The Wrong Fuse

[Optimistic Paranoid]

<p>There are a number of threads here that talk about choosing inverters, and most, if not all of them, mention the necessity of putting a fuse in the wire between the battery and the inverter.</p><p>I haven't seen any discussion of the TYPE of fuse, or the fact that not all fuses are equal.</p><p>I have a copy of the Blue Sea Catalog (electrical components) and they have several pages devoted to fuses of many types:</p><p>MAXI Fuses range from 30 to 80 amps and have an interupt capacity of 1,000 amps.</p><p>MIDI or AMI fuses range from 30 to 200 amps and have an interupt capacity of 5,000 amps.</p><p>MEGA or AMG fuses range from 100 to 300 amps and have an interupt capacity of 2,000 amps.</p><p>ANL fuses range from 35 to 750 amps and have an interupt capacity of 6,000 amps. The Blue Sea catalog says that these</p><p>"meet the ABYC requirements for main dc circuit protection on large battery banks."</p><p>Class T fuses range from 225 to 400 amps and have an interupt capacity of 20,000 amps. The Blue Sea catalog says that these</p><p>"Have an extremely fast short circuit response." and are <strong><span style="text-decoration: underline;">"RECOMENDED BY MOST INVERTER MANUFACTURERS." </span></strong></p><p>Wikipedia says the following about interupting capacity:</p><p><em>"Breaking capacity or interrupting capacity is the current that </em><em>a fuse, circuit breaker, or other electrical apparatus is able to interrupt without being destroyed or causing an electric arc </em><em>with unacceptable duration. The prospective short circuit current which can occur under short circuit conditions should not exceed the rated breaking capacity of the apparatus. Otherwise breaking of the current cannot be guaranteed."</em></p><p>We know that under certain conditions of voltage and amperage, electricty can flow across an air gap. Spark plugs are an obvious example, and arc welders depend on this fact to melt steel and weld metals.</p><p>It looks to me like if a powerful inverter fails due to a short circuit, and you have it fused with the wrong kind of fuse, the fuse may blow, but the current may arc across the blown fuse and continue to flow, causing a fire which will probably destroy your vehicle.</p>

 

[highdesertranger]

not saying you don't need the correct fuse but never heard of a 12v system jumping a blown fuse.&nbsp; to jump a gap you need allot of volts not allot of amps.&nbsp; &nbsp;I am not an electrical engineer.&nbsp; so I might be wrong.&nbsp; I say this from experience not a book.&nbsp; so maybe someone else could chime in.&nbsp; highdesertranger&nbsp;

 

[DollarJoe67]

highdesertranger is correct.&nbsp; Without getting too technical, very high voltage is required to create a path for current to travel.<br><br>Voltage can "jump"&nbsp; between two differences of potential.&nbsp; Like the zap when you touch a door handle.&nbsp; Current is highly less likely.&nbsp; Current flows through the path of least Resistance.&nbsp; Air is high Resistance vs. solid wire.&nbsp; It would require extremely high power output (and source)&nbsp;to continuously arc through air and cause damage.&nbsp; I am unfamiliar with interrupt ratings, but a 1000 amp interrupt rating seems to require that much current to "jump" and cause damage.<br><br>For instance:<br>A 30amp fuse (500amp interrupt) blows&nbsp;with a&nbsp;200 amp surge and equipment is protected.&nbsp;&nbsp;A 600 amp surge will blow the fuse but may still cause damage to equipment. <br><br>The hypothetical amount of current and voltage&nbsp;we are considering is more likely to cause a direct short from an improper fuse rather than arc through air after blowing the correct fuse.<br><br>Also, batteries don't push current.&nbsp; A load draws current.&nbsp; So, unless an inverter itself&nbsp;places a load on the battery, they may be connected all day and night without worry.&nbsp; But the moment a radio, lamp, or other device is plugged into the inverter and turned on, current flows.&nbsp; Thus the protection of a fuse or circuit breaker.&nbsp; With all this being said,&nbsp; the "correct" fuse is dependent on the unique load(s) connected to the inverter or the maximum current capacity of the inverter, whichever is less.<br><br>Hope that helps.

