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Problem Grounding a Generator

I recently acquired a small portable back up generator for nuisance outages. Only paid $200 new. It is 2500 watts continuous, 3250 surge rated and no 220 volt outlet, so I won't be hooking it up to a transfer switch.

Even though I'm going the extension cord method, I decided to properly ground it, and use an inline GFI and surge protector. I'm willing to risk using a back up computer with it, and I am hopeful that a surge protector might clean up the "dirty" generator power.

Now the problem. I drove an 8 foot ground rod into typical soil, used a correct copper clamp (nothing improvised), and connected the rod to the generator ground lug with a single strand 12 gauge copper wire.

Trouble is, the generator outlets still tests as ungrounded.

I immediately suspected miswired outlets on the generator. So I tried checking the resistance from the power receptacle outlet ground to both the generator frame and the generator ground lug. Both tests resulted in zero ohms of resistance. I'm thinking that would eliminate miswiring on the generator as the cause.

I then removed both ends of the ground wire connection, checked and reconnected them.

Same result, tests as open ground with a three prong plug in tester. I also tested the tester, and it shows grounded outlets in my house as properly grounded.

I checked the resistance from the generator end of the ground wire to the ground rod itself. It was also zero ohms.

So it seems to be a case of it has to work, but it doesn't work. Any suggestions are welcome.

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Tony Sivori
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Reply to
Tony Sivori
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You may have high resistance soil, and need more than one ground rod. Code typically calls for three, wired together.

Reply to
PeterD

It sounds as though the generator's neutral leg is not tied to ground at the generator. The outlet tester isn't actually testing that the ground prong is anywhere near ground, merely that current can flow between the hot and the ground leg. (In a standard residential setup, the neutral leg is tied to ground at the main breaker box, and all neutral and ground wires go to the common neutral/ground bar. The outlet tester would still show the outlet is wired properly even if the ground leg were not actually connected to ground.)

By the way, the surge suppressor will do pretty much nothing to clean up the generator output power. It's also rather likely that the output is reasonably clean to begin with; certainly far better than a typical "modified sine wave" inverter output, which is a combination of a couple of square waves. A typical computer, with a switching power supply, is not picky at all about power quality, as the first stage in the power supply is a rectifier and reservoir capacitor, changing whatever the input is into (unregulated) high voltage DC power. They run just fine from the above mentioned modified sine wave inverters, potentially even a bit more efficiently than from sinusoidal inputs.

Anyhow, you could tie the "neutral" leg of the generator output to ground at the generator, which would be safe provided the generator frame is actually grounded, and which would make the outlet tester happier. Safer, IMHO, is just using the GFI and not worrying about it more. Indeed, code permits GFIs to be used on existing ungrounded circuits to allow three-pronged devices to be plugged in despite the lack of a safety ground conductor (albeit with certain cautionary labeling required).

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Andrew Erickson

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Reply to
Andrew Erickson

I think you are probably right.

A quick Google shows that multiple ground rods are often necessary. Although I found the NEC calls for multiple rods only when a single rod installation exceeds it says 25 ohms. And it seems that every other possible reason has been eliminated.

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Tony Sivori
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Reply to
Tony Sivori

I think I know what the problem is. In a house, at the electrical panel, the neutral wire is connected to the ground. Your tester maybe assuming this and that is why it is not showing a ground connection. Your generator probably has a floating output (neutral not connected to the frame ground). Which is why the tested shows no ground. This will not pose a problem when using the generator, it is still grounded, it's just that the neutral is not connected to ground.

Shaun

Reply to
Shaun

I wasn't aware of that. Thanks.

That much I did know. I even knew that the only place the neutral can be grounded is at the junction box.

A test with the outlet tester and an ungrounded outlet confirms that you're right.

I've heard a few anecdotes of generator power damaging televisions and DVD players. I thought a pair (one on each outlet) of cheap surge protectors might help.

Unless it would add safety, I'm not at all worried about bonding the neutral to ground.

I may do that. One of the reasons I was dead set on proper grounding is that I knew that the surge protectors can't work without a ground. If the surge protectors are useless and unneeded, I could leave the ground connected as is, and just use the inline GFI.

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Tony Sivori
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Reply to
Tony Sivori

You're right about the floating neutral.

I found a couple of OSHA pages discussing portable generator grounding and neutral bonding.

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although it seems to contradict this one:

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Tony Sivori
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Reply to
Tony Sivori

On 11/15/2009 8:06 PM Tony Sivori spake thus:

Don't say "junction box", as that statement will make all electricians wince. You meant to say "breaker panel" or "distribution panel", right?

