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?

[time2roll]

4' in plain sight, well protected and no danger you can skip the fuse IMO.
But a catastrophic failure would be a massive internal short and at least 900 amps available from the battery to push the magic smoke out.

[Pauljdav]

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.

[Dennis_M_M]

Electricity is made of smoke, when the smoke leaks out it does not work.

But seriously, where there is smoke there can be fire, the fuse protects you from the potential of fire from a dead short.

[joshuajim]

MEXICOWANDERER wrote:
"What Is The WORST Thing That Can Happen?"

Is the prime directive of electrical safety design.

At one time there were special breakers that supposedly could detect arcing while current still was under the max amp trip point, and break the circuit. I've been out of circulation too long I guess. I do not know if these breakers were successful and if they still exist.

Is this what you are referring to? Arc Fault breaker

They are required on bedroom circuits where I live.

[MEXICOWANDERER]

When not having a plan goes bad...

[Chris_Bryant]

mena661 wrote:
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?

The converter side is rarely fused, because the converter is inherently current limited.
The battery side is always fused though.

[eb145]

tenbear wrote:
As a couple of others have said, the battery that the converter is connected to is capable of supplying much more than 55A. I fused my #4 wire with an 80A fuse.

I have heard this situation (an unfused wire connected to battery positive terminal that gets shorted to ground somewhere due to frayed insulation or cut wire), is referred to as a "welder". No way to stop the "welding" from that loose #4 wire till battery dies or something melts in the path of the current.

Ed

[tenbear]

As a couple of others have said, the battery that the converter is connected to is capable of supplying much more than 55A. I fused my #4 wire with an 80A fuse.

[BFL13]

One rule for cars to car audio amp is that the battery fuse must be within 18" of the battery on the pos line. That means the 18" of wire is unprotected, while the rest of the wire to the amp is.

Supposedly that 18" will be in the open , visible, not go into an RV's wall, where it could melt unseen.

There was some argy bargy a while ago about Xantrex's inverter installation instructions requiring fusing depending on the battery's short circuit amps ( IIRC) but nobody could find out what nntheir battery's spec was for that. it looked like Xantrex was way over the top on that requirement , since nobody else required that.

Meanwhile, Randy, at bestconverters.com says in his own write- up for a PowerMax Converter manual (since PowerMax itself doesn't seem to have such a thing 🙂 ) that it does not need a battery fuse at all since it is a "current limited device." Mena seems to be zeroing in on that aspect.

Personally, I gave up on the whole business, and have 150a ANLs on my two #4 wires in parallel on my 2000w inverter (per spec) so I just keep adding any pos wires to the battery on the outer end of those. So whatever happens, if it reaches 150a the ANL on that line will blow. If something like 60a blows too bad I guess. Actually the slide and jacks have their own 30a fuses, the converter has a 50a, and the LP alarm/stereo has a 15a, so that leaves the solar with no fuses except the 150a.

So sue me! It is too complicated 🙂

[wa8yxm]

It is not just the wire.. For example there is that 55 amp CONVERTER, what happens if you accidently plug the batteries in backwards (They are called "Reverse Polarity Fuses" since that's what they are protecting against)

and as Murphy can tell you a 25 dollar transistor will blow in order to protect a 25 cent fuse.

[bdosborn]

mena661 wrote:

MEXICOWANDERER wrote:
What happens if the converter gets mad, and I mean really REALLY mad at you and throws a hissy fit? A direct short with 4AWG is awful, like battery exploding time. Yeah fuses are a pain but what they may protect against is infinitely worse. Remember, Kelly said Murphy was an optimist...

So in this case, you're saying that there could be more than 55 amps drawn on a 55 amp converter? If this is the case, then I finally understand.

A group 27 battery can supply up to 5,000 amps to a solidly bolted fault. It's typical to design for a short circuit level of 65,000 amps for a 480V UPS battery pile.

Bruce

[RJsfishin]

You don't "must have to" do anything when it come to a fuse.
I've never used a fuse on any heavy circuit, like a large inverter etc.
But I have confidence that my installations will not allow a short circuit to happen under normal condition.. The factory MFGs don't use fuses in the battery cable/starter circuits either. If its good for them, its good for me.
Speaking of fuses, I don't have any fuse between the converter and the batteries. Apparently the MFG figured it didn't need one. And who would I be to 2nd guess them.:)

[MEXICOWANDERER]

"What Is The WORST Thing That Can Happen?"

Is the prime directive of electrical safety design.

At one time there were special breakers that supposedly could detect arcing while current still was under the max amp trip point, and break the circuit. I've been out of circulation too long I guess. I do not know if these breakers were successful and if they still exist.

[Gdetrailer]

mena661 wrote:
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?

It is a trick question folks.

Converter will only supply 55A in this case BUT the BATTERIES CAN supply considerably MORE current than the wire can handle.

The fuse at the converter end is to ensure IF the CONVERTER SHORTS that the draw can not exceed the FUSE size on the converter end of things. The fuse on the converter end also ensures all the devices connected Via the fuse panel can not exceed the wire rating from the battery. This fuse also protects the wire from the converter to battery in case of short on the wire.

The fuse or breaker on the battery end is there to protect the wire from the battery to the converter from a short the entire length of that wire. Without that fuse or breaker you could easily have a fire since the batteries will supply more current than the wire could handle.

This is a case of TWO POWER SOURCES folks (battery is a power source and the converter is a power source), when you have any power source there MUST be a fuse or breaker to protect the wire from overheating.

[wildtoad]

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. Thus a fuse sitting between the power source and say a radio is there to protect the radio. A circuit breaker is there to not only protect the devices that may be on the circuit in the event of a short, but also to keep from overloading the wiring and melting. Imagine a 15 amp circuit using the appropriate sized wire and you decide to put 4 coffee pots and a microwave on that circuit at the same time. Without the breaker the draw of current from the five devices would most likely melt the wire and bad things happen.

I guess you could put big honking wire everywhere but not realistic plus you would still need a breaker or fuse.