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Posted By: SP4RX Really Weird Generator Situation - 02/22/08 04:50 AM
The new construction building I'm working on had a problem the other night. The concrete finishers were working late and were the only ones left on-site so some of the details are unclear.

First off, the site's electricity is supplied by a diesel powered, 3-phase, 208/120V Y generator. There is a piece of cab-tire feeding a temporary panel inside the building. The panel is a single phase FPE.

Now to the problem. What the site super told me happened is that the concrete finishers had 2 portable 1000 W M.H. fixtures operating on a single extension cord plugged into a 15 A cct. The GFCI plug that the extension cord was plugged into is at the temporary panel. Somehow the plug managed to melt down and catch fire. The fire was contained to the single plug, and completely destroyed 1/2 of the receptacle...you could look right through where that half of the receptacle was and see the back of the box.

I show up yesterday morning and the super tells me what had happened the previous night, so I check out the damage. The first thing I did was to use my voltage sensor to be sure that the conductor feeding the destroyed plug is dead. However, I couldn't get the sensor within 6 or 8 inches of the plug or panel without it beeping and flashing.
This got me wondering because that's kind of the way my voltage sensor reacts to 600/347V.

I pulled off the panel cover to take some meter readings and things get really weird. While I was removing the panel cover I could see slight arcing a couple times in the burnt-out box. Once the cover was off I could clearly see that the wire from the fire-plug went to a breaker that had been turned off. So then I'm wondering what's causing the arcing, the neutral or ground? I metered things out and here's what was measured:
A phase - Neutral--------------120V
B phase - Neutral--------------120V
A phase - B phase--------------208V
so far so good...but wait
A phase - Ground---------------140V
B phase - Ground---------------333V
Neutral - Ground---------------266V

I guess that explains the arcing I saw...the neutral was arcing to ground.

So how is this possible?

The generator has a digital display that displayed normal voltages (for L-N and L-L) and frequency.

There was a ground wire going from the generator to a ground rod pounded at the generator. I checked the ground connections at the generator and the rod and they were good.

I metered the cab-tire to see if there were any opens/shorts but it was good.

The only thing I could see that might cause the situation was a possible problem with the generator so I recommended that the super have a service technician come out and take a look at it. He decided to have it picked up and taken off-site yesterday. Today the supplier calls and says that the generator is fine. What gives??

I'm completely stumped and really want to know what caused this to happen.

If any of you have any input/suggestions/ideas, I'm all ears.


Thanks for listening.


Shawn.






Posted By: sparkyinak Re: Really Weird Generator Situation - 02/22/08 05:17 AM
The generator lost its main bond is my first thought but what about the C phase readings?

A-C?
B-C?
C-N?
C-G?
Posted By: homer Re: Really Weird Generator Situation - 02/22/08 06:51 AM
The problem could be in the cab-tire. At first glance it souncs like I know what I'm talking about, except I have no clue what cab-tire is.
Is that generator is multivoltage / multiphase unit ??

if so.,, it possiblty that one phase at the generator got mess up ?? this can happend with combation zigzag/wye connection.

Merci,Marc
Posted By: jkraft Re: Really Weird Generator Situation - 02/22/08 01:57 PM
Uh what's a cab-tire?
Posted By: Jonno Re: Really Weird Generator Situation - 02/22/08 02:12 PM
cabtire is slang for rubber insulated cord (type S, SO etc).
Posted By: SP4RX Re: Really Weird Generator Situation - 02/23/08 01:03 AM
Quote: "The problem could be in the cab-tire. At first glance it souncs like I know what I'm talking about, except I have no clue what cab-tire is."

LOL I would expect nothing less from somebody who chooses "homer" as a nick. smile

Everybody around here refers to "flexible cable having a heavy rubber or neoprene outer sheathing" as cab-tire, don't ask me why.


As to the generator problem,Sparkyinak nailed it...no neutral bonding at the generator. I was thinking that the problem would be more along the lines of what frenchy was getting at, but at least it was a simple fix. The repair place called the site super yesterday and told him they had inspected the gen-set and it was working properly, and they would return it. When I got to the site this morning the generator was back. I opened the cover on the termination cabinet to re-attach the cab-tire and the first thing I noticed was a bonding jumper going from the neutral bus to the ground lug. I know this jumper was not there two days ago because I looked for one. I mistakenly assumed that the bonding had been done inside the unit somewhere.

I'm really surprised how much of a difference can be made by removing the neutral bond, I never would have guessed it would be this dramatic.

