AC disconnect fuse keeps blowing - puzzled!

HarveyJ

I found this Forum and joined when I Googled “Fuses in my solar array disconnect box keep blowing”.

My name is Harvey, and I live in Las Vegas. I currently have an 18.7 KW solar system (68 panels, 325 W per panel). This array was installed by a local company (Today’s Energy Store) in February of 2015, and has been functioning without issues since the array came on-line in March of 2015. Unfortunately, in the last several months, the fuses in my Disconnect Box have blown on several occasions. In both cases, my system was off-line for several days due to my inattention. If you are anything like me, downtime is akin to throwing money away. The ROI cost model that I used to justify this system depends on squeezing the most energy out of the available sunny Las Vegas days. When I first installed the system, I looked at my power generation at least daily (maybe hourly ). But, since everything was running so well, I turned my attention to other projects and it has recently cost me several days of “bankable energy”. Currently, my only solutions is to be more attentive (monitor my power generation several times a day) and keep several boxes of spare fuses on hand. I’m a retired Electrical Engineer, and I hate problems that can’t be resolved except through the use of “band aid solutions” (i.e., more boxes of fuses).

I’m hoping someone on this Forum can explain to me “the why” in terms that can be directly tied to basic electrical fundamentals. I have read this entire content of this particular forum topic and will follow-up on some of the suggestion as they may relate to my system, So far, nothing has “jumped out” at me. Here are a few questions…

• The fuse blows because the current through it (in either direction) exceeds some threshold for a certain time period that is sufficient to melt the fuse and open the circuit. Obviously, the bulk of the current flowing through the fuse is from the solar panels to the electrical grid. Some power (I wouldn’t think very much) must flow from the grid to the micro inverters to maintain phase synchronization.  Is this a fair representation?

• What circumstances would create excessive currents in either direction?

• I’m currently using fuse protection (it is not exactly clear what I’m protecting – my solar panels/micro inverters or the electrical grid). Can I replace the fuses with circuit breakers? This would at least be easier than playing the “fuse replacement game”.

• What do you guys use – “regular-blow or slow-blow”?

• When I put in this system, I replaced a lot (100s) of incandescent lights with LEDs. In the last few months, the LEDs will occasionally “flicker”. This could have been happening for years, but the cool down time of an incandescent bulb might have masked the flickering. Is it possible the “flickering” might be reflective of something that is happening on the grid that is related to the fuse problem?

Anyway, thanks in advance for any help.

Vic

Hi HarveyJ,  Welcome to Forum,

OK,  are there AC Combiners on a roof (or otherwise exposed to the elements) that collect a number of Microinverter's outputs,  or just what is the nature of the way that the AC outputs of the inverters are collected and,  what is the nature of the fuses, holders and boxes that hold the fuses?

If the fuses are outdoor and exposed,  are those fuses rated for such service?  Temperature cycling can take a toll on fuses which are exposed,  especially on rooftops.

Also,  you might want to check the cable connections to the fuse holders,  to make certain that these connections are torqued tightly,  and to spec  --  loose connections usually result in excess resistance which can cause heating,  that could damage fuses over an extended time periods.

Thanks for additional info that you can provide.   Vic

HarveyJ

Vic,

I'm 71 years old, and my wife has a moratorium on my climbing around on the roof. She forced me to stop putting up Christmas decorations about 5 years ago. She loves to decorate for the holidays so she found a company that comes over with a crew of five young men in their mid-20s with a cheery-picker in toe. In late November, they cover the house in LED icicles and wrap our four 25 ft. palm trees in LED strings. Shirley (my wife) and I take care of the ground level decorations (i.e., wreaths, large tree ornaments, inflatable Santa complete with reindeers, flood lamps, etc.). In mid January, the crew come over and reverse the whole process. It's expensive (about $300 per season), but so far I've dodged a fall, broken leg, broken hip or worse. Each time I mention the decorating cost, she reminds me that the last time either of us had to be transported to the ER with a minor problem, the bill started at $10,000 not including months of rehabilitation. Therefore, "I no longer do roofs".

