Portable Generator. What is the Hazard?

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mkgrady

Senior Member
Location
Massachusetts
I keep trying to understand the danger of wiring a seperatley derived portable generator to a panel without switching the neutral. I understand the difference between separately derived and non-seperatley derived. I understand automatic and manual transfer switches, interlocks, etc. I have tried drawing out a sketch of a service including the distribution transformer, service entrance, GECs, EGCs and how they connect or don't connect during a transfer. I just can't follow how a ground fault while running on a generator can be hazardous. I get that in these situations the bond between neutral and the EGC at the generator can be removed to make it code compliant and I get that this generator should no longer be used as a stand alone power system. What am I missing? Has anybody seen a sketch that shows this dangerous current path that needs to be avoided?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I'm not sure you'll find such a sketch. IMO this "danger" is more theoretical than actually existing in the real world. Maybe someone will have some data to prove me wrong. For the record I unbonded the neutral in my own generator to use it as a non-SDS.

A few years ago someone suggested on this forum that the generator manufacturers should add a switch to the unit so that it can easily be converted from a bonded to an un-bonded neutral. I like that idea. :cool:
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
I keep trying to understand the danger of wiring a separately derived portable generator to a panel without switching the neutral.
(my bolding in the quote)
It's not clear to me which configuration is the one that you're concerned about. If a system is separately derived then you are switching all conductors including the neutral. If you aren't switching the neutral then the system is non-separately derived. Do you mean connecting a portable generator with the neutral-EGC bond (if any) removed and using a transfer switch that does not switch the neutral? If so I'm not aware of any particular dangers as long as the EGC and its connections are adequate.
The only thing I can think of that's similar to your concern is when you don't have an adequate grounding electrode for a separately derived generator, then re-striking ground faults might induce higher voltages by storing energy in the capacitance or resonant LC elements of the system. This is not likely to be a problem for small systems like a residence.
 

mkgrady

Senior Member
Location
Massachusetts
I'm not sure you'll find such a sketch. IMO this "danger" is more theoretical than actually existing in the real world. Maybe someone will have some data to prove me wrong. For the record I unbonded the neutral in my own generator to use it as a non-SDS.

A few years ago someone suggested on this forum that the generator manufacturers should add a switch to the unit so that it can easily be converted from a bonded to an un-bonded neutral. I like that idea. :cool:
The switch you mention would be great except only an electrician would know what to do with it. I like the idea
 

mkgrady

Senior Member
Location
Massachusetts
(my bolding in the quote)
It's not clear to me which configuration is the one that you're concerned about. If a system is separately derived then you are switching all conductors including the neutral. If you aren't switching the neutral then the system is non-separately derived. Do you mean connecting a portable generator with the neutral-EGC bond (if any) removed and using a transfer switch that does not switch the neutral? If so I'm not aware of any particular dangers as long as the EGC and its connections are adequate.
The only thing I can think of that's similar to your concern is when you don't have an adequate grounding electrode for a separately derived generator, then re-striking ground faults might induce higher voltages by storing energy in the capacitance or resonant LC elements of the system. This is not likely to be a problem for small systems like a residence.
I’m talking about the theoretical danger of using a generator with a neutral to EGC bond and using that generator to power a home without switching the utility neutral off when the hot legs are transfered to the generator. I call it a danger because doing this as I understand it is a code violation. I just can’t see the hazard.
 

bwat

EE
Location
NC
Occupation
EE
I haven’t given much thought to other implications, but for starters wouldn’t that then create two neutral to egc bonds? One at the service disco and one at the generator. So you’ll then have current flowing on the impacted egc without any faults.
 

mkgrady

Senior Member
Location
Massachusetts
I haven’t given much thought to other implications, but for starters wouldn’t that then create two neutral to egc bonds? One at the service disco and one at the generator. So you’ll then have current flowing on the impacted egc without any faults.
I agree but I’m trying to picture what makes that unsafe. How does someone actually get shocked? To prevent this situation one would remove the bond jumper at the generator but now the generator becomes a hazard if it is used as a stand alone generator. In that case the metal frame could become energized.
 

Strombea

Senior Member
No one will get shocked but it is still creating a parallel neutral which is always against code. Usually Grounds are ran smaller than neutral so in event of neutral break, The gen will still feed Full amperage on EGC. I agree not a safety issue necessarily but its the same reason you don’t bond on sub panels.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
It's a code violation because they don't want normal neutral current to run on an EGC. That's the entire reason it's a code violation. It would take more than one failure for it to create a real danger but the code is pretty consistent about not wanting normal current running on an EGC.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
These generators are typcially connected with a cord and plug. Since the cord will have an equal sized EGC and neutral there is no way that the EGC could be overloaded even if the nuetral were somehow disconnected. The NEC is specific about not having the generator neutral bonded when used as a non-SDS but the danger of having the neutral bonded is probably not very great. I would guess that after a natural disaster like a hurricane that there are thousands of portable generators with bonded neutrals providing power to homes and businesses without incident.
 
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