AC disconnect fuse keeps blowing - puzzled!

Hi! This is my first post. I've been a long-time lurker on this forum. Maybe you guys have some insightful thoughts for me.

I am based in North Texas. I have a 4.5 kW DC system (Sunny Boy 4000 TL US + 18 x 1SolTec 250W polys in two strings of 9 each). My system is wired into the grid via line-side tap and hence doesn't go through my breaker box. System has been up for the past 9 months and running flawlessly.

I have a 30A Eaton AC disconnect switch between the lineside tap and the inverter, equipped with two 30A time-delay fuses. A couple of days ago the fuses had blown around 9am in the morning. I've replaced the fuses and two days later the same thing happened, oddly at around 9am also. The inverter displays fault code "801 - grid failure - check fuse."

Conditions at 9am: 80 deg. F, inverter starts ramping up production quickly but still only at 1300ish Watts, a third of the maximum wattage. Wiring going from inverter to line-side tap are AWG 8, well above the 30A rating of the disconnect switch.

What could be causing this?

Here is what I have done: Measured grid voltage at time of fuse blowing: 253V. Perhaps a tad high? Should it even matter with fuses? I also checked all screw terminals at the disconnect, and everything is fine. Just replaced the fuses again and the system is working just fine.

Solar installer has been notified and technician will be on site soon to take a look at it.
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Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,049 admin
    Re: AC disconnect fuse keeps blowing - puzzled!

    I have no idea... The minimum fuse/breaker for your system should be:

    4,0000 watts * 1/212 VAC minimum * 1.25 NEC derating = 23.6 Amp fuse/breaker/branch circuit wiring

    So, a pair of 30 amp slow blow fuses should be no problem.

    Running at 253 VAC actually helps a bit because it lowers the average current:

    4,000 watts * 1/253 volts = 15.8 amps typical maximum (depending on how the inverter is rated)

    Nominal rating should be:

    4,000 watts / 240 VAC * 1.25 = 20.8 amps, or a minimum of 25 amp circuit

    Do you have the exact brand/model of slow blow fuses--Sometimes you have to look the specifications up on the web to make sure you have the "right parts".

    Also, does the fuse socket look clean and "normal"? If you see browning/burning, corroded/oxidized sockets-holders--That could heat up the fuse and cause a trip.

    SMA is one of the top quality manufacturers--That side of the system should be fine.

    Just to be sure--This is the AC side of your system--Not the DC/Solar array side (your post sure indicates this is the AC side).

    Good Luck!
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • callmeismaelcallmeismael Registered Users Posts: 12
    Re: AC disconnect fuse keeps blowing - puzzled!

    Hi Bill! Yes, exact matching fuses, both times. Until the technician gets here, I have now put in the regular non-delay fuses since they're considerably cheaper.
    I have now tightened the grip/socket around the fuses by slightly tightening it with pliers. We'll see if that helps. The first pair of fuses came out fairly easily although there was enough a grip on them.

    No corrosion on the fuse sockets. There is some paste on the moving conducting parts of the disconnect but I've seen that paste before on other electric devices such as circuit breaker stabs (e.g. Square D homeline breakers have it on them also).

    Still puzzled!
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: AC disconnect fuse keeps blowing - puzzled!

    What else happens when the fuses blow? As in are there any consistent loads coming on at that time?

    Be sure the neutral lines are checked also.

    And why is this not using a 240 VAC breaker to connect the inverter?
  • BB.BB. Super Moderators, Administrators Posts: 29,049 admin
    Re: AC disconnect fuse keeps blowing - puzzled!

    There is no reason for slow blow fuses on a GT inverter AC output--Motors, maybe. See if you can get the exact part number/brand for the fuses... I wonder if they are rated "different" for steady state loads.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: AC disconnect fuse keeps blowing - puzzled!
    Hi Bill! Yes, exact matching fuses, both times. Until the technician gets here, I have now put in the regular non-delay fuses since they're considerably cheaper.
    I have now tightened the grip/socket around the fuses by slightly tightening it with pliers. We'll see if that helps. The first pair of fuses came out fairly easily although there was enough a grip on them.

    No corrosion on the fuse sockets. There is some paste on the moving conducting parts of the disconnect but I've seen that paste before on other electric devices such as circuit breaker stabs (e.g. Square D homeline breakers have it on them also).

