Residential generator - seperately derived or not

[ron]

I will have a portable generator that will be on occasion connected as a separately derived source and at times not a separately derived source.
When I have a tool directly plugged into the generator mounted receptacle, it will be a separately derived system. When I have the generator recept connected to house wiring through a 2 pole transfer switch (neutral not switched), it will not be a separately derived source.
250.34(C) dictates that as a separately derived source, the grounded conductor must be bonded to the generator frame. As a non-separately derived source, it would not have its grounded conductor bonded to the frame.
Remember since all loads will be connected through the generator mounted receptacles, no connection of the frame to a grounding electrode system is required 250.34(C).
OK, now the theory question. How does the functionality and safety characteristics change if the neutral to frame remains connected or unconnected (only one way or the other) when the gen is used as a non-separately derived source and a separately derived source. I cannot grasp why it is necessary to change the generator from bonded to unbonded if I plug the tool into the gen directly or into a recept in my home (which is plugged into the gen through the xfer switch).

As a side note, this same question has been cross posted on other forums.

 

[Ed MacLaren]

Re: Residential generator - seperately derived or not

If I read your question correctly, you are asking why the generator neutral should be isolated from it's frame when the transfer switch does not switch the neutral.

The equipment enclosures would be bonded to the neutral in two places, permitting unbalanced load current to flow on the EGCs, while on utility power.

Ed

 

[ron]

Re: Residential generator - seperately derived or not

Ed,
Nicely done, thanks. In this particular installation the generator is portable and will be unplugged from the 2 pole xfer switch when not in use, but you point is clearly made.
Again thanks, a picture is worth 1,000 words.

 

[bennie]

Re: Residential generator - seperately derived or not

Ed: What is the origin of this schematic, who is the author?

 

[Ed MacLaren]

Re: Residential generator - seperately derived or not

Bennie,

What is the origin of this schematic, who is the author?

I am. It was originally produced for an apprentice training seminar. All of my sketches are offered to illustrate basic principles only, and are not intended to represent any actual installation.

The basic information illustrated is from a Square D transfer switch application note on their website.

I recently modified it to show the problem identified by Ron who started this thread.

Ed

[ October 08, 2003, 11:18 PM: Message edited by: Ed MacLaren ]