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Reply to
David Nebenzahl

A slip of the tongue, or fingers, if you prefer.

I've found some more pages on the pros and cons of bonding a portable generator floating neutral to ground.

formatting link

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Tony Sivori
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Reply to
Tony Sivori

And good luck trying to clean up the generator output if you intend on using a UPS in the circuit. My generator is 2500/3250 surge like yours. And it is brushless. But there was nothing I could do to filter the output well enough where my two 1000va APC UPS units would come back off battery. Both are used in my A/V gear after a about with nuisance outages of short duration but short enough to need to reset a lot of things. Also I had two 9 foot copper ground rods, one on either side of the house connected inside to my ham radio stuff. When hooked to an APC multi outlet filter with a wiring fault indicator, the indicator did not light while on generator power. So I guess in my case the two rods were enough. And they sure reduced my noise level on the HF bands, the main reason for their installation.

Reply to
Meat Plow

If clean power is a requirement, can't you add inside your 'distribution panel' a couple of multistage EMI filters? Then, for the 2kHz to 150kHz band, do your own?

Sequence is GEN, EMI Filter, EMI Filter, Custom Filter, your stuff of course, put the breakers and over voltage in there in the appropriate place.

EMI filters have Y caps, so...

Reply to
Robert Macy

Not much. It may eliminate voltage spikes which rise significantly above the nominal voltage level (although if there are enough of these, they may burn up the MOV!). If it also has a "noise filter" in it, it may reduce radio-frequency hash.

It will probably not, unfortunately, turn the rather ratty-looking power waveform coming out of the generator into a clean sinusoid.

The three-prong testers work by detecting the fact that the "ground" and "neutral" wires are bonded together, back at the main circuit-breaker panel. This allows a tiny trickle of current to flow from the "hot" pin, through the neon bulb in the tester, to the "ground" pin, and back to the panel via the ground wiring. [I infer that this current flow isn't enough to trip any GFI in the circuit.]

The tester does not indicated "grounded" when you plug it into your generator, because the generator does not strap its "neutral" and "ground" together within the generator shell.

I understand that this is normal - that in a typical gas-powered generator, the whole AC wiring system may be "floated" - it's insulated away from ground. This actually decreases the shock hazard... if you were to accidentally touch the "hot" lead, there would be no way for current to flow through your body and back to the generator via ground, since the generator doesn't connect ground and neutral together.

In that case, your grounding arrangement is good.

I'd suggest checking your generator manual, to see if you can determine whether it's designed to have a "floating neutral". I suspect that it is. If so, don't modify it.

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Reply to
Dave Platt

No relation to "wtom" ?

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Reply to
Baron

I've read elsewhere that the problem with generators and UPS units, is that the generator doesn't hold an exact 60 Hz. You might find a generator's frequency will range as much as 63 Hz to 59 Hz from no load to full load. The UPS likely depends on a crystal controlled oscillator to determine if 60 Hz power is present.

I wonder if inverter generators like the little Honda EU1000i, will work with a UPS?

Fred

Reply to
Fred McKenzie

Probably would work better. My UPS, a high end high power one, tells me the line frequency on it's statistics page. Interesting watching the statistics, and seeing line voltage variations, etc.

Reply to
PeterD

These two UPS models have a sensitivity adjustment but it's only accessable via the USB connection and APC Powerchute software. I should install it in my laptop and set them both lower but I've nbot had to break out the generator since a few years ago when a storm ripped its way through here and knocked out the juice for 3 days.

My main focus was to keep the fridge going and a couple lights. I shut the APC units off and hooked up direct to a multi outlet box calculating the wattage of everything on the generator to just und

2000 watts. We had a couple lights and tv and a laptop and DSL modem/wireless router + the fridge. Was ok if you didn't trip over all the extension cords :)
Reply to
Meat Plow

Mine is a KingCraft from Aldi. Would you happen to have the same one?

I'd guess that I'm grounded with the one 8 foot rod. I just didn't realize how limited the cheap three prong outlet testers are. Live and learn.

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Tony Sivori
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Reply to
Tony Sivori

wtom? Is that a nym?

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Tony Sivori
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Reply to
Tony Sivori

Not much in the manual. No mention of the floating neutral, just a statement not to connect it directly to any structure's electrical system.

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Tony Sivori
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Reply to
Tony Sivori

No. A troll.

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Michael A. Terrell

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