    Still puzzled!
    One other possibility to consider is that something might be happening on the POCO grid at that time each day, such as switching a voltage regulator tap or switching power factor correction capacitor banks in or out. If there is a fault in that equipment it could cause a short term transient voltage on the grid which could cause the GTI to try to source far too much current when the grid waveform changes suddenly and before the protective circuitry cuts the GTI off from the grid.
    It might be difficult to catch that without using a power line monitor that takes samples many times per second or is set up to catch transients.
    SMA SB 3000, old BP panels.
  • DanS26DanS26 Solar Expert Posts: 239 ✭✭✭
    Re: AC disconnect fuse keeps blowing - puzzled!

    Just a guess, but 9am each time might be an indicator of thermal expansion which causes a fault or short L to L or L to N or L to G. Check all wiring closely especially sharp bends or anywhere a line could have been nicked during installation. Look in places where the sun might be causing heating. Loose connections could also cause heating and wire movement.
    18.2kW Kyocera panels; 2 Fronius 7.5kW inverters; Nyle hot water; Steffes ETS; Great Lakes RO; Generac 10kW w/ATS, TED Pro System monitoring with PVOutput.org
  • callmeismaelcallmeismael Registered Users Posts: 12
    Re: AC disconnect fuse keeps blowing - puzzled!

    Cariboocoot: This is not using a regular breaker as it is a line-side tap. It's not going through my breaker box but into a separate junction box on my outside wall that connects the meter with my breaker panel. I don't know what you mean by not using a 240 VAC breaker? Aren't the fuses fulfilling the same role? One fuse on each hot leg? Isn't that the same as a regular double-pole breaker? Also, no significant loads present when it happened. This morning, consumption of the house was approx. 300 watts, production was about 1000 watts.

    BB: I agree that the heavy duty fuses aren't required. No compressor/motor startup current to support, obviously. But shouldn't that make them more robust towards short-lived grid spikes?

    inetdog: This sounds like a possibility that there is something going on with the grid. I've had it happen before that the inverter would occassionally disconnect from the grid for approx. 5 minutes or so and then reconnect. Perhaps overvoltage or something that it didn't like. However, those incidents happened before, only rarely, and don't seem to be connected to the current problem.


    The first set of fuses that blew: Edison time delay dual element rated at 250VAC or less / #ECNR30
    The replacement fuses: Trionic / Ferraz Shawmut dual-element time delay 250 VAC / #TR39R
    The third set (just put it in this afternoon and still running): Cooper/Bussmann general purpose 30A / #NON-30

    Here is a link to my egauge monitor:
    http://egauge7082.egaug.es/

    It has happened on June 11 & 13 around 9am, although not at the exact same time.
  • callmeismaelcallmeismael Registered Users Posts: 12
    Re: AC disconnect fuse keeps blowing - puzzled!

    DanS26: Thermal expansion, this is certainly something I need to check. And yes, this is the east side of the house, so morning heat-up is possible even though I have wide overhangs.
    But then, wouldn't I see a momentary power spike on my egauge monitor due to a short circuit? It captures the entire house consumption plus grid import and export.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: AC disconnect fuse keeps blowing - puzzled!
    Cariboocoot: This is not using a regular breaker as it is a line-side tap. It's not going through my breaker box but into a separate junction box on my outside wall that connects the meter with my breaker panel. I don't know what you mean by not using a 240 VAC breaker? Aren't the fuses fulfilling the same role? One fuse on each hot leg? Isn't that the same as a regular double-pole breaker? Also, no significant loads present when it happened. This morning, consumption of the house was approx. 300 watts, production was about 1000 watts.

    Just because it is a line-side tap is no reason not to use a circuit breaker. The advantage being you would not be replacing anything when it trips, just resetting the breaker. This is not an application where fuses offer any advantage at all.

    Inetdog may be correct that the cause of failure could be the utility doing something and not your household. if so, good luck getting them to admit it or correct it.
  • DanS26DanS26 Solar Expert Posts: 239 ✭✭✭
    Re: AC disconnect fuse keeps blowing - puzzled!
    DanS26: Thermal expansion, this is certainly something I need to check. And yes, this is the east side of the house, so morning heat-up is possible even though I have wide overhangs.
    But then, wouldn't I see a momentary power spike on my egauge monitor due to a short circuit? It captures the entire house consumption plus grid import and export.