 

[Optimistic Paranoid]

<p>I'm not an electrical engineer, or a licensed electrician, or anything like that; just a guy who's been tinkering with electrical stuff for the last forty years. Since I haven't killed myself or anyone else and haven't set anything on fire, and the stuff I put together usually works, I would go so far as to call myself a succesful tinkerer. But I definitely don't claim to know it all.</p><p>An inverter takes 12 volt dc and changes it into 120 volt ac. Big inverters actually have a grounding lug that is supposed to be attached to the vehicle ground (chassis). The ABYC code specifies that this grounding wire be the same size as the dc positive and negative feeds.</p><p>So if a dead short develops, and the unit starts dumping 120 volts to the chassis ground - and from there back up into your battery bank, which is also attached to the chasssis ground, how does that change the calculations as to how much current and voltage is going through the fuse to the inverter? I don't know, but it can't be good.</p><p>Could a feedback loop occur so that the current and voltage keeps going up, up, up?</p><p>The Blue Sea catalog said that, in addition to having 20,000 volts interupting capacity, Class T fuses also have a "very fast response to short circuits which protect high amperage electronic equipment such as inverters."</p><p>Maybe it's their very fast response time rather than their high interupt capacity that makes them the fuse of choice for inverters?</p><p>Regards</p><p>John</p>

 

[Optimistic Paranoid]

<p>In the book RV ELECTRICAL SYSTEMS: A Basic Guide to Troubleshooting, Repair, and Improvement by Bill and Jan Moeller, there is a short section on DISASTER FUSES.</p><p><em>"An important item - usually missing - is a fuse or circuit breaker to protect the RV from possible destruction in the event of a major electrical problem. Batteries can short out internally from excessive heat or overcharging, literally melting down. If converter/chargers and inverters short out, the resultant rush in current flow can cause wiring to become so red hot that insulation melts. These situations can result in fires, and a fire can quickly consume an RV</em></p><p><em>A fuse to prevent these occurrences is aptly named a DISASTER FUSE. The amperage of the disater fuse should depend on the number of batteries in the system or the maximum current output of the equipment it is to protect. (Manufacturers recomend that one Group 27 battery should not sustain a load of more than 50 amperes for more than a short interval) We use a 100 ampere fuse to protect our system because we have two Group 27 batteries (i.e., 50 amperes for each battery).</em></p><p><em>The disaster fuse should be mounted in the negative line going from the batery bank's minus post to the chassis ground post."</em></p><p>I've had this book for several years, and personally, I never got why you would put a fuse in the ground wire when you've already got a fuse in the hot wire. Now that we're considering how a short circuit in an inverter would go to chassis ground, the lightbulb in my head starts to come on.</p><p>Regards</p><p>John</p>

 

[DollarJoe67]

Very good information.&nbsp; That's a "Final Destination" scenario.&nbsp; Seems your question was answered.&nbsp; To prevent the disaster scenario, place a&nbsp;fuse on the ground and hot line.&nbsp; A full size RV is a house on wheels so it definitely could draw enough current for the disaster scenario.<br><br>Essentially, the chassis becomes a short between battery and inverter because both are grounded to the chassis.&nbsp; BATTERY&gt;INVERTER&gt;CHASSIS&gt;BATTERY (Assuming the malfunction has ocurred)<br><br>The protected circuit would look like this:<br>CHASSIS&gt;FUSE/CB&gt;BATTERY&gt;FUSE/CB&gt;INVERTER&gt;CHASSIS<br><br>I liked "<em>Batteries can short out internally from excessive heat or overcharging, literally melting down. If converter/chargers and inverters short out, the resultant rush in current flow can cause wiring to become so red hot that insulation melts"</em> because it identifies the battery as the failing component.

 

[Zil]

For huose batteries, the marine industry and the solar industry use ANL fuses sized to protect the gauge of battery cable wiring, located as close to the battery positive post as possible. This is to protect from catastrophic failure (direct short). Components each have their own fuse, sized to the equipment specs. <br><br>There is another discussion about the best make of fuses.

