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Posted By: LateGreatJC Portable generator grounding question - 10/24/10 04:32 PM
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.
Posted By: gfretwell Re: Portable generator grounding question - 10/24/10 05:05 PM
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.
Posted By: LarryC Re: Portable generator grounding question - 10/24/10 05:44 PM
Remember you, the structure, already have a local ground at the service entrance.
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.
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?
It puts me at the same potential as the generator chassis?
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.
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?
Posted By: gfretwell Re: Portable generator grounding question - 10/27/10 03:31 AM
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.
Posted By: resqcapt19 Re: Portable generator grounding question - 10/27/10 12:58 PM
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;
Posted By: KJay Re: Portable generator grounding question - 10/27/10 03:16 PM
My portable generator pulls double duty as a home backup and is also occasionally used on the job, so I always just leave the neutral bonded to the frame.
Doesn’t OSHA require a generator used on a jobsite to have the neutral bonded to the frame?

Maybe manufacturers could put a switch on their portable generators to change the configuration, instead of having to manually remove and install a jumper wire. Then again, that might just make things more confusing.

IMO, no matter what perceived hazard exits because of this situation, from a safety standpoint, things are still light-years ahead of where they were 10-years ago.
It’s got to be much safer than backfeeding into a dryer receptacle, like so many people used to do before all these home transfer switches and generator-ready load centers became readily available.
Posted By: gfretwell Re: Portable generator grounding question - 10/27/10 06:07 PM
I suppose we should look at why we ground services

250.4(A)

Quote
(1) Electrical System Grounding. Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.


Those are not scenarios that are present with a portable generator. You are not driving a rod so you are not going to ground the system anyway. Bonding the neutral really serves no purpose.

If you are connecting to the house with switched neutral transfer equipment (taking the MBJ out of the circuit) you would need to bond the neutral in the generator but if the transfer equipment is only switching the ungrounded conductors you should leave the generator unbonded.
Posted By: SteveFehr Re: Portable generator grounding question - 11/02/10 06:17 PM
The neutral must be bonded once, and exactly once, in any given electrical system. If the neutral is not bonded, equipment that relies upon the neutral being close to ground (like old ranges and dryers) can be become dangerously energized during fault conditions. Additionally, ground the neutral effectively ties the hot wires to ground; if it was left to float, instead of -120 and +120 phases, you may end up with -5V and +245V, which is considerably more dangerous.

The neutral cannot be bonded in multiple places because the neutral current will travel through the ground system, which creates all sorts of other safety issues. And yes, this is considered a "ground fault" just as much as accidentally shorting a neutral somewhere to ground is.

ALL small portable generators with receptacles on the side have an internal ground-neutral bond because they're intended to run standalone. When you grab one of these off-the-shelf and plug it into a house, you've now created a double-neutral bond, with neutral current running through the generator cable's ground wire, and potentially energizing the generator chassis. You need to do one of two things to remedy this:

1) Disconnect the ground/neutral bond in the generator. I don't like this, because it makes it dangerous to use as a standalone generator. If you do this, disable the outlets!

2) Install a transfer switch that transfers the neutral. (Note: have to move the ground/neutral bond point to the line side of the transfer switch; you can't leave it in the panel.)

The ground rod on the generator is simply for extra safety, and ensuring the generator chassis and ground wires are all at the same potential as the earth.
Posted By: gfretwell Re: Portable generator grounding question - 11/02/10 07:37 PM
Nobody has given me a compelling reason to ever bond the neutral in a portable generator that never gets connected to premises wiring. If anything you are creating a hazard that didn't exist. Like an ungrounded delta, the first fault is free.
Posted By: KJay Re: Portable generator grounding question - 11/04/10 07:20 PM
Originally Posted by gfretwell
Nobody has given me a compelling reason to ever bond the neutral in a portable generator that never gets connected to premises wiring. If anything you are creating a hazard that didn't exist. Like an ungrounded delta, the first fault is free.


