Question utilizing an old portable military light genset with GFCI breaker.

aksala · October 2019

I am off-grid with a conext XW+ inverter plus solar and battery bank with generator backup. I have a simple 3 pole manual disconnect switch outside the house of which I only use 2 for the hot leads. The disconnect was installed by the electrician as there was only a generator running the house initially through the build and at final inspection. All off-grid equipment was installed by me after the build.

So the electrician used the disconnect switch as the place to bond N-G. That's all fine and I haven't had any issues running my two gas generators to charge batteries or even in bypass and power the house. However now I have a problem with this new to me diesel generator. My portable gas gensets are not GFCI and so have no issue with neutral not being switched. But this generator has one 30amp GFCI breaker on the 240 outlet. It also has a 20amp non GFCI and a couple of 120V outlets also non-GFCI.

If I try to use the GFCI 30amp circuit the breaker on the genset trips, as it should. The cord from the genset has N and G tied to a busbar in the disconnect which is how it was done by the electrician. So my question is whether I should mess around trying to figure out where to break the N-G bonding on the genset or just remove the GFCI breaker and replace it with a standard 30A breaker and move on? I'm thinking that simply replacing the breaker would be the best bet as an interim measure.

Since I am completely off grid I was thinking of eventually wiring L1 and L2 right to the AC1 input of the inverter as I can configure it as a generator input. Then I can still use the disconnect switch with one of my portable gas sets if I need to service the diesel etc., but am confused as to how I would then run ground and neutral from the diesel set.

mcgivor · October 2019

There should be only one neutral ground bonding point, that should be on the load side of the inverter in a distribution panel. The disconnect installed by the electrican would have been the single point during construction perhaps with a floating generator, so not currently needed, the generator may have a NG bond, check continuity between N & G.

Since the AC input of the inverter only needs both live conductors to supply 240V, the neutral is redundant, neutral will be reestablished on the split output of the inverter. This would be true only if all loads run through the inverter without any taps before the AC input.

Should GFCI protection be needed for house loads GFCI breakers could be installed in the distribution.

aksala · October 2019

@mcgivor

Yes, I'm sure there is a N-G bond on the genset. I have GFI circuits in the house panel already. Too many if you ask me, but it was wired to code. I'm not sure if my other gas generators have N-G bonding but they never gave me a problem. Then again, they didn't have any GFI breakers either. I'm just not sure where the N-G bond IS on this generator, and no manual. It's a military surplus, a nice little Kubota 905-E turning a Kohler 7.5kw alternator. But then everything runs to a Murphy panel and it was rigged to run two 1KW sodium vapor lamps. It's not clear to me why they put a GFI breaker only on the 14-30R and not on the other 240 20A receptacle. But I'll have to snip the tie wraps and trace some wires. Everything is black wire, but numbered. Of course I have nothing to tell me what the numbers are.

There is a large ground stud on the back of the Murphy console and I can trace an 8 AWG ground lead from the block to the stud, but whether any of the dozens of leads attached to that stud are a neutral from the alternator I don't know yet. I'll have to pull the cover off when I get a chance and trace things out.

But what I was wondering is what is the quick fix? Assuming I'm just plugging the set into the house, not trying to use the generator with anything else plugged into the outlets. I've seen some suggestions elsewhere that dropping the ground from the disconnect would work in the interim.

BB. · October 2019

From what little genset experience I have--Generally the smaller generators (and AC inverters) have floating (no neutral+ground bond)--Typically 3,000 Watt or less(?).

For larger gensets (and inverters), the come from the factory configured with Ground bonded Neutral at the genset (inverter) output. And the solution would be to lift the Neutral+ground bond at the genset.

The GFI is probably tripping because you have a parallel current path between the generator and the main panel. Basically, the Neutral and the Green Wire ground are both carrying the "return" current leg.

GFI basically measure the "total current" of the Hot and Neutral legs, the "Ground Wire" current is ignored.

For GFI, the H+N go through a couple coils of wire (a simple transformer), and the GFI is set to trip if the current going "out the hot" and "back in on the Neutral" is over ~10 milliamps (0.010 amps--specifics vary between types of GFIs and Country of installation), the breaker trips.

