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Author Topic: Ground loop with a portable generator  (Read 1506 times)

Steven Cohen

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Ground loop with a portable generator
« on: September 21, 2017, 11:49:30 am »

Greetings,

I have a question about grounding a 3K watt Yamaha inverter generator during power outages. I am planning on bonding the generator from its supplied ground screw to a bare copper ground wire that runs between my central A/C and pool pump. The main breaker will be off during a power outage. I also would like to run a metal security chain between the metal handles on the generator and an aluminum fence. My question is, do I need to be concerned with a potential ground loop by the generator being bonded to both the copper ground wire and the aluminum fence, assuming the fence is bonded to ground at some place?

Thanks in advance for any advice,
Steve 
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Phil Graham

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Re: Ground loop with a portable generator
« Reply #1 on: September 21, 2017, 01:05:49 pm »

Greetings,

I have a question about grounding a 3K watt Yamaha inverter generator during power outages. I am planning on bonding the generator from its supplied ground screw to a bare copper ground wire that runs between my central A/C and pool pump...

Steve,

The Yamaha inverter generator in question is a floating neutral generator. A floating neutral generator does not have the frame, nor the EGC (i.e. ground/green wire) bonded to the neutral return of the inverter. Per the portable generators manufacturing association, floating neutral generators, like the Yamaha, are not designed to be operated connected to the GEC and ground rod when operating cord and plug equipment in a standalone fashion. Doing so can create result in a secondary fault current path through the earth in some cases. Please let me know if this makes sense, as it is a subtle point.

See reply #4 (http://forums.prosoundweb.com/index.php/topic,164927.msg1519827.html#msg1519827) for clarification of what happens when tied to the mains panel, in lieu of service supply from the utility.

P.S. the attached file is actually a PDF from the portable generator manufacturers association explains the secondary fault current path you could create during standalone use. I've had to change the file extension to get it to upload. Please change it back to .pdf and it will open correctly.

Edit: Clarified the use cases between standalone generator use, and panel tied generator use.
« Last Edit: September 21, 2017, 03:15:34 pm by Phil Graham »
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Stephen Swaffer

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Re: Ground loop with a portable generator
« Reply #2 on: September 21, 2017, 01:43:52 pm »

Phil,

This does not make sense to me.  The green ground terminal he mentions is connected to the frame of the genny.  The NEC and the owners manual require this to be attaced to a GEC (grounding electrode conductor) to a grounding electrode.  Bonding in the manner the OP describes can do nothing but improve safety.

I would be interested in reading their explanation-but having trouble getting the file into the correct format.
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Steve Swaffer

Phil Graham

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Re: Ground loop with a portable generator
« Reply #3 on: September 21, 2017, 02:20:07 pm »

Phil,

This does not make sense to me.  The green ground terminal he mentions is connected to the frame of the genny.  The NEC and the owners manual require this to be attaced to a GEC (grounding electrode conductor) to a grounding electrode.  Bonding in the manner the OP describes can do nothing but improve safety.

I would be interested in reading their explanation-but having trouble getting the file into the correct format.

Stephen,

Take the attached file, and change the extension to .pdf, and it should open for you in your PDF viewer of choice.

I discussed this whole topic at great length with the PGMA back in 2013 in the course of writing a series of articles on small generators for FOH. details surrounding all of that that are contained in the linked thread, including discussion of the double fault case:
https://soundforums.net/forum/low-earth-orbit/lighting-electrical/8463-nfpa-decision-on-new-section-nec-445-20-for-small-15kw-portable-generators

I think if you read that thread, it will clarify PGMA's thinking.
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Phil Graham

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Re: Ground loop with a portable generator
« Reply #4 on: September 21, 2017, 03:09:06 pm »

Some further thoughts, as I probably have muddied the OPs original q:

If the OP is connecting the generator to their main panel by means of a transfer switch, or open mains breaker, then the Yamaha will already have the single EGC / Grounded conductor / GEC / panel enclosure bond point at the main panel. This point will therefore tie the Yamaha to the grounding electrode. For this use case you would not want a second grounded -> egc bond point at the generator, to prevent objectionable current via the generator's EGC, per NEC 250.6. It is this use case why the manufacturers float the generator neutral.

