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Phil Rhodes

Generator grounding (one for Guy?)

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Hi folks


A few times in the past we've discussed the practicalities and safety implications of grounding generators, or not grounding generators.


In some parts of the world, I've read that it's actually illegal to ground generators. Some sources say this is the case in Los Angeles. I am not familiar with the rationale behind this but I assume it has something to do with reducing the potential for injury in the (vanishingly rare) circumstance that a piece of equipment has a live-to-case short and its safety earth is open circuit. Some sources also suggest that this rule was brought in to avoid safety issues caused by people driving stakes in very deep in order to get a reasonable connection to the dry terrain of southern California, and running into water or gas pipes, but that sounds a bit apocryphal to me. Sources also say that people in New York routinely do ground generators. Presumably the safety rules are established on a state-by-state basis?


In the UK as far as I'm aware it's a requirement to do it although people often don't for some reason, but I'm not sure. I'm not that familiar with British Standard 7909, "code of practice for temporary electrical supplies." The relevant phraseology is in BS7430:1998, "code of practice for earthing," among other things. For that I'd have to pay £254, and I'm not that dedicated to becoming a professional electrician.


Any info gratefully received.



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If a 25-year-old memory from a weapons range is good enough, yes, we used to earth the 25kW ones at Shoeburyness. A pointed length of angle-iron IIRC, but it was usually so wet that you could have pushed it in with one finger. No pounding required.
Edited by Mark Dunn

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Good read. Covers Why and When and also exemptions to the rule. http://www.lightingandsoundamerica.com/mailing/plasaprotocol/pspring12_shadowandlight.pdf


As you can see by this article, to ground or not to ground is a hotly debated issue in the industry. The bottom line is that you must do what the Authority Having Jurisdiction (AHJ) requires. In the city of Los Angeles the AHJ requires that portable generators not be grounded, but instead isolated from earth. In the city of Boston they do - end of debate.


What is still debated is whether putt-putts, like the Honda EU generators, should be grounded or not since AHJs generally don’t extend their authority to include portable generators under 10kw (some do however.) What fuels the debate is a general ignorance that there are two distinct types of portable gas generators – those with Floating Neutrals and those with Bonded Neutrals. Which type of generator you are using, and how you are using it, determines whether it should be earth grounded with a ground electrode.


“Floating Neutral”, “Grounded Neutral”, “Bonded Neutral”, “Floating Ground”, “Earth Ground”, “Ground Fault”: if you are unfamiliar with the meaning of these terms I would suggest you read an article I wrote for our company newsletter on the use of portable generators in motion picture lighting before proceeding further. Neutral Bonded generators have their Neutral bonded to the frame of the generator. Neutral Bonded generators offer a high degree of protection against Ground Faults (if there was a fault to the frame via the Equipment Grounding System, the generator’s circuit breaker would trip eliminating the fault), and so they can be operated without an Earth Ground in what is called a Floating Ground condition. Crawford Studio generators are Neutral bonded, but most Honda portable gas generators are not.


In most Honda portable gas generators the neutral circuit is not bonded to the frame of the generator or to the earth ground lead; and are commonly called Floating Neutral generators. The floating neutral configuration is common for applications such as connection to a recreational vehicle and connection to home power where the transfer switch does not switch out the neutral to ground connection. When used as a stand-alone power source (a “Separately Derived System” in NEC parlance), Floating Neutral generators require, IMO, that at least the frame of the generator to be earth grounded.


Portable generators, like the Honda EU series, offer a false sense of security IMO. Their receptacles have ground pins and many of them now have GFCIs. But, absent a bonded neutral, fault current won’t to go to the Equipment Grounding Conductor (EGC) to trip a breaker and clear a fault. A neutral-ground bond is also essential to GFCIs operating reliably.


Simply using a GFCI on a Floating Neutral generator will not ensure a safe system, and can in fact be misleading. A GFCI will not operate reliably if one side of the winding is not grounded to the generator frame because Fault Current has no path back to the winding to complete the circuit. Only when neutral is bonded to the EGC, will current go to the EGC to complete the circuit when there is a current leak. In other words, a complete circuit is required to create an imbalance and cause the GFCI to trip. GFCI test circuits can also be misleading when they are used on Floating Neutral generators. On a Floating Neutral generator, the test button will draw power from the Hot through the CT and back through the CT again to the neutral via a Current Limiting Resister. The discrepancy caused by the Current Limiting Resister in the test will initiate the GFCI to trip even though there is in fact no Ground Fault Circuit for Fault Current to go to if there were a Fault. The false positive received by GFCI test circuits on Floating Neutral generators does nothing to eliminate faulty equipment.


OSHA spells this requirement out in fact sheet DSTM 10/2005:


"effective bonding of the neutral conductor to the generator's frame is also a concern for the safe use of the equipment. As with grounding terminal connections, proper bonding of the neutral terminal of a power receptacle may be confirmed via testing by a competent electrician with the correct equipment, and the ohmic resistance should measure near zero and must not be intermittent, which indicates a loose connection."


For those interested, I wrote a four part series on ground fault protection with the Honda inverter generators for Protocol. If you can’t find the print edition of the magazine, there are links to it and other articles in my “Production Power on a Budget” series in Protocol.


Guy Holt, Gaffer, ScreenLight & Grip, Lighting Rentals & Sales in Boston

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