    Line side tap...your egauge is downstream from the fault...would not pick it up
    18.2kW Kyocera panels; 2 Fronius 7.5kW inverters; Nyle hot water; Steffes ETS; Great Lakes RO; Generac 10kW w/ATS, TED Pro System monitoring with PVOutput.org
  • BB.BB. Super Moderators, Administrators Posts: 29,049 admin
    Re: AC disconnect fuse keeps blowing - puzzled!

    The time delay fuses look "normal" -- Should never blow with 1.25 (or 80%) NEC derating factor.

    Nick in a wire, arcing at the L1/L2 connections at the inverter (or somewhere down stream of the fuses) would seem to be it... But if you also have a breaker--I would have expected that to trip first--well before the fuses would.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • callmeismaelcallmeismael Registered Users Posts: 12
    Re: AC disconnect fuse keeps blowing - puzzled!

    Some good suggestions! I need to inspect the wire some more and look for kinks and lesions. However, since this system has been running for 9 months and we have had days with much more sun and higher temperature (has been cloudy during the past couple of mornings), it's hard for me to imagine that this is emerging now (as an expansion problem). Wouldn't I see burn marks and/or smell burned wire shell? I remember when my circuit breaker in the kitchen once tripped because two wires in an outlet box had touched and shorted out. I could smell that even after the fact.

    And, yes, I agree that a circuit breaker at the AC disconnect would make much more sense.
    I am tempted to get a pair of 30amp fuses rated for 600V and see if that would solve the problem. Perhaps the 250V fuses are sensitive to voltage above 250? I don't know what their tolerance is.
  • BB.BB. Super Moderators, Administrators Posts: 29,049 admin
    Re: AC disconnect fuse keeps blowing - puzzled!

    Nope--250 VAC fuses are fine for the job.... 600 VDC will not change a thing.

    I have had old (decades old) slow blow fuse fail on heater systems and such... Otherwise--Sounds like something may be wrong (sliced insulation when pulling wire in conduit). Do electricians "highpot" AC branch circuits (should withstand 1,800 VAC).

    Damage of wiring when pulled (sliced by sharp conduit, etc.)--Does happen.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • DanS26DanS26 Solar Expert Posts: 239 ✭✭✭
    Re: AC disconnect fuse keeps blowing - puzzled!

    Do both L1 and L2 fuses blow? Could be a good clue on what is going on.

    If you are satisfied that wiring is not the problem, then the next most probable source of the fault is the inverter.
    18.2kW Kyocera panels; 2 Fronius 7.5kW inverters; Nyle hot water; Steffes ETS; Great Lakes RO; Generac 10kW w/ATS, TED Pro System monitoring with PVOutput.org
  • BB.BB. Super Moderators, Administrators Posts: 29,049 admin
    Re: AC disconnect fuse keeps blowing - puzzled!

    I did have a GT inverter fail (after ~1 year of operation)--It tripped the AC branch circuit breaker. Turned the breaker back on--And the GT inverter failed to operate (error message about AC power failure on LCD panel).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • DanS26DanS26 Solar Expert Posts: 239 ✭✭✭
    Re: AC disconnect fuse keeps blowing - puzzled!

    Another quick question...is your egauge monitor on the inverter output side set up to record current flow in only one direction? If yes could explain why it does not pick up high current flow in the opposite direction.
    18.2kW Kyocera panels; 2 Fronius 7.5kW inverters; Nyle hot water; Steffes ETS; Great Lakes RO; Generac 10kW w/ATS, TED Pro System monitoring with PVOutput.org
  • callmeismaelcallmeismael Registered Users Posts: 12
    Re: AC disconnect fuse keeps blowing - puzzled!

    RE: egauge - it measures bi-directional. I know that because at night I can see the actual measurements of what the inverter pulls from the grid, approx 1.2 Wh per hour. If there were short-circuit current flowing from the meter towards the inverter egauge should pick up on it. (egauge claims to be revenue-grade, so it is very sensitive to small amounts of current, measures down to mWh)

    System has been running fine today. I haven't inspected all the wiring yet. Only the visible parts. I am still hesitant about pulling the wiring out of the conduit. Will wait with that until the next occurence, I guess. I hope it's not the inverter's fault. I would hate to have it replaced that early.

