Blown battery fuse question

Steve961Steve961 Posts: 93Solar Expert ✭✭✭✭
I think I may have a problem with my wiring and am looking for some answers. I was installing a GFCI outlet at my cabin, which has a small solar system in it, this weekend and ended up blowing the fuse on my battery. The outlet was wired correctly, but when it was pushed into the box one of the wires came loose and contacted the ground wire. The fuse most likely blew when the circuit breaker was switched back on.

The strange thing about the battery fuse blowing is that there were two much smaller circuit breakers in line before the main battery fuse. This is what makes me think I might have a wiring problem somewhere. And while the fuse did do its job, I'd much rather have a breaker trip than replace a $15 dollar fuse if there's a short.

Here's my power flow from the battery to the outlet:
  1. 104 AH AGM battery. The negative terminal is connected to the ground rod.
  2. Blue Sea terminal fuse block with 80 amp fuse.
  3. Bussmann 60 amp circuit breaker.
  4. Morningstar SureSine 300 watt Inverter. Connected to the ground rod.
  5. Iota automatic transfer switch.
  6. Square-D QO load center. Power is backfed through a 2-pole 15 amp QO breaker. All other circuits run through 15 amp QO breakers. Neutral is bonded to ground only here, and the load center itself is also connected to the ground rod.


Thanks.

Steve

Comments

  • BB.BB. Posts: 24,533Super Moderators admin
    Re: Blown battery fuse question

    How big was the batter fuse? 60 amps or so?

    At the AC outlet, you only have AC Hot, AC Neutral, and a Green Wire Ground that goes back to the ground rod?

    And AC Neutral is bonded to AC Green Wire/Wire to ground rod?

    DC battery negative (or negative bus) is tied to the ground rod?

    There are no other DC 12 volt battery cables in the area of the AC GFI outlet you were working on?

    Where is the DC fuse installed? AC inverter DC input? Or is this to a some sort of charge controller--or just a "master battery bank fuse"????

    My guess is that the Inverter AC Neutral Bonding to DC negative Bonding is not what you think it is... About the only thing I can think of off hand that would pop a 60 amp fuse in the battery bus would be if the Battery Bank was positively grounded and you somehow touched a battery return / negative lead which you thought was at Earth Ground to the AC side of earth ground--Causing current through the fuse.

    Or, you somehow had a 12 + cable from the fuse going near the AC wiring box and you shorted the DC + to earth ground and you caught the 12 volt + cable while working on the AC box. You could also have a nick on a +12 volt cable (after the battery fuse) near some grounded metal... And while working on the outlet, the 12 volt cable was somehow shorted to ground as you were moving AC wires/outlet around (kicked the cable with your foot, etc.)...

    The Morning Star AC inverter DC input is electrically isolated from the AC output... Is it possible that you connected the Iniverter's "Neutral" to the +12 volt battery bus instead of the -12 volt (grounded return bus)? If miss-wired, touching the AC Neutral to Earth ground/metal box could pop the fuse.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Steve961Steve961 Posts: 93Solar Expert ✭✭✭✭
    Re: Blown battery fuse question

    Bill:

    The battery fuse is an 80 amp Blue Sea terminal fuse attached to the positive battery terminal.

    The AC outlet only has an AC neutral, AC hot, and a ground wire that's bonded to the ground bus in the load center which is itself bonded to the neutral bus with a 12 gauge wire.

    The AC neutral is bonded to the ground only at the load center.

    The DC battery negative is tied to ground.

    The cabin is not wired for DC, so there are no DC wires alongside any AC wires. DC is only used for my water pump and a single DC outlet at my power center.

    The main DC fuses for this are the 80 amp at the battery, and another 60 amp breaker between the battery and inverter. There are some smaller DC fuses for the charge controllers, pump, and single DC outlet, although I doubt they could be involved with this.

    My wires are all well organized and color coded, and any error would definitely stand out. I know for a fact that my battery is not positively grounded. I have also tested the AC outlets with a GE outlet tester and they all test as correct.

    Here's a pic of my power center if that would help.

    Attachment not found.
  • vtmapsvtmaps Posts: 3,416Solar Expert ✭✭✭✭
    Re: Blown battery fuse question
    Steve961 wrote: »
    The outlet was wired correctly, but when it was pushed into the box one of the wires came loose and contacted the ground wire. The fuse most likely blew when the circuit breaker was switched back on.

    The strange thing about the battery fuse blowing is that there were two much smaller circuit breakers in line before the main battery fuse.

    I am not surprised that the fast acting fuse blew before the slower breakers. What is most confusing is that any DC fuse or breaker blew at all. If your problem was really with the GFCI, I would expect that the suresine 300 would fault before anything on the DC side.