Greg, Wouldn’t a fault to ground in a piece of equipment being powered up not have a return path back to the source?
I know some portables, like mine, don’t have the GFCI protected duplex outlets on them. I use a plug-in GFCI adapter, but I’m sure there are still a few people out there who don’t.
Posted By: gfretwell Re: Portable generator grounding question - 11/04/10 10:13 PM
If there is no path back to the source, where is the danger? That is the theory of isolation transformers on lab benches.
In that situation there is no "hot" or "neutral", just 2 conductors with some volts between them. There is no reference to ground. You can be sitting on a metal bench in a puddle and there is no fault path unless you get phase to phase.

We ground services because of the things that can happen to overhead lines (lightning strikes, contact with high voltage secondaries, surges etc). You don't have that exposure with a portable generator.
Posted By: LarryC Re: Portable generator grounding question - 11/04/10 10:15 PM
Quote
Wouldn’t a fault to ground in a piece of equipment being powered up not have a return path back to the source?


Actually, there probably enough leakage current via capacitance coupling to produce enough current to shock or kill a person. Without the bond, there will never be enough current to trip any overcurrent protective device.

Plus without a reference to earth, the output voltages can float anywhere above or below earth potential. This could result in the failing of insulation in extreme circumstances.
Posted By: gfretwell Re: Portable generator grounding question - 11/04/10 10:49 PM
I would be curious just exactly how much capacitive coupling there really is but since small Honda generators are shipped without a bonded neutral and we have not seen dead bodies dropping all over I doubt it is a problem.
Again, if nothing is grounded, where is the fault path?
I imagine the EGC would measure 60v to each phase and zero to a water pipe. A megohm resistor to either phase would make the 60v go away.
Posted By: KJay Re: Portable generator grounding question - 11/04/10 11:34 PM
Wouldn’t the guy standing in the puddle with one hand hand on the grounded building steel and leaning up against a piece of ungrounded equipment with its frame energized become a conductor and provide the path to ground?
Posted By: gfretwell Re: Portable generator grounding question - 11/05/10 12:26 AM
There would be a single path to ground but no return path to complete the circuit. No current would flow.
Posted By: KJay Re: Portable generator grounding question - 11/05/10 02:09 AM
Okay, but just to clarify things, most generators have the yoke of their 3-wire receptacles in direct contact with a metal panel that is also connected to the generator frame. There usually is no way to bond the receptacle grounding terminal to the neutral conductor of the generator receptacle without also bonding the frame and vice versa. There may be some that have this ability, but it’s not a very common feature from what I’ve seen.
I don’t see why putting yourself between the line and "neutral" wouldn’t result in current flow or why giving it a second path to an independent "ground" couldn’t still allow enough flow to cause trouble for someone.
My generator is pretty typical. Each side of the 20A/120V duplex is connected to one leg of the generator, so 240V between hots. With a 500W load [halogen work light] connected to one half of the duplex receptacle, I can read 56V AC from the hot of the other half of the receptacle with no connected load to an independent building ground using my Fluke 337 meter.
It seems that the hazard could possibly increase when multiple loads are connected to the receptacles at the same time.
Posted By: gfretwell Re: Portable generator grounding question - 11/05/10 03:17 AM
If neither winding of the generator is connected to anything but the current carrying conductors in the receptacle there is no path to the frame or to ground. The output is totally isolated from both. If you touch ther center tap of the winding that we think of as neutral and one of the other generator windings you have a fault but none of these windings will create a fault to the EGC pin, ground or to the generator frame
Posted By: LarryC Re: Portable generator grounding question - 11/05/10 08:04 AM
How much extension cord would be required to capacitively couple 5 mA of current thru a short between a phase and earth from a unbonded small generator?

How much crappy abused cheap extension cable is required to leak 5 mA of current thru a defective power tool thru a sweaty person to a return path via the crappy abused extension cord?

I guess it is all dependent on how much nannying we are willing to tolerate. In most cases, there is probably not a problem bonding or floating the neutral with respect to earth when using "small" generators.

When it comes to connecting to an existing structure that is supposed to be NEC compliant, requiring the earth neutral bond will be necessary, if for nothing else, because of the size and variety of loads in the structure. Plus the potential for interaction with other services that may be subject to voltage transients such as cable TV, phone service, isolated water or gas/propane supplies, etc.

Larry C
Posted By: KJay Re: Portable generator grounding question - 11/05/10 11:05 AM
I think there may be some confusion about what I’m saying. The last two paragraphs highlighted in the regulation below seem to describe what I’m driving at.

