Not the same ol transfer switch question!

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davedottcom

davedottcom

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Not sure about the Ground lug on the outside of the Generator Frame...here's the set up:

1)The Auto Transfer switch does Not switch (Or Bond) the grounds & neutrals.
(According to our good friend, Mr. Holt, if the Transfer switch does Not open the grounded neutral conductor the Generator is Not a seperately derived system)

2)The (70Amp) generator does Not bond the grounds and neutrals either.(Bond is from Ground lug to frame only)

So far so good!

3) The #10 Ground (pre-wired whip from factory) runs from the Generator to the transfer switch. Transfer Switch is grounded from the main panel.
Editted to add: This transfer switch is also a load center. (grounds and neutrals still seperate at this point!)
(Grounds & Neutrals bonded in Main Panel only)

So far so good still!

4)There is a lug on the outside of the Generator for grounding the frame to something else!
But this would create a parallel ground since it is already grounded by the #10 in the "whip".

Is this #10 (pre-wire) ground from the Auto/transfer switch all that is needed for a 15K Generator???

I can't figure out how the ground lug on the outside of the Gen. could be utilized without creating a paralleled ground.
:confused:

Dave

Editted to add: This transfer switch is also a load center. (grounds and neutrals still seperate at this point!)

[ April 26, 2005, 04:00 PM: Message edited by: davedottcom ]
 
Re: Not the same ol transfer switch question!

you are allowed to install a supplemental ground on the generator as long as the frame of the generator is not bonded to the neutral. check 250.54 & 250.58 for supplemental grounds
 
Re: Not the same ol transfer switch question!

The ground rod is for lighting strikes, the manufacturer of the generator generally usually asks for this rod in the installation requirements.
 
Re: Not the same ol transfer switch question!

I just read 250.54 about 2 seconds ago!
It says as long as the Earth isn't being used as an effective ground-fault path a sup. grnd. rod is ok. And the #10 would be the effective ground-fault path...the rod would be supplimentary.

That makes sense.

Thanks,

Dave
 
Re: Not the same ol transfer switch question!

There is no problem with parallel paths for Equipment Grounding Conductors. You can have as many bonds as you like between the frames and external metal parts of everything in sight. The reason this is true is that this path, or series of parallel paths, will not ever carry current, unless and until there is a fault in the system. At that time, the fault current through these many paths will cause the breaker to trip and terminate the event.

But there is a problem with parallel paths, when you bond the neutral to the frame (or EGC, or other external metal parts) at more than one location. The problem occurs because the neutral carries current on a normal, routine basis. If the neutral is in parallel with an EGC path, then the EGC will also carry current (on a no longer normal but still routine basis). That means that you could touch the metal frame of any item, or touch a metal conduit, and receive a shock.
 
Re: Not the same ol transfer switch question!

Charlie, I didn't notice you replied until now. Thanks.
The ground and neutrals are ONLY bonded in the main panel. I made sure today when I went back to look at the job again. Ohmed out the frame of the Genny to it's Neutral lug too...Nothing. :)

I understand the danger with bonding at two points, but I also thought the parallel EGC was a violation. I guess I just misunderstood and it was stuck in my head!

Well, what you said makes sense, and that's good news...I can run the EGC from the Genny's frame right to the service's ground rod. :)

Of course now I wont get to use my handy Dandy Makita Ground Rod Driver! :(
That's my favorite tool! (8' ground rods, all the way down in 20 seconds...give or take a second!)

Thanks again Charlie,

Dave
 
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