    Are you sure the problem was with the GFCI? Was everything working before you installed the GFCI?

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • BB.BB. Posts: 24,533Super Moderators admin
    Re: Blown battery fuse question

    I don't think you have utility power--It looks like you have an AC transfer switch and an AC battery charger--Is that to a backup genset?

    Was the generator running at the time?

    There is a two terminal device to the lower left of the AC inverter with some black cables going to it... Is that a Shunt (precision power resistor) or is that a fuse block? Or something else.

    You have a clean installation--I don't see any obvious issues.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Steve961Steve961 Posts: 93Solar Expert ✭✭✭✭
    Re: Blown battery fuse question

    vtMaps:

    I don't think the GFCI outlet itself had anything to do with this, it's just that I was installing one when it happened. The problem is that either the neutral or hot wire, I wish I remembered which one, contacted the ground wire when the outlet was pressed into the box, and when the circuit breaker was enabled it blew the fuse.


    Bill:

    I do not have utility power, but I do have the ability to hook up a Generator to power the cabin and charge the battery. The generator was not hooked up, and I have not even used it for close to a year.

    The two terminal device is just a negative bus bar (Blue Sea) I installed for wiring convenience.


    Thanks for your help.

    Steve
  • BB.BB. Posts: 24,533Super Moderators admin
    Re: Blown battery fuse question

    Which circuit breaker (AC to outlet or DC breaker to inverter)?

    Turn off the DC input to the AC inverter (all AC power off) and take a volt meter and measure the various voltages with respect to your green wire safety ground. Inside your main panel would be most convenient--Just measure green wire ground to Hot and Neutral). If you cannot find any + or - 12 volts--I am not sure what happened.

    It is possible that shorting the output of the MorningStar AC inverter popped the fuse...:

    600 watt surge * 0.85 inverter eff * 1/12 volt battery bus = 50 amps on the DC bus...

    If you had >>30 amps on other 12 volt loads--Or the Morning star could pull >>80 amps for a second (not sure it can)--It is possible the AC short to ground popped the DC fuse...

    If you see a +12 or -12 VDC bias to any AC wires--Then you have something miss-wired between "Neutral" and earth ground (battery somehow in circuit providing output).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Posts: 17,615Banned
    Re: Blown battery fuse question

    How about: a freaky, weird coincidence where the inverter "hot" just happened to be on full (+) output when it contacted the ground which is tied to DC (-) creating enough current across the Blue Sea fuse just long enough to fry it?

    The question now is, with the accidental short removed and the fuse replaced does everything work now?
  • Steve961Steve961 Posts: 93Solar Expert ✭✭✭✭
    Re: Blown battery fuse question

    Bill:

    The circuit breaker I disabled to work on the outlet was the AC to outlet breaker in the load center.

    I will test the wires this weekend when I get back up to my cabin and report back.

    BTW, I didn't have any DC loads running at the time, aside from my analog voltmeter. This also happened mid day when my roof array would have been generating about 7.5 amps or so. My battery was also nearly fully charged as best I could tell.


    Thanks for the ideas.

    Steve
  • Steve961Steve961 Posts: 93Solar Expert ✭✭✭✭
    Re: Blown battery fuse question
    The question now is, with the accidental short removed and the fuse replaced does everything work now?

    When I fixed the short and replaced the fuse everything worked just fine. The funny thing is it did take a bit to figure out the fuse was blown. Since my panels were producing power the inverter kept working and continued to power 2-3 LED lights I had on in the cabin. It's only when I tried to turn on my 45 watt Vornado fan that the SureSine stopped working.
  • CariboocootCariboocoot Posts: 17,615Banned
    Re: Blown battery fuse question
    Steve961 wrote: »
    When I fixed the short and replaced the fuse everything worked just fine. The funny thing is it did take a bit to figure out the fuse was blown. Since my panels were producing power the inverter kept working and continued to power 2-3 LED lights I had on in the cabin. It's only when I tried to turn on my 45 watt Vornado fan that the SureSine stopped working.

    Okay, you just described a flaw in the wiring. The charge controller should go directly to the battery through the controller fuse. The inverter should come off the battery through the inverter fuse. You don't want a situation where the charge controller goes to the inverter and then to the battery. It will work electrically, but it makes it hard to figure things out when something goes wrong/doesn't work right.
  • Steve961Steve961 Posts: 93Solar Expert ✭✭✭✭
    Re: Blown battery fuse question
    Okay, you just described a flaw in the wiring. The charge controller should go directly to the battery through the controller fuse. The inverter should come off the battery through the inverter fuse. You don't want a situation where the charge controller goes to the inverter and then to the battery. It will work electrically, but it makes it hard to figure things out when something goes wrong/doesn't work right.

