Help with measurements of N-G bonding on inverter generators for article

[phaset]

Everyone,

I'm hoping a few of you can help me out with some measurements for an upcoming article. I am the technical column writer for FOH Magazine, the leading professional audio trade magazine. Because inverter generators tend to be different beasts, I want to write a sidebar for the article specifically about them. This pertains primarily to the Honda EU series generators, specifically the EU6500i, which is quite prevalent as a small generator in the professional audio (and film) context. I'm trying to confirm the lack of a low impedance current path between N/G on this generator, among other things.

And yes, I've already reached out to Honda Power Systems US, as their headquarters are <5 miles from my day gig (chief engineer of an industrial lighting company). The fellow I talked to was knowledgeable, but couldn't clarify all the fine points.

See these two threads on soundforums.net for context:
GFCIs small generators and NEC / request for inverter genset measurements
http://soundforums.net/lighting-electrical/7329-nec-2013-draft-osha-generators-details-discussion-long.html
http://soundforums.net/lighting-electrical/7327-inverter-generator-owners-please-help-my-foh-article.html

 

[tom baker]

Did you measure n to g?

 

[Smart $]

I downloaded one of the four manuals listed on their website for the model spec'd.

The schematic shows no direct connection between output neutral and generator/receptacle ground. It shows a ground connection to the contained inverter unit but only indicates it as connecting to the case. There is also no indication that any of the generator windings are grounded.

Unless you send me a unit p), I can provide no further analysis.

 

[phaset]

EU6500i N-G measurement

EU6500i N-G measurement

Did you measure n to g?

Tom,

I did on the 6500 that was rented for a festival I helped with 2 weekends ago. It indicated that ground and neutral are not bonded. I was primarily looking for separate confirmation, and perhaps experience with some of the other inverter generator models (e.g. Yamaha) out there. This unit's inverter is differentially configured with +120V to neutral and -120V to neutral. Since you could eat off the equipment from this particular generator rental company, and that the configuration matches Honda's manual, I'm pretty sure its correct.

It is, of course, one thing to think something correct in your head, and another thing to assert it in print to a circulation of more than 20,000 people.

For the use case of a home with a transfer switch that transfers only the hots, and does not produce a separately derived source, Honda's configuration makes sense. As used in the pro audio or film world as a small separately derived source, the lack of bonding and GFCIs presents potential issues.

-Phil Graham

 

[phaset]

I downloaded one of the four manuals listed on their website for the model spec'd.

The schematic shows no direct connection between output neutral and generator/receptacle ground. It shows a ground connection to the contained inverter unit but only indicates it as connecting to the case. There is also no indication that any of the generator windings are grounded.

Unless you send me a unit p), I can provide no further analysis.

Smart $,

I see this exactly the same way from Honda's information, so I'll run with what they show.

-phil g

 

[phaset]

First generator article is up

First generator article is up

All,

Thanks for the discussion on the forum about inverter generators for pro audio. The first FOH article in the series is up. If you want to read it online, go to the link and then choose page 50 to go directly to the article's start:
http://digitaleditiononline.com/publication/?i=163172

Looking back, as a newbie to Mike Holt forums, I should have posted this in bonding vs. grounding sub-forum. This first article amounts to a differentiation between grounding and bonding, with a mind towards portable production event power.

It suffices to say that the behind the scenes details for small generators and the NEC are a very engaged topic at the moment, and we just happened to start this series coincident with that discussion. We'll have plenty of interesting thing to discuss in FOH as the series progresses.

Cheers,

-Phil

 

[GoldDigger]

All,

Thanks for the discussion on the forum about inverter generators for pro audio. The first FOH article in the series is up. If you want to read it online, go to the link and then choose page 50 to go directly to the article's start:
http://digitaleditiononline.com/publication/?i=163172

Looking back, as a newbie to Mike Holt forums, I should have posted this in bonding vs. grounding sub-forum. This first article amounts to a differentiation between grounding and bonding, with a mind towards portable production event power.

It suffices to say that the behind the scenes details for small generators and the NEC are a very engaged topic at the moment, and we just happened to start this series coincident with that discussion. We'll have plenty of interesting thing to discuss in FOH as the series progresses.

Cheers,

-Phil

Thanks for the follow up, Phil. I appreciate that.

One of the places where the bonding issue does come up frequently is when a PV inverter is added to an RV with an existing internal charger/inverter (power convertor) with a transfer switch for shore power. The shore power connection is expected to have a ground-neutral bond, but the neutral is interrupted by the transfer switch integral to the convertor. The convertor (if it is a Modified Square Wave variety) may drive the RV wiring with a symmetric AC waveform, similar to a "balanced" supply described in the NEC. The hot and neutral of the 120V are both varying by 60V AC from ground.

If a generator or PV system that has a ground-neutral bond is connected using anything but a switched-neutral transfer switch, it will let the magic smoke out of the convertor and maybe some of the wiring.

 

[phaset]

Thanks for the follow up, Phil. I appreciate that.

One of the places where the bonding issue does come up frequently is when a PV inverter is added to an RV with an existing internal charger/inverter (power convertor) with a transfer switch for shore power. The shore power connection is expected to have a ground-neutral bond, but the neutral is interrupted by the transfer switch integral to the convertor. The convertor (if it is a Modified Square Wave variety) may drive the RV wiring with a symmetric AC waveform, similar to a "balanced" supply described in the NEC. The hot and neutral of the 120V are both varying by 60V AC from ground.

If a generator or PV system that has a ground-neutral bond is connected using anything but a switched-neutral transfer switch, it will let the magic smoke out of the convertor and maybe some of the wiring.

GoldDigger,

That seems like yet another edge case that needs considered in these kinds of situations, and certainly gets at the heart of the matter in the difficulty of configuring NG bonding on various devices. In my research for the article above, one of the things I read through was UL458, which allows the "balanced" +/- 60V to ground inverter configuration you speak of. The smaller Hondas (EU 2000/3000) have inverters configured this way.

In the case of the larger inverter generators (>5kw) I investigated (Yamaha and Honda), they are both configured without a post-inverter NG bond. This appears to be done with the mindset that the generator neutral, service neutral, generator ground, and service ground will be bonded in the main service panel. This is a totally reasonable configuration for providing household backup power where there is only a swap of the ungrounded conductors via the transfer switch, and the generator is not an SDS.

In the case of standalone (i.e. SDS-like) usage, though, the lack of NG bonding is problematic. There is no fault-current return path to the inverter, let alone a low impedance one, and thus no engagement of the (non-gfci) overcurrent device. Then there is the energizing of the generator frame up to the inverter line potential.

Because standalone usage defines portable audio/visual production events (The Honda EU6500is DOMINATES small events in this space), it is a circumstance where a NG bond on the generator, or a fault current detecting device (e.g. gfci), is needed. The motion picture folks here: http://www.screenlightandgrip.com/html/hdfilmstrip4lg.html have addressed this by using a transformer after the generator and doing the NG bond at the secondary SDS.

I've been in communication with the portable generator manufacturers association (http://www.pgmaonline.com/) about ng bonding, and other topics. I also sent them a fellow who had a Honda EU6500 denied by the AHJ at a production event because of the above discussion.

PGMA has evidently been busy with Tentative Interim Amendment (TIA) 1097 about proposed NEC 445.20, which closed for comment last Friday, but hopefully they can get back to me when the dust settles from that:
http://www.nfpa.org/Assets/files/AboutTheCodes/70/ProposedTIA1097_NFPA70.pdf