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transfer switches+ theory

3429 Views 6 Replies 5 Participants Last post by  bmart
Hi all,

I was hoping to get some insight on something I dont quite understand.
When Installing transfer switches " like the small gentran manual ones".
I understand that the 3 position switches isolate the hot legs so they do not backfeed into the utility lines. I also understand that in a 240 volt circuit that the neutral carries the unbalanced portion of the load from the two hots. What I dont understand is why doesnt the neutral backfeed into the utility lines, the neutrals from the transfer switches are not switched and hook to the busbar in the panels with the other neutrals, sooo why doesnt the unbalanced portion of the neutral carry into the utility lines. Is it because it returns to the source? Any and all insight into this will be greatly appreciated.
Thanks in advance.
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The current seeks the source, it is being supplied from.

Speaking of 3 phase 4 wire or single phase 3 wire, there are two types of ATS a 3/2* pole only the phase conductors transfer and a 4/3* pole the phase and neutral conductors transfer. With a 3 pole ATS the neutral conductor in the generator MUST NOT BE BONDED TO GROUND. In a 4 pole ATS the neutral is bonded to ground as a 4 pole ATS is a SDS separately derived system.

* 3/2 3 pole 3 phase 4 wire, 2 pole single phase 3 wire
* 4/3 4 pole 3 phase 4 wire, 3 pole single phase 3 wire
In order to complete any electrical circuit, you must have a path from the source through the load and back to the source.

When you are running the genny, the genny is the source, not the utility. Any electron flow will want to go back to the genny.
Thanks alot for the replys, I kinda figured it must be something along those lines but wanted to make sure I had my facts straight
In a 4 pole ATS the neutral is bonded to ground as a 4 pole ATS is a SDS separately derived system.
Good point Brian.
clarification of grounding for transfer switch

Meant for that to be a new thread, sorry
Hi New spark: I raised the same question several years ago, but never pursued the answer, I would imagine if your neutral is bonded to ground thru a cold water pipe, building steel or earth driven ground rods this current would take a path to the nearest ground in your house; however if this wasn't the case, it could backfeed. I have backfed my house with a generator and transfer switch and had no problems, however if I was a Lineman, I would be concerned.

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