    I see your point, but that's why I had a 60 amp circuit breaker dedicated to just the inverter. I also have separate dedicated fuses for each charge controller. The 80 amp terminal battery fuse was meant as a last ditch fail safe for the battery. I truly thought the 60 amp circuit breaker would trip before the 80 amp battery fuse blew. I could remove the 80 amp battery fuse which would then do what you want, but I wouldn't be comfortable with it. I would also need to have two battery switches rather than the single one I have now.

    Thanks.

    Steve
  • CariboocootCariboocoot Posts: 17,615Banned
    Re: Blown battery fuse question

    60 Amps @ 12 Volts = 720 Watts. That will never trip on a 300 Watt inverter.

    As a rule, breakers will take longer to trip in an over-current condition than fuses will.
  • Steve961Steve961 Posts: 93Solar Expert ✭✭✭✭
    Re: Blown battery fuse question
    60 Amps @ 12 Volts = 720 Watts. That will never trip on a 300 Watt inverter.

    As a rule, breakers will take longer to trip in an over-current condition than fuses will.

    I tried to size the breaker for the surge capacity of the inverter, which is 600 watts. I had researched this site before getting my breaker and it seems 60 amps is not out of line. I can certainly install a smaller breaker as I can't use more than 300 watts at a time anyways. Could I also install a 100 amp fuse at the battery to create a greater differential between the breaker and fuse? I currently have 6 gauge wire feeding the inverter and DC panel.
  • CariboocootCariboocoot Posts: 17,615Banned
    Re: Blown battery fuse question

    Using the surge rating for sizing breakers is often a mistake. The problem being the inverter can't maintain the surge rating for as long as it takes to actually trip the breaker anyway. Following the NEC derating rule then results in a breaker that provides no protection because the inverter will probably fry before over-current is maintained long enough to trigger the breaker.

    Morningstar actually recommends a 100 Amp fuse on these! They also suggest a 3 Amp on the output which is pretty much a wasted effort too.

    Were it me I would rely on the fact the inverter is never going to be fully loaded, much less overloaded, and size it for nominal Voltage (not minimal Voltage) @ maximum continuous output, or 300/12 = 25 Amps, and "size up" from there to a 30 Amp breaker. That will be sufficient in most cases. A lot of this has to do with interpretation and application and someone is going to say I'm wrong because my plan doesn't follow the minimum Voltage/maximum power rule plus NEC derating. Okay, I've already said it doesn't so don't tell me it doesn't. :roll:

    Oh, and don't scrimp on the wires! They should always be as large as you can fit in the terminals; the fuse/breaker needs to be the weakest point in the circuit. 6 AWG should handle 60 Amps.

    Your battery fuse did what it was supposed to do: popped when the current got too high and kept the wires from frying. If you go up to 100 Amp you're pushing the max current limit of the 6 AWG.

    I can't see your battery switch clearly. Is it ON/OFF or 1/2/BOTH type?
  • Steve961Steve961 Posts: 93Solar Expert ✭✭✭✭
    Re: Blown battery fuse question
    I can't see your battery switch clearly. Is it ON/OFF or 1/2/BOTH type?

    It's a 1/2/Both type, and it's set to 1.
  • CariboocootCariboocoot Posts: 17,615Banned
    Re: Blown battery fuse question
    Steve961 wrote: »
    It's a 1/2/Both type, and it's set to 1.

    Well then, could you wire it so there is the 80 Amp fuse on the battery feeding the center of the switch, then have #1 go through the appropriate size fuse/breaker to the charge controller and #2 through the appropriate size fuse/breaker to the inverter? That way in OFF nothing is connected, in #1 the controller is hooked up, in #2 the inverter is hooked up, and BOTH would be normal operations.

    Possible?
  • Steve961Steve961 Posts: 93Solar Expert ✭✭✭✭
    Re: Blown battery fuse question
    Well then, could you wire it so there is the 80 Amp fuse on the battery feeding the center of the switch, then have #1 go through the appropriate size fuse/breaker to the charge controller and #2 through the appropriate size fuse/breaker to the inverter? That way in OFF nothing is connected, in #1 the controller is hooked up, in #2 the inverter is hooked up, and BOTH would be normal operations.

    Possible?

    In looking at the battery switch wiring diagram, I think it would behave exactly as it is now. I'll probably just switch out my 60 amp breaker for a 30 or 40 amp version - that would be the easiest thing to do.
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