Vehicle-or trailer mounted generator (grounding not required)

Dsg319

Senior Member
Location
West Virginia
Occupation
Journeyman Electrician
I know that such generator is not required to be grounded (earth) if it is only supplying receptacles mounted on the vehicle, trailer, or the generator itself.

If the correct conditions are met.
Such as the generator frame is bonded to the vehicle and or trailer frame, and the normally non current carrying metal parts of equipment and the EGC terminals of the receptacles are connected to the generator frame.

Also the generator manufacturer is required to bond neutral conductor to the generator frame under these following conditions.

1.) The generator is a component of a separately derived system.

2.)A grounding type receptacles is mounted on the generator or is supplied by the generator.

3.) A receptacle on the generator is supplied by the generator with GFCI protection.

With having said all that, I understand when just using cord and plug equipment From the generator receptacles grounding is not required.

hypothetical scenario.... 20kw BONDED gen, trailer mounted genset, feeding a 100amp distribution panel from (not a receptacle) but from load terminals on the genset with OCPD.

Temporarily supplying power to part of a station, no ATS or nothing like that just temp power.

Should the genset than be required to be grounded since it’s supplying Power from the terminals and not a recep?
 
What does grounded mean here?

The generator needs an EGC as a fault-current path, it follows the output feeders.
It needs a SBJ if there isn't one somewhere else (also the fault-current path).
It does not need it's own GES but that does no harm (and makes some AHJ's happier). To me this is more of a physical location call- if the trailer is next to the building and that has a GES, I wouldn't add another; if it's on a field next to a job trailer, I would.

BTW, I've never had a trailer mount generator come with it's own SBJ, but I don't think I've used one smaller than about 50kva in a while.
 

Dsg319

Senior Member
Location
West Virginia
Occupation
Journeyman Electrician
What does grounded mean here?

The generator needs an EGC as a fault-current path, it follows the output feeders.
It needs a SBJ if there isn't one somewhere else (also the fault-current path).
It does not need it's own GES but that does no harm (and makes some AHJ's happier). To me this is more of a physical location call- if the trailer is next to the building and that has a GES, I wouldn't add another; if it's on a field next to a job trailer, I would.

BTW, I've never had a trailer mount generator come with it's own SBJ, but I don't think I've used one smaller than about 50kva in a while.
Grounded as in having its own grounding electrode system.
 

Dsg319

Senior Member
Location
West Virginia
Occupation
Journeyman Electrician
The more I think about it my own question confuses me. I guess what I’m getting at is I just received an email last night about OSHA finally agreeing that physically grounding a portable genset on a job site poses more harm than it does good. (Earth return path for fault current, not opening OCPD.)

And I just was wondering about the scenario I created with the 20KW genset feeding a panel from load terminals. But by than I guess it would be more along the lines of a SDS even if temporary.
 

kwired

Electron manager
Location
NE Nebraska
With grounded utility systems (and usually MGN systems) you have a huge network of grounding electrodes that gives a pretty good ground reference to said systems, even if you lose connection to the GES at your premises you still have pretty stable voltage to ground.

With a stand alone source like a portable generator there is no ground reference until you make one. A single electrode will be fairly weak link most the time, but once you connect that generator to a premises wiring system that is also connected to a grounded utility system it kind of becomes part of that utility system when it comes to whatever conductor is grounded. Grounding in those cases is typically done in the premises wiring and a EGC running back to the generator is for bonding the generator frame to the premises grounding system so that genset frame and other premises grounded components are at same potential.

When running generator stand alone it is safe to be "ungrounded" as there is no reference to ground on said system - until one is made. One conductor faults to ground - the system then becomes grounded, but there is no current as there is no path for ground current to flow until there is a second connection to ground in the same system conductor - that is what becomes a hazard as you can get in the path. If another system conductor faults to ground you have high current flow and overcurrent devices will open the circuit.
 

Todd0x1

Senior Member
Location
CA
This is something that has driven me nuts forever. Towable generator with camlock connectors with various cables and distribution boxes plugged into it. A grounding electrode does pretty much nothing here. This setup is generally considered to meet 250.34 for cord and plug connected equipment. Some municipalities and inspectors still insist on a grounding electrode.
 
This is one place where I really hate the word grounded, it's so vague as to be useless. A lot of confusion would be saved if people treated most generators* as transformers and bonded accordingly.

*I'm not counting the small/portable ones with just receptacles
 

rambojoe

Wireman
Location
phoenix az
Occupation
Wireman
If it is seasonal, look in the manual and see if there is provisions to simply bond the chassis internaly- then just rely on the egc through the wip.
Then ground rods at the distribution.
Expect faults at the panel, not the gen., and keep faults at the closest source for equipment protection...
 

Dsg319

Senior Member
Location
West Virginia
Occupation
Journeyman Electrician
This is one place where I really hate the word grounded, it's so vague as to be useless. A lot of confusion would be saved if people treated most generators* as transformers and bonded accordingly.

*I'm not counting the small/portable ones with just receptacles
When I say grounded, I mean literally copper in the dirt.
 

kwired

Electron manager
Location
NE Nebraska
As long as the button on a gfi tester trips...which, for safety or forest service inspections or god forbid a building/electrical dude will surely find..and waste the day ;)
If no neutral to EGC bond that same tester should have indicator lights that indicate "open ground", which should be considered normal for an ungrounded system, but yes many inspectors especially safety inspectors won't understand what they are dealing with anyway. Driving a rod and attaching to frame of generator won't change a thing with the tester either, but will confuse the hell out of the inspector.
 

rambojoe

Wireman
Location
phoenix az
Occupation
Wireman
If no neutral to EGC bond that same tester should have indicator lights that indicate "open ground", which should be considered normal for an ungrounded system, but yes many inspectors especially safety inspectors won't understand what they are dealing with anyway. Driving a rod and attaching to frame of generator won't change a thing with the tester either, but will confuse the hell out of the inspector.
Agreed. If memory serves, i bet the vehicle gen is bonded..either way, at the loadcenter is best. Safety first, always- but as far as forest service goes, which was my ahj, you can always confuse them with logic and win approval.
The "no gfi test trip" always comes up with hospital or elevator back up gen...at least a few times. 800mill job and it fails for something stupid like that :(
 

kwired

Electron manager
Location
NE Nebraska
Agreed. If memory serves, i bet the vehicle gen is bonded..either way, at the loadcenter is best. Safety first, always- but as far as forest service goes, which was my ahj, you can always confuse them with logic and win approval.
The "no gfi test trip" always comes up with hospital or elevator back up gen...at least a few times. 800mill job and it fails for something stupid like that :(
One situation where person doing the testing can get lit up is actual grounded system, test showing no EGC, metallic weatherproof cover or some other similar situation where you are likely to be touching the cover (or other item) then when you press the test button the cover becomes energized and you happen to be making a path to something that is grounded.

I didn't learn this the hard way but know of someone that did.
 
Top