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#196721 - 10/24/10 09:32 AM Portable generator grounding question
LateGreatJC Offline
New Member

Registered: 08/27/10
Posts: 6
Loc: Missouri, USA
I've been reading everything I can find about wiring portable generators for backup power for houses. One thing I don't understand concerns the neutral. A common wiring method is to isolate the neutral and ground at the generator frame and solidly bond the generator neutral to the service neutral in the transfer switch, which is considered to effectively ground the generator (because the service ground is grounded at the transformer). This seems wrong to me. What if the meter is fed from an overhead service that is torn down during a storm? The service neutral (ground) is no longer grounded, nor is it grounded at the generator. This would seem to be an unsafe situation. Plus, since the return current from a 120V circuit would have 2 paths, mightn't it be possible to have some backfeed on the service ground, possibly endangering the lineman? If someone can answer this I have a couple of other questions.

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#196722 - 10/24/10 10:05 AM Re: Portable generator grounding question [Re: LateGreatJC]
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
The generator is grounded in your scenario by the grounding electrode conductor, not the neutral going back to the transformer. The power company also grounds the transformer to protect their people.
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#196725 - 10/24/10 10:44 AM Re: Portable generator grounding question [Re: LateGreatJC]
LarryC Offline
Member

Registered: 07/05/04
Posts: 776
Loc: Winchester, NH, US
Remember you, the structure, already have a local ground at the service entrance.

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#196730 - 10/24/10 01:43 PM Re: Portable generator grounding question [Re: LarryC]
LateGreatJC Offline
New Member

Registered: 08/27/10
Posts: 6
Loc: Missouri, USA
It's my understanding that the GEC is only used to handle over voltage problems. The grounded wire is what's used to clear faults, that's why the GEC, the grounding and grounded wires are all bonded together at the service disconnect. If a grounding wire was only connected to a made electrode and a fault occured, there may not be enough current flow to trip a breaker. So in the scenerio above, with the generator neutral isolated from the ground and no service grounded wire, a fault to the circuit grounding wire would have one path back to the generator chassis (becoming a shock hazard) and a 2nd path from the transfer switch cabinet to the service panel and the GEC. Since the neutral is bonded to ground here and bonded to the generator neutral in the transfer cabinet, the fault will travel back to the generator neutral. I assume this would cause enough current flow to trip the overload on the generator. I suppose that would work, but it just doesn't 'feel' right. Makes more sense to completely isolate the 2 different power sources. But I guess that's why I'm not an electrical engineer.

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#196731 - 10/24/10 03:00 PM Re: Portable generator grounding question [Re: LateGreatJC]
renosteinke Offline
Cat Servant
Member

Registered: 01/22/05
Posts: 5305
Loc: Blue Collar Country
First, be clear on one thing: no one truly 'understands' electricity. ALL of our models have flaws, and 'unusual' situations usually underscore the weakensses of our models.

I am a strong supporter of transfer-switched neutrals specifically because of questions like these. I'd rather avoid the uncertainty.

Here's grounding in a nutshell: Electricity only wants to "go home." It does not want to go anywhere else.

For power from the power company, this means it wants to go back to the transformer that made it.

For generator-made power, this means it wants to get back to the generator.

For lightning, this means back to the planet surface.

The 'neutral' is nothing but a path 'home.'

Care to guess where the ground rod fits into all of this?

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#196732 - 10/24/10 03:33 PM Re: Portable generator grounding question [Re: renosteinke]
Alan Belson Offline
Member

Registered: 03/23/05
Posts: 1801
Loc: Mayenne N. France
It puts me at the same potential as the generator chassis?
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#196733 - 10/24/10 07:34 PM Re: Portable generator grounding question [Re: Alan Belson]
renosteinke Offline
Cat Servant
Member

Registered: 01/22/05
Posts: 5305
Loc: Blue Collar Country
Alan, you know better ...

With a portable generator, there is no way to assure a good connection between the generator and the ground you're standing on- if for no other reason than that the generator has rubber feet.

So you drive a ground rod. What have you accomplished, apart from taking 'portable' out of the equation?

No, our 'ground' is of interest only in clearing faults. That's where the ground wire, and the neutral/ground connection at the transfer switch comes into the picture. Any current that comes from the generator wants only to return to that generator - not to go anywhere else.

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#196774 - 10/26/10 08:20 PM Re: Portable generator grounding question [Re: renosteinke]
LateGreatJC Offline
New Member

Registered: 08/27/10
Posts: 6
Loc: Missouri, USA
renosteinke - thanks for your reply. Your explanation of the ground returning to its source makes perfect sense and I agree completely. Another reason I don't like sharing the neutrals is that you're required to isolate the neutral and ground in the generator. Since it's portable, it can be used for other applications where an appliance or tool can be plugged directly into the generator. Would it be dangerous to use it like this with the neutral and ground isolated? Would the overload trip on a fault to the grounding wire? It would be easy for the user to forget to change this connection for each change of use.
Similar problem with connecting the generator to a GEC. Likely the generator will be stored elsewhere and moved to the house connection point when it's needed, requiring the user to remember to make that connection. And if the GEC is for handling overvoltage problems, where is the overvoltage going to come from anyway?

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#196776 - 10/26/10 08:31 PM Re: Portable generator grounding question [Re: LateGreatJC]
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
I am still not sure where the danger is if the ground and neutral is isolated in a truly portable generator. The small Hondas are made this way. There is no real connection between the power and ground to provide the hazard.
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Greg Fretwell

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#196781 - 10/27/10 05:58 AM Re: Portable generator grounding question [Re: gfretwell]
resqcapt19 Offline
Member

Registered: 11/10/00
Posts: 2209
Loc: IL
While I don't see a hazard with the generators that do not have their neutral bonded, there is no way to use those in an NEC application because 250.20 will require a grounded system in most cases. As far as what is an NEC application, given the definition of premises wiring...most everything;
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Don(resqcapt19)

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