AC disconnect fuse keeps blowing - puzzled!

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.

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.

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.

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.

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!

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.