It's kind of ironic - when you're in your 20s and can't come close to affording even a modest solar array, you have no problem going topside; when you can afford to write a check for a 18.7 KW, turnkey solar installation, you're too old to climb up on the roof to admire it. Getting old is a bitch.

Since I'm "roof restricted", it may be a few days for me to research my system and get back with answers to your questions. Here's what I do know...

• Each solar panel (68 total) has its own dedicated micro inverter
• There are 4 cable runs from the roof each containing 4 wires (black, red, white, green or cooper) running to one side of my disconnect box (contains the only fuses - everything else is circuit breakers). I haven't a clue how these cables are connected to the 68 micro inverters on the roof (remember that the roof is "off limits to me"  ) - see above. Is this the function of the AC combiners mentioned in your post? Is this device or devices active or passive?
• There is a meter that monitors solar production
• There is a residential 400 A Service Panel with an integrated net-meter which monitors both grid consumption and solar production
• All of the above components are mounted outside
• There are 3 distribution panels (100A, 100A, 200A) mounted inside and contain the circuit breakers for everything in the house

Thanks for your timely response; I look forward to confining our conversation.

Harvey

Vic

Hi Harvey,

Fine on your situation ...

SO,  the Fuses that are blowing are not on the roof,  and assume that you are able to access the fuses,  and have been replacing them yourself (?).

What is the part number for those fuses?

More later,   Thanks,   Vic

Dave Angelini

There is a warranty right?  Why don't you contact the installer?  Just curious.

DanS26

Which fuses are blowing.......both hot lines at the same time? or only one and the same line every time?  Any wire insulation look heat damaged?   Any varmints running around on your roof chewing on things?

mike95490

Fuses/grippers  If the clamps are not really secure to the fuses, they have some resistance and can heat up.  Usually big cartridge fuses have copper or silver conductive grease on the contacts, to seal air out and they last 30+ years.  If you are needing to re-tension the grippers to hold the fuses in, I would consider contact heating to be an issue.

HarveyJ

Thanks to everyone...

Your questions and dialog have forced me to scramble in the last few days to get a bit smarter about my solar system. Since all I did was write a check for the entire installation, there was no incentive for me to look at the "nuts and bolts" of the system. As long as my electrical bill continued to come in at about $35/month - normally $400 to $700/month). I was only paying attention to the financials. It was akin to buying a washing machine without understanding what was going on "under the hood". What I discovered when I dug a bit deeper was that my solar system (from an electronic and electrical standpoint) is infinitely simpler than a modern washing machine. Shame on me for not getting back to "engineering 101 basics". The upside to my lack of knowledge was finding this Forum and a lot of great people willing to take time to comment.

Anyway, here's an update on what I know...

• My solar panels are SolarWorld Model SW 275 (275 watts each) - I have 68 panels. The shade factor on the overall array is a bit over 98%
• Each panel is connected to my home grid via an Enphase Model M250 micro inverter. This allows each panel to "do its own thing" to maximize energy harvesting Also, this allow individual Wi-Fi monitoring of each panel/micro inverter. Any failure of a solar panel or micro inverter is "self isolating" and can be identified via  Envoy's Communications Gateway in conjunction with monitoring software that will run on any PC or Smart Device.
• The components (i.e., panels and micro inverters) are covered by a 25 year replacement warranty. On the surface this sounds great. What else do you have in your house that comes with a 25 year warranty (maybe your roof)? The problem is that solar energy is not yet a mature industry, and one never knows who'll be in business down the road.
• As local solar energy companies continue to install more and more systems, their work load, as it relates to monitoring system performance for their customers, increases. Currently, this monitoring (in my case) is a "free service" that was folded into the original system cost. But, it stands to reason that if you aren't paying monthly for monitoring services, you're not as high a priority as a "new customer" with a checkbook in his hand. Don't get me wrong, if I were an owner of a solar energy company trying to keep my "doors open", I would prioritized in a similar manner. Since my system is so easy to monitor, I'm guilty of not doing my daily due diligence - that has changed.