 

[James AKA Lynx]

Fuse as close to the device as you can.<br><br>Check your manual or the manufacture for the proper fuse and type.<br><br>Please fuse and not breaker. I have found them not to be reliable in the long run on my boat..<br><br>James AKA Lynx

 

[bee]

I would be curious to see who these "inverter manufactures" are that specify class T fuses. &nbsp;Sounds like marketing bullcrap to me, look at the price difference. &nbsp;Not saying that a fast blow fuse isn't safer. &nbsp;But I am sure ANL are fast enough, ABYC seems to think they are fine.<br><br><br>Just to add to the post above mine, I had a circuit breaker issue today. &nbsp;It was a marine style 150w breaker. &nbsp;It turns out that my volt meter shows 12v whether it was on or off, not saying that it wasn't working, but it made diagnosing my problem really confusing till I figured that out.

 

[Timmymac]

Putting a fuse or circuit breaker in the negative wiring is usually a bad idea.&nbsp; The reason is that most of us are using negative grounded systems, and the fuse/breaker will exhibit a voltage drop under load.&nbsp; So, the ground at one point in your system may not be the same as the ground at another point.&nbsp; These differences in the ground voltage cause what's called a 'ground loop' which can cause noise in audio equipment, and can cause difficult to diagnose issues in digital equipment (imagine that when the printer starts printing, your laptop spontaneously reboots).&nbsp; <br><br>All fuses should be in the positive wiring, and disaster fuses should be located within 12 inches of the battery terminal (as close as practical).&nbsp; Always, Always, Always install a high current rating fuse at the battery.&nbsp; An electrical fire will ruin your day.<br><br>One thing that is frequently overlooked is fusing the solar panel.&nbsp; In Hulk, I installed a 30 amp fuse in the positive power wire of the solar panel.&nbsp; In the event of a short circuit in the wiring, the fuse will blow before the wiring and solar panel are damaged.&nbsp; Of course, this doesn't protect against faults internal to the solar panel, but wiring issues do happen...<br><br>

 

[Zil]

If the wiring is proper, a catastrophic fuse can be used on the negative cable at&nbsp; the battery bank, or on the positive at the bank. This is to protect against a major short. Must be sized to protect the max gauge wire in your system. This is a separate fuse from the smaller fuses used at each 12v device.<br><br>While 12v fuses may not arc they can burst and ignite fuel vapors. (as in the hold of a boat)

 

[cygvan]

i use an mrbf terminal fuse block attached to the battery post with mrbf fuses.<br><br>the model i use is a dual position block and i have a 100amp fuse for 12v distribution line and 200amp for hot lead to inverter (per manuf).<br><br>i found them very convenient and easy to install.

 

[FALCON]

Regarding Class T Fuses for Inverters, I spoke with technical support at Blue Sea and then with an engineer at Samlex today. Here is what I learned:

FROM BLUE SEA - CLASS T FUSE SIZES:
The reason I called is because I tried to use a Class T fuse, as specified in their manual, and ran into sizing trouble. I bought a Blue Sea Class T fuse holder and a Bussman 70a Class T fuse. That 70a Bussman class T fuse doesnt fit in that Blue Sea Class T fuse holder. Why? Because there are actually two different sizes of Class T fuses. The "smaller" size is generally less than 200a, and the "larger" sizes are over 200a. Blue Sea used to make fuses and holders in both size ranges but they sold very few in the lower range because people bought a different (cheaper) type of fuse. So now they only make the "large" holder and fuses. I'm using quotations for the sizes because my "small" 70a Bussman fuse is actually LONGER than what would fit in the "Large" Blue Sea Class T fuse holder. I learned all of this by calling Blue Sea.