You could bond the generator's frame to the GEC, of course, which is presumably what the AC -> pool pump wire is acting as.

---

The NEC compliance of all "floating neutral" generators is contingent on their being no fault current possibility, which is assured by the floating neutral under single fault conditions. The moment that a fault current could flow (e.g. via a second fault through ground), then there must be a low impedance path for that fault current, which their standard egc configuration does not provide. Hence not grounding the floating neutral generator in the event of standalone use with cord and plug connnected equipment.
« Last Edit: September 21, 2017, 03:20:08 pm by Phil Graham »
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Steven Cohen

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Re: Ground loop with a portable generator
« Reply #5 on: September 21, 2017, 03:17:26 pm »

Thanks Phil for taking the time to respond.
 I am glad you posted the link to the thread from the SoundForum Network as I was planning on using an external GFCI device to female end of an extension cord. If I understand the floating ground principle correctly, the GFCI would not provide any additional protection as a GFCI needs a bonded ground to function correctly.
I think my best option at this point is to feed a manual transfer switch from the generatorís 125 volt, 30-amp twist lock receptacle to my homeís main electrical service panel. Coincidentally, I have a 35 year old Federal Pacific electrical service panel that needs to replaced due to being a fire hazard, I will have the manual transfer switch installed at the same time.


Some further thoughts:

The NEC compliance of all "floating neutral" generators is contingent on their being no fault current possibility, which is assured by the floating neutral under single fault conditions. The moment that a fault current could flow (e.g. via a second fault through ground), then there must be a low impedance path for that fault current, which their standard egc configuration does not provide. Hence not grounding the floating neutral generator in the event of standalone use.

Further, if the OP are connecting the generator to their main panel by means of a transfer switch, or open mains breaker, then the Yamaha will already have the single EGC / Grounded conductor / GEC bond point in the main panel. This point will therefore tie the Yamaha to the grounding electrode. For this case you would not want a second grounded -> egc bond point at the generator, to prevent objectionable current via the generator's EGC, per NEC 250.6. You could bond the generator's frame to the GEC, of course.
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Scott Holtzman

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Re: Ground loop with a portable generator
« Reply #6 on: September 21, 2017, 03:27:02 pm »

If I understand the floating ground principle correctly, the GFCI would not provide any additional protection as a GFCI needs a bonded ground to function correctly.


GFCI does not require a ground to function, it detects a difference between the forward and return load.
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Phil Graham

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Re: Ground loop with a portable generator
« Reply #7 on: September 21, 2017, 03:29:33 pm »

Thanks Phil for taking the time to respond.
 I am glad you posted the link to the thread from the SoundForum Network as I was planning on using an external GFCI device to female end of an extension cord. If I understand the floating ground principle correctly, the GFCI would not provide any additional protection as a GFCI needs a bonded ground to function correctly.

This is part of of Guy's discussion on "equivalent protection," in the event of a double fault, in the SFN thread. But that is in the context of a standalone floating neutral generator with double fault conditions. A GFCI merely looks for current difference, ground or no, but Guy's point is that this specific fault case bypasses the GFCI's ability to detect the current difference.

I re-organized reply #4 for clarity while you were posting your reply, so maybe have a look at it again for your intended use case.

Quote
I think my best option at this point is to feed a manual transfer switch from the generatorís 125 volt, 30-amp twist lock receptacle to my homeís main electrical service panel...

If you tie the generator into your mains panel, then the EGC -> GEC -> grounded (i.e. neutral) bond is done in the panel, and your Yamaha becomes a "normal" generator with N/G bonding, by means of the panel. You don't want a second N/G bonding at the genny, as that would give two parallel current paths between the panel and the gennie. But because the generator is already tied to the GEC and grounding electrode via the panel, there's no problems with further bonding the generator frame to the GEC, or the fence to the GEC.
« Last Edit: September 21, 2017, 03:42:07 pm by Phil Graham »
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Stephen Swaffer

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Re: Ground loop with a portable generator
« Reply #8 on: September 21, 2017, 04:43:10 pm »

My understanding is that he was using the ground screw on the genny-which is essentially the frame of the genny to bond to the fence/egc.  This has absolutely NOTHING to do with ground-neutral bonding-which on that genny could only be accomplished by an internal modification or some sort of a G-N adapter as has been discussed in other threads.  Making sure the fence is in fact bonded to the install (not just earth grounded) adds a significant measure of safety to this setup-even if the genny is connected through a transfer switch and thus bonded to ground.  There was a recent electrocution (Kansas, I believe) of a child that in all liklihood would have been prevented had the fence been bonded to the electrical service (no matter if that is a genny or mains).