    I tightened the grip on the fuse socket. I am not sure whether that made a difference or not. I would love to think so but probably not a decisive factor. Inverter measured grid voltage at a max. of 250V this morning around 9 am (the time of the past two occurences), that's less than the 253V I measured after a previous incident. Again, not sure if that matters at all.
  • callmeismaelcallmeismael Registered Users Posts: 12
    Re: AC disconnect fuse keeps blowing - puzzled!
    DanS26 wrote: »
    Do both L1 and L2 fuses blow? Could be a good clue on what is going on.

    If you are satisfied that wiring is not the problem, then the next most probable source of the fault is the inverter.

    Yes, both L1 and L2 were blown. Could this be taken as an indication that this is NOT a line to ground OR line to neutral short?
  • BB.BB. Super Moderators, Administrators Posts: 29,049 admin
    Re: AC disconnect fuse keeps blowing - puzzled!

    Pretty much a Line to Line "short"... Even a bad/weak fuse holder or running fuses are near their rated current would not normally cause a pair of fuses to blow.

    Your line voltage is no problem. Nominal AC line is ~212 to 264 VAC for a 220/240 volt mains.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • DanS26DanS26 Solar Expert Posts: 239 ✭✭✭
    Re: AC disconnect fuse keeps blowing - puzzled!
    Yes, both L1 and L2 were blown. Could this be taken as an indication that this is NOT a line to ground OR line to neutral short?

    Yes, line to line as Bill says.

    Have you delved into all the fault codes on the inverter? Sometimes the true fault is buried under the latest fault code and exists in history.
    18.2kW Kyocera panels; 2 Fronius 7.5kW inverters; Nyle hot water; Steffes ETS; Great Lakes RO; Generac 10kW w/ATS, TED Pro System monitoring with PVOutput.org
  • callmeismaelcallmeismael Registered Users Posts: 12
    Re: AC disconnect fuse keeps blowing - puzzled!

    How can I retrieve past faults from the inverter?

    I double-checked all the visible wiring, and everything looked great. Today went by without any problems.
  • DanS26DanS26 Solar Expert Posts: 239 ✭✭✭
    Re: AC disconnect fuse keeps blowing - puzzled!
    How can I retrieve past faults from the inverter?

    I double-checked all the visible wiring, and everything looked great. Today went by without any problems.

    Qualification..I'm not familiar with the SMA transformerless inverters.....but Page 15 of the User Manual (see Items H and K) indicates error events are being recorded and thus a history exists. It may only be accessible by a SMA qualified technician.

    http://files.sma.de/dl/18726/SB3-5TLUS22-BA-en-13W.pdf
    18.2kW Kyocera panels; 2 Fronius 7.5kW inverters; Nyle hot water; Steffes ETS; Great Lakes RO; Generac 10kW w/ATS, TED Pro System monitoring with PVOutput.org
  • callmeismaelcallmeismael Registered Users Posts: 12
    Re: AC disconnect fuse keeps blowing - puzzled!

    Now, here's another thing that got me thinking. I believe that the preceding nights to these two events we had thunderstorms in the area. The second one saw an extremely close and loud lightning strike. The power went out for a while. Is it possible that these fuses got somehow compromised by this and then the next morning, after a bit of load was placed on them, finally blew? Would that explain it? None of my circuit breakers tripped, though, and no devices were damaged.
  • callmeismaelcallmeismael Registered Users Posts: 12
    Re: AC disconnect fuse keeps blowing - puzzled!

    Solar technician will check my system next week. He said he had a couple of ideas what it could be. The third pair of fuses is still going strong. System has been operating normally. I hope it's nothing major. I will post an update to this thread next week. Thanks for all of your helpful suggestions. I will share them with the technician.
  • callmeismaelcallmeismael Registered Users Posts: 12
    Re: AC disconnect fuse keeps blowing - puzzled!

    Technician came and looked at the unit. Couldn't find anything. Called SMA and was advised to not even bother reading out fault codes but just to install the replacement unit they will ship. No further questions asked by SMA. They noted that they had not encountered this problem before, and it was agreed upon that the short must have happened somewhere between inverter and AC disconnect (which are connected by two ft. of wire). Wire is fine, disconnect switch is fine.

