Dead short on diesel generator not tripping breakers?

I have a off grid shop running in a Kubota kj13 diesel generator, 12.5 kwn1800rpm diesel machine. Problem is with a dead short the breakers are not tripping. I had a wire heat up and short at a Romex staple when I wasn't paying attention a few days back, the gen puffed some black smoke then stopped putting out power. Hopefully not to much damage i brought it to a repair shop. Just can't figure out why the breakers wouldn't trip? The gen has a built in breaker and the building has sub panel with properly sized breakers for the wiring? What gives aren't breakers there for this exact purpose?

Not familiar with Kubota generators, but I'll give it a try.

If the romex staple was downstream of the breaker box, the breaker is too high amperage and the generator not at fault. If the romex staple is upstream of the breaker box, it's wired wrong. The generator built in breaker is for the entire output of the generator and it would take an extremely heavy load to trip it.

Short was in a circuit before a recelticle after the subpanel breaker, that breaker was sized for the wire size (20 amp and #12 Romex) the subpanel is down stream of the onboard breaker 60amp which was factory installed on the generator. everything is wired properly, properly for on grid power system, but not sure if since gen is providing power some thing needs to be different. I would think that the onboard breaker would trip if the short was strong enough to cause the generator to **** the bed?? Thanks for the help

I agree, the generator breaker should trip before the generator goes up in smoke, whether it's a short or overload. The 20 amp breaker should also trip immediately with a short... so what happened?

What made the romex heat up in the first place? did you notice the voltage or frequency (engine speed) drop just before the incident? Was the whole generator close to full capacity or just that circuit? What brand is the 20 amp breaker? Single phase or three phase generator, and 20 amp circuit?

Generator was near full load, the circuit that had the short was a new addition to my shop with a constant load that was turned on with no problems. About two hours later, the short happened, gen puffed black smoke and power out stopped. The short was located where a metal electrical staple had been hammered in a little to tight, I assume over the two hours the Romex warmed up enough to compromise the already thin insulation from the staple damage.
The subpanel is a regular BR style panel with cutler hammer breakers. Nothing tripped. Generator is single phase. Generator is at an industrial motor shop being looked at now and I'm curious what the damage is. They said hopefully just a voltage regulator and not damage to the gen head.

Still scratching my head as to why neither breaker in the system tripped? Do I need some type of arc fault breaker or gfci receptacle in line?

Inguess I'll order a new breaker for the generator to be safe but...

Depending on how long of total wire run/wire size is from the genset to the load/short can affect how fast things trip, and you mentioned the setup was running at pretty good load... this in turn will heat up the wiring and make it harder to trip at designed load.

If it was an arc fault (instead of a full short) that would explain why the breaker didn't trip. Was there a lot of metal vaporized out of the wires and staple? It might be possible that the arcing caused the damage to whatever went up in smoke in the generator? The motor shop, or smokstak will know better than I do if that's likely.

You can certainly load test the breaker if you doubt it's tripping ability. Water heater elements are one relatively easy way to absorb a lot of amps.

I've always heard QO and CH breakers are the better grade, I don't have "statistically significant" experience.

Well just heard the generator head is smoked. They want 3000$ to rewind it. I'm pretty frustrated and can't see spending that much money on this thing. probably won't look into testing the onboard breaker. Not really to clear on the difference between arc and ground fault but it was a direct short across a 120v circuit not sure if the hot was shorting to the neutral or to ground the whole area was black?? Anyone have a good source of reasonable priced generator heads?

Ouch!!!

How old was this genset? any chance of some kind of warranty or recall that would cover this from the mfg?


Don't know on new heads...3g's sounds a bit high for a rewind... maybe a new whole head... but a rewind??? if it was me I would be on the lookout for similar genset with motor problems that I could pick up cheap... but you would still have the problem of why it smoked in the first place.

Yeah I thought it was spendy as well. I am going down there in the am to get part number and see if I can find a replacement gen head for that price, looking online I'm seeing them for less then that new but I'm not exact on the gen head model number. The gen is not ancient, but definitely out of warranty. Kubota only gives 2000hrs to the original owner, which I'm not. And yeah although I know why it smoked I don't know why it didn't trip the breaker.

I would think taking the head to an actual shop that does rewinding would only set you back a few hundred or less if you can find someone that deals with regular public and not just wholesale.