Since my previous post, I understand that my energy company has replaced the "fuse based" disconnect box with a "circuit breaker based" disconnect box". I will verify this later today. I still don't understand the mechanism that would cause a fuse/circuit breaker to open. In a previous post someone commented that the voltage out of their solar panels was unusually high. Another poster responded that this was a good thing. Since power (W) is the product of voltage (V or E) and current (I), you can "tradeoff" wire insulation capability against wire gauge. That is, you could reduce the gauge of your wire by raising the voltage and reducing the current. This is the fundamental bedrock of modern power distribution. We raise the voltage (100,000 volts or more) on distributions lines in order to be able to use fairly small wire to ship energy throughout the grid. But, if your voltage on a solar array increase, the current will also increase (i.e., Ohms Law I=E/R) -  this will blow fuses because they react to current. Is that what is happening in my case? Once I understand this, I can "bore-down" on the problem. I know that this is not "rocket science" because I use to do "rocket science". For a good part of my career, I designed electronic systems for the US unmanned space program (i.e., going to Mars, mapping Venus, Jupiter, Saturn, etc.) at the Jet Propulsion Laboratory.

Thanks for your timely responses; I look forward to continuing our conversations.

Harvey

westbranch

Harvey, on a specific PV, when the output increases say in winter on a cold! Day, it is only on the voltage side, generaĺly there is no increase in Amps for that panel, hence you can use,say, a 10 A CB FOR 5 PANELS IN SERIES, not so in the combiner where your amperage adds up
Hth

HarveyJ

Hth,

I'm wondering if we are having a semantics problem here. If you are measuring the voltage across any device that creates an electrical potential (i.e., generator, battery, solar panel, etc.) and that device is connected to a constant load, there are 3 elements in play - the voltage across the device (E), the current flowing from the device (I) and load attached to the device (R). The value of each of these are interrelated by Ohms Law which states that E=IR (which can be rewritten as I=E/R or R=E/I). This relationship was discovered in 1781and is the first thing "beat into the heads" of aspiring electrical engineering student in their freshman year. Therefore, if the load (R) is constant voltage cannot increase without a corresponding increase in current (I). Is there a possibility the voltage is being measured in one place (i.e., the terminals of the solar panel) and the current (I) is being measured at another place (i.e., the output of some inverter device)?

Harvey

westbranch

Sorry, I did not finish my thoughts, and forgot  that you had those micro inverters, so are you  able to detect the time of day when these fuses burn out?  I do not know if get really cold AM temps, just around sunrise, which would possible cause those MI's to get a whack of V and have to drop that to match the grid, by upping the Amperage..???

HarveyJ

Hth,

Ironically, all of my outages have occurred around 12:00 noon; the outage ends when I replace fuses or flip breakers. Going forward, I need to do a better job of logging outages and come up with an automated annunciator when my solar system drops off line. The problem is that when an outage occurs, an alarm doesn't go off. In other words, the utility grid picks up the load as if nothing happened - this is what you want to happen. If I open my monitoring software, I can see the outage down to the solar panel/micro inverter. The problem is that for over 12 months this system has worked flawlessly, and I got complacent (i.e., not checking the software often enough during the day). Now I have two issues - engineering an automated indication when my the solar drops out, and attempting to see if I can do something to prevent this from every happening by understand the failure mode.

You guys have been a great help...

• You forced me to get smart about my system (I think I'm pretty much there)
• You have given me an initial check-list of things to investigate (just getting started on this)

As I tie-up loose ends, I'll keep everyone appraised.

Thanks again,

Harvey

rlschell

I realize the discussions here are old, but I just started having a similar problem and was wondering if these folks every found definitive solutions?