    On the one hand, I am happy that SMA wants their customers to be happy. On the other hand, the system has been working flawlessly since these two occurences, and I wonder if we're just throwing equipment at the problem (as we keep wondering what the problem might be).

    Our electrical delivery company is doing a large-scale upgrade of our neighborhood's grid next week. Who knows if there were problems with the grid that caused this. Hopefully, I'll never find out (again).

    I wished I had more to report!
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: AC disconnect fuse keeps blowing - puzzled!

    Our electrical delivery company is doing a large-scale upgrade of our neighborhood's grid next week. Who knows if there were problems with the grid that caused this. Hopefully, I'll never find out (again).

    I wished I had more to report!

    Oh boy; that's always fun. After the 'upgrade' you may have new and different problems!

    I really hope you don't.
  • BB.BB. Super Moderators, Administrators Posts: 29,049 admin
    Re: AC disconnect fuse keeps blowing - puzzled!

    SMA makes quality equipment.

    And there should not be any way that the utility can "fault" the SMA and blow the fuses--Without taking out other appliances/circuits in your home.

    Hopefully all goes well for you. I had a ~5 year old GT inverter replaced because of possibly long term reliability issues from manufacturing problems--Was working just fine. The "new and improved" unit faulted (actually tripped the 20 amp branch breakers to the GT inverter--Sort of like yours) after about 1 year (and remained dead). The third unit (old/new stock) is working well years later. :roll:

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • DanS26DanS26 Solar Expert Posts: 239 ✭✭✭
    Re: AC disconnect fuse keeps blowing - puzzled!

    Hard to believe that the tech did not read any of those fault codes....and then SMA replaces the unit no questions asked? Companies just don't operate like that...and stay in business in the long run. Something is missing.
    18.2kW Kyocera panels; 2 Fronius 7.5kW inverters; Nyle hot water; Steffes ETS; Great Lakes RO; Generac 10kW w/ATS, TED Pro System monitoring with PVOutput.org
  • callmeismaelcallmeismael Registered Users Posts: 12
    Re: AC disconnect fuse keeps blowing - puzzled!

    The technician told me that he was advised by SMA that a lot of the fault codes do not necessarily tell precisely what's going on, so are not helpful in finding the cause for the fault. I suppose that the SMA people didn't want to take any chances as this was a two-time line to line fault. I have no idea whether that's good business practice on SMA's or the installer's side. I am just glad that SMA will provide another inverter for me.

    Since nothing else in the house tripped, I do not really believe this is a grid fault either. It's hard to imagine for the grid to cause a short in my lines.

    Now, the wire in the conduit has not been pulled and visually inspected. However, the technician tugged at it during operation, moving it around some. He's not an electrician, so I guess he didn't feel comfortable pulling it out.

    However, I believe if wire insulation damage had been the cause of this, through thermal expansion / movement or whatever this problem would have showed up again by now. Besides, each time you burn up insulation during a short and usually increase the risk of having this reoccur dramatically. Also, I did not notice any burnt plastic smell either. Usually, you can smell a short like that.
  • DanS26DanS26 Solar Expert Posts: 239 ✭✭✭
    Re: AC disconnect fuse keeps blowing - puzzled!
    Oh boy; that's always fun. After the 'upgrade' you may have new and different problems!

    I really hope you don't.

    Here's my experience with an inverter fault. About 5 months into commissioning one of my string inverters threw a "relay" fault. Called the manufacturer and had a replacement power unit within two days. Great service. But less than 10 days later the replacement unit threw a "temperature" fault. Called the manufacturer and they said they would replace the replacement.....but this time I requested a "new" inverter power unit fresh off the line rather than a refurbished unit. They baulked at first but then relinquished.

    No problems since. But this experience does bring up the point that warranty replacement is not necessarily new equipment and that you may be bringing in someone else's problems like you said.

    It's easy to replace a string inverter power unit. I can see some major problems though for micro-inverters and optimizers when they start getting to the end of their useful lives. It's all going to come down to the labor involved....and do you replace just one or all?
    18.2kW Kyocera panels; 2 Fronius 7.5kW inverters; Nyle hot water; Steffes ETS; Great Lakes RO; Generac 10kW w/ATS, TED Pro System monitoring with PVOutput.org
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