While you could replace the GFI on the genset and it would stop the tripping... In general, it is considered improper to have current flow in the ground wire/bonding system in normal operation. There should only be current flow in the ground system if there is a fault (basically, shunt the electricity to ground/neutral to pop the source breaker).

-Bill

aksala · November 2019

@BB. - I confirmed today that G and N are tied at the 14-30R on the diesel, not on my gas unit which is an 8kw unit but probably 10 years old.

So I figure I can eventually trace the wires and find the N-G bonding on the genset but in the mean time, what about just lifting the ground from the generator at the disconnect switch? The Gen is N-G bonded, and N is bonded to G at the disconnect. Seems like it would be OK for short term until I can pull the genset into the garage and have time to trace the wires down. Thoughts on that tactic?

mcgivor · November 2019

@aksala It's not clear to me why they put a GFI breaker only on the 14-30R and not on the other 240 20A receptacle.

The reason is because the 240V receptacle has no neutral /ground reference , it is a series of two windings, the 30A on the other hand is the same but with a center tap neutral.

aksala · November 2019

@mcgivor , thanks. That makes sense.

I lifted ground from the generator in my disconnect box and the generator now runs almost fine. I'm having issues with loads coming on in the house causing the 6048 to disqualify the generator and start the qualification all over again. Never had this issue with my other two gensets, so for now I'm going to make the assumption that it's due to the bonding issue causing an imbalance. For instance, when my wife turned on the dryer I saw the AC Load jump on L2 to 45amps according to the status display from the inverter while the L1 was like 5amps. We confirmed this with a couple of appliances turning them on or off and back on. We were pulling about 2KW at that time (had oven, gas dryer, plasma TV and dishwasher all going). Dishwasher changing cycles and thus load would trip the input on the inverter or stopping and restarting the dryer or turning on the microwave. This has never happened with the other two generators as I said. If we're not cycling loads then the diesel seems to do fine and I've seen no sign of voltage sag or frequency out of tolerance etc..

Estragon · November 2019

Just a guess, but maybe the diesel is an 1800rpm vs higher (3600?) on the others? Although the lower rpm diesels tend to have better life expectancy, maybe they're more sensitive to load imbalance?

Mine is wired for 120v output to avoid the imbalance issue, and my one 240v load (water pump) is run though an autoformer.

aksala · November 2019

Estragon said:

Just a guess, but maybe the diesel is an 1800rpm vs higher (3600?) on the others? Although the lower rpm diesels tend to have better life expectancy, maybe they're more sensitive to load imbalance?

Interesting thought. I have no idea. I don't know that we've ever been charging the batteries while running so many appliances before. So I'll have to do some experimentation with the different generators and loads and see if I'm getting different results. But I've been running the system since June and using the generator quite a bit in October and I've never seen the inverter disqualify the generator.

@Dave Angelini
Say Dave, does the Conext 6848 have a means to log disqualification events for generator input? It's hard to catch if visually if there's a sag in voltage or frequency causing the disqualification. There are no associated Event codes. It just drops the input and then starts qualifying again. I did not have Generator Support enabled as it's a 7.5kw unit, but perhaps I should enable it to help balance the loads?

aksala · November 2019

Just a follow up; tonight I found this gem hidden in the manual;

Note: Do not use a GFCI equipped AC source to power either the Grid (AC1) or Generator (AC2) inputs. The AC input filters on the Conext XW+ may cause nuisance tripping of ground fault protected outputs.

mcgivor · November 2019

The GFCI requires a neutral ground bond to function, multiple points are often a source of problems, so is everything working now?

aksala · May 2020

@mcgivor busy winter, I obviously don't get to the forum much anymore. So yes everything is working with the N-G bond severed on the input from the generator. On this summer project list is building a shed for the genset to protect it a bit more from the sub zero temps this winter as well as finding and removing the GFCI protection on the genset itself as well as wiring it up to my AGS. Life will be good then.

I had a few days of grumbling when it was so damn cold the diesel didn't want to start and my portable gas generator is of course NOT bonded. So I would have to open my disconnect on the outside of the house and reconnect the ground wire in order to switch between gensets. Luckily that was less than a dozen times.

Question utilizing an old portable military light genset with GFCI breaker.

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