Date: Sat, 11 Apr 2009 12:36:58 -0400
Reply-To: pickle vanagon <greenvanagon@GMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: pickle vanagon <greenvanagon@GMAIL.COM>
Subject: Re: gfci outlet buzzes with inverter
In-Reply-To: <49E0AE53.2040503@gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
Wow... thanks for all the very detailed replies!
I guess the basic idea is that chassis ground isn't a true ground, so can't
take a bunch of extra current, but also isn't connected to the A/C output of
the inverter (even though its connected to the DC side) and so charge
shouldn't want to flow through it.
At this point, though, even if I don't need the GFCI protection on the
inverter line, I use the same outlet for shore power via a double pole
transfer switch, and I was hoping to have GFCI protection at least for shore
power.
I guess I'm considering these options:
1) keep the GFCI outlet in place, assuming the buzzing noise doesn't create
a hazardous condition (via wear or whatever).
2) replace the GFCI outlet with a regular outlet, losing GFCI protection on
both sources
3) replace the GFCI outlet with a regular outlet, but hard wire a GFCI
breaker inline to the shore power souce before the transfer switch (after
the 15a circuit breaker)
I'm not really excited about 3 at this point, since I'm nearly finished with
the A/C side of the project and have very little room left in the wiring
compartment for an inline GFCI breaker.
The buzzing is slight enough not to be a nuisance at all, so I'm happy to go
with number 1. I guess I was mostly worried that the buzzing could be
creating a hazardous condition (how can you have vibration without wear, I
was thinking).
Does anybody think that its a bad idea to leave the buzzing outlet in
place? Or does anybody think it might be worth trying another GFCI outlet
in the hopes of getting a non-buzzing one?
Thanks again for all the great replies. The list really is an amazing
resource!
-Wes
On Sat, Apr 11, 2009 at 10:50 AM, Rocket J Squirrel <
camping.elliott@gmail.com> wrote:
> The powered AC receptacle is "floating," which means that it doesn't know,
> or care, about chassis ground or earth-earth. David's right -- neither the
> hot nor the neutral wire is referenced to ground. They don't know about
> the chassis of the van, they don't care, they are not interested in going
> there.
>
> There may be some eentsy leakage through the inverter, which has one side
> of its 12V input tied to chassis, but the resistance of that path will (or
> darn should) be of such high impedance that were you to lay your tongue
> onto the AC's hot or neutral wire ("or" is an operative word here, laying
> one's tongue /across/ the two wires would cause a cosmic experience) while
> grasping an unpainted part of the van's chassis, you should be in no risk
> of electrocution.
>
> To test this theory: from the inverter's hot wire, connect a 1,500 ohm
> resistor to chassis and measure the AV voltage across that resistor.* The
> resistor simulates the conductivity of your body and is sufficiently low
> so that it will bleed down the trivial leakage (if any) within the
> inverter. If the voltage across that resistor is greater than 0.7 volts,
> then the power at the receptacle carries the risk of electrocution and
> should be treated with the same degree of respect as a household outlet.
> Repeat this test on the neutral side of the inverter's outlet.
>
> Note that if you were to do this test on a household outlet, the hot side
> would read your 120 VAC household voltage, while the neutral side would
> read essentially nothing because in a properly-wired house, even without
> the leakdown resistor, the neutral side is meant to be at the same
> potential as earth/ground so no volts there. But the output of the
> inverter, as David said, is not referenced to ground so you want to check
> the neutral side in your van, too.
>
> If neither the hot nor neutral show any significant voltage across the
> resistor then one could consider going on to the holding of the guitar
> while putting your lips on the microphone test. Me, I'd not be worried
> about doing that as long as I knew that the van was not connected to any
> shore power, and not even if a wire from the chassis of the van was
> dangling into a puddle on the ground. But I'm not advising you to do that
> because it would really mess up my day if I got a call from your widow's
> attorney referencing this message.
>
> W/r/t GFI buzzing -- you're not feeding it a nice smooth sine wave, but
> one with steps. It's gonna buzz. I doubt it would do anything useful with
> the floating output of the inverter, because a GFI compares the "send"
> current (call it the hot wire) with the "return" current (say, the
> neutral) and as long as everything going into the load (your toaster) is
> also returning out of it, then it's happy. But when your puppy knocks that
> toaster into the bathtub, some of the send current will NOT be returning
> where it should but will rather be going through whoever is in the tub to
> earth/ground. This imbalance of send and return currents will cause the
> GFI to shut off power.
>
> In the van, though, the current really doesn't want to go the
> earth/ground/chassis, so you could pitch the toaster into the tub and the
> send current would/should return right back to the neutral wire like good
> little electrons do. So the GFI would never pop. But and always in matters
> of safety, I urge that you proceed carefully before doing a full-scale
> test. In other words, use a mother in law before checking it on yourself,
> is my advice.
>
> ===========
> * This is a standard load for testing for leakage current from AC-powered
> appliances. You'll also want to use a true-rms reading meter because the
> output of an inverter is not a sine wave. More details about this at the
> bottom of http://repairfaq.ece.drexel.edu/sam/safety.htm for more
> information.
>
> --
> Mike "Rocket J Squirrel" Elliott
> 84 Westfalia: Mellow Yellow ("The Electrical Banana")
> 74 Utility Trailer. Ladybug Trailer, Inc., San Juan Capistrano
> Bend, OR
> KG6RCR
>
>
>
>
> On 4/10/2009 10:41 PM David Beierl wrote:
>
> At 08:28 PM 4/10/2009, pickle vanagon wrote:
>>
>>> I'm concerned about whether the buzzing is something to be worried about
>>> otherwise. It's faint enough that you don't really hear it unless you
>>> get
>>> down on all fours right next to the outlet. But is the vibration
>>> creating
>>> an unsafe situation?
>>>
>>
>> The maker of the GFCI should know better than we do whether that
>> stepped square wave is ok for his device...
>>
>> ...but we can say that the vibration is not going to hurt anything.
>>
>> Whether the rest of the installation is safe is a different
>> question. Tentatively, it sounds ok to me, *with the transfer
>> switch.* Keep in mind that normally neutral and ground are tied
>> together somewhere nearby, which would probably destroy the inverter
>> if it connected to such a situation; but with the outlet isolated I
>> think you're all right unless you plug in a device with a hot-ground
>> or neutral-ground short in it, which will take down the inverter.
>>
>> The inverter doesn't really have a neutral -- both terminals are hot
>> w/ respect to chassis ground, just like a (US) household 220 vac
>> circuit. Some inverters use a dummy ground terminal on their socket(s).
>>
>> Another point -- I'm quite sure I've read that modern GFCIs do not
>> measure current flowing in the ground leg, but instead algebraically
>> sum the hot and neutral legs and trip when the difference exceeds 5
>> ma. Thus they don't care where the missing current is going, and can
>> operate fine with a two-wire hookup.
>>
>>
>> --
>> David Beierl - Providence RI USA --
>> http://pws.prserv.net/synergy/Vanagon/
>> '89 Po' White Star "Scamp"
>>
>>
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