This link has some pretty decent graphics too.
Generator Regs

Grounding Requirements for Portable
and Vehicle-mounted Generators
Under the following conditions, OSHA
directs (29 CFR 1926.404(f)(3)(i)) that the
frame of a portable generator need not be
grounded (connected to earth) and that the
frame may serve as the ground (in place of
the earth):
• The generator supplies only equipment
mounted on the generator and/or cord and
plug-connected equipment through
receptacles mounted on the generator,
§ 1926.404(f)(3)(i)(A), and
The noncurrent-carrying metal parts of
equipment (such as the fuel tank, the internal
combustion engine, and the generator’s
housing) are bonded to the generator
frame, and the equipment grounding conductor
terminals (of the power receptacles
that are a part of [mounted on] the generator)
are bonded to the generator frame,
§ 1926.404(f)(3)(i)(B).
Thus, rather than connect to a grounding
electrode system, such as a driven ground
rod, the generator’s frame replaces the
grounding electrode.
Posted By: gfretwell Re: Portable generator grounding question - 11/05/10 05:43 PM
KJay, you are just saying the non current carrying metal is all bonded together, not that it is bonded to any current carrying conductor. As far as I am concerned, connecting this to the current carrying conductor, without actually grounding it, is creating an additional hazard.

Larry, I don't think anyone is saying you can connect a generator to a structure without grounding it.
Posted By: LarryC Re: Portable generator grounding question - 11/05/10 06:30 PM
Originally Posted by gfretwell
If neither winding of the generator is connected to anything but the current carrying conductors in the receptacle there is no path to the frame or to ground. The output is totally isolated from both.


I disagree with the statement that the output is totally isolated from both the frame or the ground. There will be SOME coupling. Is it enough to cause a shock or fatality? That depends.

It depends on the state of the insulation of whatever is connected to the generator and the condition of the generator itself. It depends on the applied voltage to the insulation.

It comes down to Who or What are we trying to protect?

Larry
Posted By: gfretwell Re: Portable generator grounding question - 11/05/10 06:37 PM
You say there is some coupling and then you say you will fix that by intentionally bonding it. Isn't that 100% coupled?
At that point you have to make the arbitrary decision of which current carrying conductor you want to bond. Until you bond one, they are all the same and none of them has a particular affinity to the frame of the generator.

As I said before, small Hondas are not bonded and I have not heard of any problems with them.
Posted By: LarryC Re: Portable generator grounding question - 11/05/10 09:41 PM
I am stating that there is "some" coupling. How much is dependent on the individual generator and the connected load.

By bonding the "neutral" to the frame/earth of the generator, the variability of this coupling is no longer a variable. With a known configuration, certain protective devices can act in a predictable manner. (GFCI)

Are protective devices necessary? That is a different question.

Is bonding of any of the output leads required or even desirable? It all depends on the design of the system.

Will not bonding any output lead cause a problem? In most cases of using small portable generators, I think not.

Can it cause a problem? Under the right circumstances, absolutely!

Do I think the bond is required? Right now, as long as the generator is a truly independent system, I do not believe it is necessary. As soon as it gets connected to an external system, such as an fuel supply, battery charger, remote monitoring system, block heater, an external structure like a building, hard wired phone system, or anything else that makes it not independent, then a bond MAY be required.

Larry
Posted By: gfretwell Re: Portable generator grounding question - 11/06/10 12:23 AM
I agree with all of that and when the generator is connected to a structure it must be bonded.
I suppose I just prefer 2 pole transfer equipment and leaving that bonding to the service main bonding jumper.
When you start switching neutrals you open yourself up to all sorts of potential problems.

Posted By: KJay Re: Portable generator grounding question - 11/06/10 01:59 AM
From what I'm reading, it sounds like the bigger shock hazard may not be from electrical so much, as from the possible fine OSHA may issue if someone brings a portable generator on a jobsite without the grounding terminal of the receptacles being bonded to the neutral or not having GFCI protection for them. frown
Posted By: gfretwell Re: Portable generator grounding question - 11/07/10 03:07 AM
If OSHA shows up I would pull out this rule.