Ground fault trips if the current in the two legs of the circuit show any imbalance, meaning some electricity leaked to ground, through the hair dryer to a wet hand for instance. An arc fault breaker senses the wiring arcing (like a loose connection, or wires rubbing together) and shuts it off even if it's not drawing enough amps to trip the breaker otherwise, this probably would have tripped faster than yours. A short should trip the 20 amp immediately and if it's a good short it might trip the 60 amp breaker at the same time.

I don't know what could be different on a generator that would cause this to happen??? Maybe it would have tripped immediately if that circuit was the only load? but the 60 should still trip, right?

You could keep an eye on craigslist for old generators, if you can figure out what "bell housing" your Kubota has and if any of the Onans would mate up to it. I have a couple of old gashog Onans that I got cheeeeeeeap.

What part of California are you in? would that be something that would be worth taking across the border if you couldn't find a reasonable motor shop local?

I am in very northern cal on the coast an hour from Oregon, just looking to not get hosed here. Maybe I could look into dissembling and shipping to a motor shop in another area? Ill go get the part number off of it tomorrow and see what I can find a new gen head for the machine. Thanks for the Info yall

The moral of this story as you well know is that conventional circuit breakers cannot be trusted to protect a generator head from damage. Cummins or somebody put out a paper to this effect. The time/temperature curve inside the breaker is slower than the time/temperature/damage curve actually occurring inside the generator head and things can and frequently do melt down before the breaker can open. Breakers are set up to protect wire and not generators.

I know a couple of shops if you want to PM me.

From PM:

707pc50 said:

Do you have experience with approximate price on a rewrap of a machine about this size? I can't wrap my head around 3000k for a rewrap. Id love to read the cummins article you referenced about generators and breaker protection. Id be interested if arc fault would have saved this whole dilemma. thanks

Click to expand...

I guess it depends on how the generator end is configured. Is $3000 the repair shop quote for a full repair with them doing all the legwork or is it just the price from a rebuild shop for the stator itself? If it is the total repair then go for it I say. Your machine is worth north of $11000 new and depending on how much of its life is gone it may still be worth half that. Right now you have a hunk of scrap metal, $3000 gets you a $5000 generator is one way to look at it.

https://powersuite.cummins.com/PS5/..._Asset/pdf/Commercial/technical/T-030.pdf#121

Start reading the bottom of the page referenced and continue on for many pages. I am glad I was able to find it this late at night.

Arc fault may or may not have saved you but generally they are for protecting sleeping areas in houses and there is considerable debate as to whether they are worth anything or not.

I have no idea how much those breakers that protect generators with a proper thermal curve cost. It might be cheaper just to go through the electrical system with a fine toothed comb to ensure this does not happen again.

What brand/style was the 20 amp branch breaker that did not trip?

Thanks for the info. 3k$ was to do the job. The breaker in the subpanel protecting ( ha ) the circuit was a cutler hammer breaker. I talked to an electrician tonight who said he has seen this many times where a generator won't kick the breaker on a short like this, although he said usually no damage is done because the user notices and shuts it down. This issue happened after the wire warmed up and no one was paying attention.

How was the generator connected to the building wiring? If you had a hot to ground fault and the neutral wasn't bonded to ground the circuit breakers would have no way to open. That's what I suspect because two breakers didn't open. Neutral and ground should only be bonded at one point at the electric services first over current protection device. and as far as cutler hammer breakers you can weld with them. Now ge you won't even see a spark before it opens.

If the wire heated up you have too long a run for the wire size and load. You may also have too much power draw on one leg if it's a 220 single system, it's possible you overloaded one output leg and burned that winding without tripping a dual breaker? Just throwing ideas out there.

Had a wood frame shop building burn down right around the shop I worked in, our section was all metal and all that was left standing. I got to help with the insurance investigation. Forty years of hard use had caused a subpanel feeder run in metal conduit to wear a hole in the insulation at a junction box. The magnetic field when the wires were under heavy load caused them to vibrate in the metal conduit. Dead short on one leg of a 240 three phase system, quickly turned into a fire and subsequent dead short on all three legs... twenty feet or more of steel conduit and a couple junction boxes with the sides blown out from the power, but the 200 amp main fuses held. Too long a run from the transformers to the power feed, voltage drop was so high that according to the insurance investigator there was simply no chance to develop 200 amps on any leg. Three alarm weenie roast. Just had to share.

Hope you find an economical solution.