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Re: Setting up a pole pig's wiring

Original poster: "Barton B. Anderson" <bartb@xxxxxxxxxxxxxxxx>

Hi Phil,

Tesla list wrote:

Original poster: FIFTYGUY@xxxxxxx
In a message dated 5/1/06 12:37:55 PM Eastern Daylight Time, tesla@xxxxxxxxxx writes:

>Yes, but there is no assumption. D.C. said "as the interior side of
>primary is at X1 (grounded) potential". That indicates, connecting X1
>to ground. That is clear.

Sorry, I'm a little slow, but that still doesn't clarify it to me. If anything, this raises the additional issue of which end of the *primary* winding is adjacent to the core or secondary winding. Again, unless I'm missing some conventional labelling of pole pigs, the worst case could end up:

One end of primary (LV winding), the end furthest from core, at ground potential Other end of primary (LV winding), the end closest to core or to the secondary's (HV winding) HV end, at 240V

Neither end of the LV windings are at ground. They are floating. 240 Vac would be typical in the US connecting the LV across X1 and X3. X2 is left unconnected.
I see we got our X's and H's mixed up. Thanks for being persistent.

One end of secondary (HV winding), the end furthest from core, at ground potential Other end of secondary (HV winding), the end closest to the core or to the primary's (LV winding) 240V end, at 15kV

Ideally, the core side should be at RF ground as this is the least stressful situation. I connect H2 on my pig to RF ground..

Obviously this is the highest-stress situation, and best-case is the complete opposite. My concern is calling the "H1" or "X1" labelled ends of the windings the ones closest to the core or other winding. Is there some convention that pig manufactures must follow when labelling, or should a pig owner open and verify the connections?

Not a bad idea. Popping the lid and taking a look doesn't hurt. But there is a standard for all distribution transformers. Looking at the transformer so that the LV bushings are facing you, the HV bushing H1 is on the left and H2 is on the right. The LV bushing X1 is on the right. X2 (neutral) is in the center and X3 is on the left. That is the standard for a single phase distribution transformers.

The transformer core is connected to the case. I have left the core as built and tie RF ground to the transformer case. Mains ground stops at the control panel. In other words, I make sure RF ground and mains ground are not connected (otherwise, you're going to get little shocks here and there at the control cabinet).

You're concern is certainly valid because we got our X's and H's mixed up. H2 should be at RF ground. This is the HV side closest to the core and when looking at the LV bushings, H2 is the HV bushing on the right.

On an (9, 12, or 15kV) NST, the interior ends of both HV windings are both grounded to the core. Grounding one "bushing end" puts 4.5, 6, or 7.5kV stress at the center of the HV windings to core, where there was zero stress before. If the depotted NST HV winding fails, and the core isn't still grounded, that puts the core floating at 4.5 - 7.5kV where it normally would be at ground. And thus it would put an abnormal stress to the primary winding adjacent to the core. So in that respect I don't think that connection scheme minimizes primary to secondary stress.

Yes, the center of the HV side is at the core. I'm talking about "if" the NST is unpotted, that situation could be removed. Then it could be setup just like a distribution transformer. But I don't unpot NST's (too messy). Just a possibility if someone wanted to go that route. The case of the NST would also be tied to RF ground. I set my NST under the coil and tie RF ground to the case. Mains ground is nowhere near and only back at the control.

Hope that helps clarify.

Take care,

-Phil LaBudde
-Phil LaBudde