Quote
1926.404(b)(1)(ii)

Ground-fault circuit interrupters. All 120-volt, single-phase 15- and 20-ampere receptacle outlets on construction sites, which are not a part of the permanent wiring of the building or structure and which are in use by employees, shall have approved ground-fault circuit interrupters for personnel protection. Receptacles on a two-wire, single-phase portable or vehicle-mounted generator rated not more than 5kW, where the circuit conductors of the generator are insulated from the generator frame and all other grounded surfaces, need not be protected with ground-fault circuit interrupters
Posted By: KJay Re: Portable generator grounding question - 11/07/10 04:04 AM
Greg, it looks like if you have a straight 120V two-wire generator it would fly, but from what I’ve been reading on the Q&A on the OSHA site, it sounds like if the generator is both 120/240V, it might not be allowed.

Here’s an example reply from the Q&A that has me wondering:

In the OSHA standards for portable vehicle-mounted generators, equipment grounding conductor bonding is mentioned in 1926.404(f)(3)(i)(B) and in 1926.404(f)(3)(ii)(C).Neutral conductor bonding is mentioned in 1926.404(f)(3)(iii).
The intent of 1926.404(f)(3)(iii) becomes very clear when one considers that the term “neutral" is used in its technical sense. A 120/240 volt system has a neutral and therefore must be bonded to the generator frame. A 2-wire 120 volt system has no neutral and therefore bonding is optional. Recall that neither side of a 2-wire derived system is a neutral and when one grounds either side, it becomes a grounded terminal or conductor, but it is not a neutral.
Posted By: gfretwell Re: Portable generator grounding question - 11/07/10 05:27 AM
I have still not heard a good reason why you bond the neutral in an ungrounded system. There is certainly no way to have a "ground fault". About the only thing you can have is a frame fault and you artificially created that situation when you bonded the generator.

Posted By: LarryC Re: Portable generator grounding question - 11/08/10 12:53 AM
If you have a floating 120/240 system from a portable generator and a fault developes that connects one of the "hot" legs to earth, is there more risk with the other "hot" leg now being 240 V with respect to earth?

I know that there 120 V breakers and 120/240 V breakers. What is the difference?

Larry
Posted By: gfretwell Re: Portable generator grounding question - 11/08/10 06:06 AM
Larry your "earth" question makes my case with an ungrounded generator. If you had the neutral bonded to the frame and one phase got grounded to earth, the whole machine would be 120v above earth, including the case of that drill you are holding.
You might operate a GFCI if you got in the fault path between EGC and earth but not the overcurrent device.

Posted By: LarryC Re: Portable generator grounding question - 11/10/10 02:45 AM
Originally Posted by gfretwell
Larry your "earth" question makes my case with an ungrounded generator. If you had the neutral bonded to the frame and one phase got grounded to earth, the whole machine would be 120v above earth, including the case of that drill you are holding.
You might operate a GFCI if you got in the fault path between EGC and earth but not the overcurrent device.


Greg, if you did not bond the centertap of a 120/240 generator and you had the same phase to earth fault, the case of the drill is at an unknown voltage with respect to earth.

I believe the case voltage will be dependant on how much current is flowing thru the holder of the drill.
Posted By: gfretwell Re: Portable generator grounding question - 11/10/10 03:54 AM
I would rather have the case floating than at a known 120v above the dirt I am sitting on.
Posted By: SteveFehr Re: Portable generator grounding question - 11/10/10 06:52 PM
Floating the voltages is a well known protection scheme; it used to be the norm before the advent of inexpensive GFCI outlets, and is still used in bathrooms in Europe and on naval ships, etc. It IS safer than a solidly grounded neutral due to fault tolerance; if one phase suffers a ground fault, it simply changes from a floating system to a corner-grounded system. Fault clearing is diminished, though; if there's a ground fault, you might never know until a 2nd fault occurs.

I'd just never heard of portable generators NOT bonding the neutral.
Posted By: gfretwell Re: Portable generator grounding question - 11/10/10 07:10 PM
Small Hondas were all shipped without the bonding. I haven't looked lately.
Posted By: WESTUPLACE Re: Portable generator grounding question - 11/10/10 10:13 PM
I have a Honda EU2000. There is no bond between ether of the leads although it has a grounded receptacle and a ground screw. It uses an inverter to generate power out of the multi-pole alternator. I feel real safe using tools in damp or wet locations with it.
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