FROM SAMLEX - INVERTER FUSE TYPE
I started telling the Samlex guy about the sizing problems I ran into and without me even asking whether Class T are really needed, he said that MRBF fuses and ANL fuses are both ok to use with (their) inverters. He said that the inverter has internal fusing to protect the device itself, and that the fuse is strictly for protecting the wire (and, I assume, this is at least part of the reason that other fuse types are actually ok). He did ask about my wire sizing and said that since I'm using such a large cable (2AWG) that I could actually use a 200a fuse. So now I need to decide whether to order a 200a Class T fuse (which would be the "larger" size and fit the Blue Sea Fuse holder I already have, or return my fuse and holder and use ANL or MRBF instead).

I'm wishing now that I has asked him why the manual makes it seem like it's so important to use Class T fuses.

**(I did specify at the start of the conversation that I have a 600w inverter. He didn't say so, but it could be that some of his advice was given a specific way because of the size of my inverter. I didn't get any indication of that, but I'm just throwing it in here for full clarity)

Edit - Adding - I compared Class T, ANL, and MRBF costs for the fuse blocks and fuses, and decided to use ANL. They are cheaper than the others, but not by a huge amount. I was checking just Blue Sea prices.

 

[ccbreder]

Can anyone demonstrate how the 120 volts from the inverter can get back to the battery? It can't happen in a properly wired system.

 

[Optimistic Paranoid]

Can anyone demonstrate how the 120 volts from the inverter can get back to the battery? It can't happen in a properly wired system.

It can't happen under normal circumstances.

My understanding is that it CAN happen if there is a catastrophic failure of some of the internal components inside the inverter.  Not being an electrical engineer, I am unable to say exactly which component has to fail, or otherwise explain exactly what happens when it does.

Best I can say is that DC goes in to one side of the box and AC comes out the other, and something fails which allows the DC to still be converted to AC, but instead it loops back to the DC side wiring.

Regards
John

 

[ccbreder]

Stop worrying people about that. It can't happen.

 

[Optimistic Paranoid]

Stop worrying people about that. It can't happen.

Of course it can't.  The quality of made-in-China inverters is SO high that none of the internal components could EVER fail.

Regards
John

 

[VJG1977]

Of course it can't.  The quality of made-in-China inverters is SO high that none of the internal components could EVER fail.

Regards
John

While any component could fail most of those failures would result in the shutdown of the inverter.  You are more to win the Lottery than to see 120 AC fed back to the battery.

 

[Optimistic Paranoid]

 You are more to win the Lottery than to see 120 AC fed back to the battery.

Let's not confuse the two words "unlikely" and "impossible".  They are not synonyms for the same thing.

To be fair about it, I suppose we could use a lightning rod as an analogy.

We know that lightning can strike buildings.  We know that lightning strikes can start fires that destroy buildings.  We know that lightning rods work, and protect buildings from burning down.

Yet most people DON'T have lightning rods on their houses.  They rely on the unlikelyhood of lightning striking any one building.  And most people get away with relying on that unlikelyhood.

But there are a few of us who just believe in doing things right.  A catastrophe fuse is cheap enough insurance against something that is unlikely but not impossible.  It gives us one less thing to worry about.

Regards
John

 

[VJG1977]

Let's not confuse the two words "unlikely" and "impossible".  They are not synonyms for the same thing.

To be fair about it, I suppose we could use a lightning rod as an analogy.

We know that lightning can strike buildings.  We know that lightning strikes can start fires that destroy buildings.  We know that lightning rods work, and protect buildings from burning down.

Yet most people DON'T have lightning rods on their houses.  They rely on the unlikelyhood of lightning striking any one building.  And most people get away with relying on that unlikelyhood.

But there are a few of us who just believe in doing things right.  A catastrophe fuse is cheap enough insurance against something that is unlikely but not impossible.  It gives us one less thing to worry about.

Regards
John

I am not saying, not to use fuses. Any and every conductor leaving a power source should be fused with a correctly sized fuse.  That is a size that will blow before the conductor over heats.  I have never seen a need to fuse the ground side of the battery.  Individual pieces of equipment, I trust the manufacturer to specify conductor size and fusing at the input or output and would comply. 
What I am saying is that any component failure within an inverter that fed 120 volt AC back to the 12 volt DC side should kill the inverter.  I would not be so bold as to say it could never happen.