In my case, the output from my Fronius inverter and 4.7 kW array runs through some short, convoluted conduit into a production meter and then the AC disconnect fuse box. The system is 7 years old and has worked flawlessly.

Recently every afternoon, one of the fuses is blowing in the AC disconnect box and the inverter shows an error indicating it is not seeing the grid (yeah, the fuse is blown). It's always the same fuse in the same hot line and always in the afternoon, for maybe 4 times in a row now. When I put in a new fuse in the evening, the inverter starts up and runs normally and continues to do so into the next morning. No other error codes or any sign of inverter issues. 

Here are a few items to consider:

1. In looking over the wiring, I found the neutral line at the AC disconnect was VERY loosely connected by a wire nut that basically fell off and allowed an open neutral. For some reason, the ground from the inverter is not connected all the way to through to the fuse box either. I fixed the open neutral, replaced the fuse, and... it blew again the next afternoon. So, that wasn't it.

2. The township-approved schematic for the wiring shows that run to be 10 ga. and it originally was. When the inspector came, he made the electrician change it to 6 ga. The 6 ga. is really tightly stuffed in the conduit and the insulation is seriously scuffed up from pulling it through. It really doesn't fit well and may be damaged.

Given that this started occurring with hot weather and only occurs during the heat of the afternoon, I'm suspecting thermal expansion and a possible short circuit related to item #2 above. 

Thoughts?

Dave Angelini

Separate the wiring from the equipment it goes to, make sure everything is off and meg the wiring. A megger is different than an ohmeter and will help you find out if the insulation is bad. The one below is pricey and for a Pro. You can find something less or maybe hire a good electrician. Good Luck

https://www.amazon.com/Fluke-1507-Megohmmeter-Insulation-Resistance/dp/B000X4O9WI/ref=sr_1_1?keywords=megger&qid=1559773046&s=amazon-devices&sr=8-1

mcgivor

 Repair the neutral/ ground  issues first, if the problem persists, check the grid voltage, it may be that the voltage is low which will cause current to increase. Rationale, hot weather could be related to air conditioning use by others,  overloading the grid thus causing voltage to drop.

BB.

I have had a Xantrex GT series grid tied inverter that ran for a few years just fine... Then, one day, (I guess) a couple switching transistors shorted on, and it would trip the branch circuit breaker (240 VAC pair) in my main panel. Everytime I restored power, it kicked off the branch circuit breaker again.

The bad neutral/ground connections can cause the inverter to fault (depending on brand/model/age, the inverter may measure voltages between neutral/hots/ground and if they are not "correct", it will shutdown output until fixed).

I don't think the bad connections will cause the inverter to fault in, otherwise normal, operation (of course, there are rare occurances that can happen).

If the wire was "too large" and/or the conduit and box penetrations were not deburred and grommeted to reduce the chances of insulation failure, then following Dave's suggestion to test/inspect the wiring is probably needed. Trying to move wiring/add slack, etc. in boxes/pulls may temporarily cure a short--And confirm what/where you are looking for problems (wiring vs GT inverter vs Breakers vs other issues).

-Bill

rlschell

As a follow up to my earlier post, I have good news! I pulled out all of the wiring between the inverter and AC disconnect and rewired with 10 ga. as originally approved. Upon carefully examining the 6 ga wire I pulled out, aside from a lot of insulation abrasion, there were no obvious signs of any shorted or burned areas. Naturally, I paid careful attention to making sure the neutral AND ground were fully connected at every point and each metal junction box was grounded. 

After several hot, sunny days with near full power output from the system - no blown fuses! So far, so good. 

So the lesson is, faulty neutral or ground may cause strange problems. In my case, imbalanced current on the hot lines making one fuse blow repeatedly. The fact that this was happening every afternoon was likely related to peak power output and not temperature as I had incorrectly speculated.  

BB.

Thank you for letting us know how it all worked out... And glad it did for you!

Take care,
-Bill