This generator has been on-site for about 6 weeks in this condition, I'm surprised nothing bad has happened.

Thanks for the responses!

Shawn.

p.s. the gen-set was feeding a single phase panel so C phase was not being used

Posted By: WireNuts29 Re: Really Weird Generator Situation - 02/23/08 02:28 AM
It seems to me even though you solved the main problem there is more trouble here. a three phase genset, feeding a single phase panel. Not to mention the Fpe panel, you are asking for a fire. you're lucky it was isolated to one receptacle
Posted By: SP4RX Re: Really Weird Generator Situation - 02/23/08 03:56 AM
I'm not sure I follow...where is the problem? If the generator is only feeding 2 phases to the panel, the panel operates at 120/208. The panel is rated for 120/240. The third phase never leaves the load side of the breaker at the generator. The only thing I wondered about when first seeing this setup was how the imbalanced electrical load (2 phases out of 3) would affect the mechanical load on the diesel engine.

We provide all of our sites with temporary panels, and they're always 120/240. This GC bought a new generator and for some reason decided to buy a 3 phase model, although I can't figure out why. I think what happened here is that the temp panel was already built and on-site when the new genny showed up...our guy went to the site and just connected the cable from the panel to the new unit.

If you think there may be issues with this setup I would really like to know what concerns you so I can prevent any further problems.

Posted By: sparkyinak Re: Really Weird Generator Situation - 02/23/08 04:59 AM
running a three phase genset on only 2 phases is not the healthiest for the genset. It is like running a 3 cylinder engine on only 2. It is not as bad on a wye as it is on a delta (so I have been told). Other factors include the size of the genset and the loads on it.
Posted By: SteveFehr Re: Really Weird Generator Situation - 02/25/08 12:08 PM
Originally Posted by SP4RX
I'm really surprised how much of a difference can be made by removing the neutral bond, I never would have guessed it would be this dramatic.

This generator has been on-site for about 6 weeks in this condition, I'm surprised nothing bad has happened.

Thanks for the responses!

Shawn.

p.s. the gen-set was feeding a single phase panel so C phase was not being used

If the system is balanced but the neutral is unbonded, there is still some natural capacitance to ground and small amounts of leakage current, etc, and the system will generally float close to ground. If the system is highly unbalanaced, though... like if you take a 3-phase generator and only use 1-phase, though, it will float well off ground, creating dangerous voltages. I'm glad nobody got hurt!
Posted By: jdevlin Re: Really Weird Generator Situation - 02/25/08 02:56 PM
Originally Posted by WireNuts29
Not to mention the Fpe panel, you are asking for a fire. you're lucky it was isolated to one receptacle


FPE panels are very common in Canada. We don't have the issue the USA panels had.
Posted By: WireNuts29 Re: Really Weird Generator Situation - 02/25/08 10:23 PM
My concern was the unbalanced load. possibly putting an un due strain on the motor, and cutting its expected life. Also I'm not disagreeing, but did Fed pacific use all the quality panel parts in Canada and ship the bum ones throughout the U.S ?? I've witnessed a 20amp 277volt short dead to ground and the breaker never tripped, it welded the mc jacket to the ceiling grid and made some bad burn marks? I don't know why nothing tripped? but it got my attention. and I always triple check Fp breakers cause some in my expierience don't snap open when you shut them off. Just my2 cents.
Posted By: SP4RX Re: Really Weird Generator Situation - 02/26/08 01:12 AM
At work this morning the site super told me that he is really surprised that there have been no problems with the generator since Friday. Three whole days! Before the bonding jumper was installed, the gen-set's electronic controller would shut the power off due to over-voltage, under-voltage, and other improper readings. This would sometimes happen several times a day.


As far as FPE being reliable up here; I wouldn't want to be in a situation where the only thing protecting me from harm is a FPE breaker.

I'm not sayin', I'm just sayin'.


Shawn.
Posted By: SteveFehr Re: Really Weird Generator Situation - 02/26/08 11:44 AM
Originally Posted by WireNuts29
My concern was the unbalanced load. possibly putting an un due strain on the motor, and cutting its expected life.
Most 3-phase generators are designed to handle 100% imbalance, but must be derated; you're not going to get 50kW out of a 50kW 3-phase generator if you only use one phase, you're going to get about 28kW. The engine doesn't care, and the generator is just cables on a rotor, which is still balanced from an electromechanical standpoint. There's really nothing electrically or mechanically damageing about it aside from running the engine at less than half load all the time, and all the bad things that happen when you run a lightly loaded diesel engine too long.
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