and that is still subject to a shorted condition and the loss of smoke.

[BFL13]

mena661 wrote:

roam1 wrote:
Depends on where the fuse is.
If its near the power source like the battery or the distribution panel, it is there to protect the wiring. That wiring may be supporting many devices, like lights, sensors, stereo......If it is near the device, like the small fuse near your radio, it is there to protect the device. Both are prudent!

Sorry I've been absent here but this seems like the most reasonable explanation for having a fuse. I don't have a fuse on the converter output. I do have one on the battery output but it's sized for the inverter load. I will say I'm still confused on the need for a catastrophic fuse cause I don't understand how a shorted battery can send current outside of the battery if there's no device to draw it. Can someone explain that one?

My version of that is that the battery is its own draw 🙂

You have a wire from pos to neg posts (American) or neg to pos (Canadian) on the battery posts making a circuit through a device which is off (no draw), but the connections are still there.

Now inside the batt a cell shorts its pos and neg plates. This "closes" the loop (instead of the device being turned on) and "makes the circuit" using the battery posts, not the internal cell plates that are shorted.

Now on standby, waiting to hear how it really works! 🙂

[MEXICOWANDERER]

The owner of a 70' trawler under construction "borrowed" my 1/2" Snap-On" combination wrench and shorted it out against the hull. Trouble was there were 8 8-volt batteries (32 volt system). The wrench instantly turned white hot after a blue flash. It burned a huge hole through a 5/16" steel hull.

"You owe me a wrench John"

"No way. And how much is it going to cost to repair three melted terminals on my new batteries, and the wiring?"

"A hun-- I mean, three hundred dollars" (this was thirty years ago).

He "knew-it-all" "I-don't-gotta-disconnect no stinking ground cables"

"Ignore" and "Ignorance" are two closely related words...

[Bob_Landry]

Some of these posts are simply amazing.

[bdosborn]

MEXICOWANDERER wrote:
Delivered

For

The

Shortest

Of

Whiles

Only if you have a fuse. Don't you remember when the wrench you were tightening the battery + terminal with touched the car body? 😉 I welded the wrench to the fender and burned myself trying to tear it off.

Bruce

[MEXICOWANDERER]

Delivered

For

The

Shortest

Of

Whiles

And Joshuajim thank you. Yup, I'm a wooly mammoth caught in the big freeze. No longer receptacles but outlets huh? Been NEC code since 1999. Where the %$#@! is my buggy-whip?

[bdosborn]

Battery cable to the inverter rubs against a grounded metal frame until insulation is breached. Boom, 5000 amps of short circuit current is delivered in a most energetic fashion.

Bruce

[mena661]

roam1 wrote:
Depends on where the fuse is.
If its near the power source like the battery or the distribution panel, it is there to protect the wiring. That wiring may be supporting many devices, like lights, sensors, stereo......If it is near the device, like the small fuse near your radio, it is there to protect the device. Both are prudent!

Sorry I've been absent here but this seems like the most reasonable explanation for having a fuse. I don't have a fuse on the converter output. I do have one on the battery output but it's sized for the inverter load. I will say I'm still confused on the need for a catastrophic fuse cause I don't understand how a shorted battery can send current outside of the battery if there's no device to draw it. Can someone explain that one?

[Gjac]

Pauljdav wrote:
No fuse and the worst thing that will happen?
Fire, The battery would put out enough amps to cause a fire if the wire shorted to the trailer frame.

While working on my steps this winter I kept blowing 15 amp fuses to my ignition wire. After much searching using long wires and alligator clips I traced wire from the fuse box to the point of origin I never saw the problem until I opened my slide and found the wire had been caught by the slide mechanism and grounded to the frame. The mechanism was also rubbing on my house battery cable which could have had the same fate. I never would have thought about this as a problem until it happened to me.

[roam1]

Depends on where the fuse is.
If its near the power source like the battery or the distribution panel, it is there to protect the wiring. That wiring may be supporting many devices, like lights, sensors, stereo......If it is near the device, like the small fuse near your radio, it is there to protect the device. Both are prudent!