I would argue that a GFCI still provides protection in the case of a double fault.  Sure-it won't detect current flowing from line-line.  GFCI's never will, but if fault current takes an  alternate path bypassing any part of the proper circuit of a GFCI it will still trip.  If there is no path for fault current to follow to trip the GFCI, then there is no path for fault current to follow to electrocute somebody.  I'd rather have the GFCI present in case a fault occurs that I am not aware of-either in the genny or something attached to the genny-that essentially bonds one side of the circuit making it a "neutral".

Stephen,

Not sure what panel you are installing.  Several manufacturers make UL listed kits to allow a breaker in the panel to be interlocked with the main and backfed to be used as a transfer switch.  This is usually far more cost effective than a seperate transfer switch and very easy to incorporate during an upgrade like this.  I've installed several and never had an inspector have an issue.  Just an option to explore.
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Steve Swaffer

Jonathan Johnson

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Re: Ground loop with a portable generator
« Reply #9 on: September 22, 2017, 12:32:40 pm »

Red flags flew up in my mind when I read this...

...The main breaker will be off during a power outage...

This made it sound as though you were planning to hook the generator up with a double-male cord plugged in to the generator and an outlet somewhere in the house.

Connecting a generator to premises wiring by means of a double-male cord or wiring into a standard panel is a big no-no. Even if you plan to turn the main breaker is off, there is too much danger of backfeeding electricity into the power grid, which can endanger utility workers and potentially overload your generator.

So I was relieved when I read this later post:

... I was planning on using an external GFCI device to female end of an extension cord. ...

...I think my best option at this point is to feed a manual transfer switch from the generatorís 125 volt, 30-amp twist lock receptacle to my homeís main electrical service panel. Coincidentally, I have a 35 year old Federal Pacific electrical service panel that needs to replaced due to being a fire hazard, I will have the manual transfer switch installed at the same time.

The second part of this is good. A properly installed transfer switch ensures that electricity from a generator can never be inadvertently backfed into the power grid. Besides, if the "far" end of a double-male cord comes unplugged, now you've got exposed live conductors trying to poke things.

When connecting a transfer switch for standby power, it's a good idea to keep a light connected to the utility power and turned on so that you'll know when the utility power comes back on.

* * * * *

If the neutral of the generator is internally bonded to the frame, do not use the onboard GFCI to feed a transfer switch. That's because on the utility or home side of the transfer switch, the ground and neutral will be bonded.

In other words, when feeding a transfer switch, you'll have a ground-neutral bond on both sides of the GFCI. That means there's a potential return path for "neutral" current around the GFCI sensing circuitry from the premises G-N bond, through the equipment grounding conductor of the cord, and the generator's G-N bond. This will cause the GFCI to trip even when there isn't a fault.

If the generator does not have an internal G-N bond, then using the onboard GFCI to feed a transfer switch should not be a problem.

If you're using the generator to power multiple devices, it's recommended to bond ground and neutral. In a floating system, a fault in a single device isn't necessarily a problem, but if you have faults in multiple devices, the lack of a ground-neutral bond at the generator can result in voltage potential between the devices which could cause ground currents in audio interconnects or power cords, or potentially personnel should they come between two pieces of faulty equipment.

If you have audio devices powered by different power sources, such as two generators, a generator and shore power, or shore power from two different houses, you should ensure that the grounds of all power sources are externally bonded to prevent ground currents in audio interconnects or even personnel. (Audio isolation transformers with ground lift can also help.)
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Stop confusing the issue with facts and logic!
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