Re: Grounding pigs

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Esondrmn-at-aol-dot-com>

In a message dated 2/16/02 9:53:14 AM Pacific Standard Time, tesla-at-pupman-dot-com
writes:

Bart,

My thoughts on this is to disconnect the neutral lug from the case, then earth
ground the case.  My reasoning is that a single variac acts like an
autotransformer, not an isolation transformer.  Even in the off position, the
brush is connected to one side of the 240 volt AC line.  If the pig center lug
was grounded to the same ground as the electrical service, you would have 120
volts across one primary of the pig with the variac set to zero, but with power
applied to it.

Ed Sonderman

>
> << Hi David - 
>
> Why "must" you remove the strap? There is still 240 across the 2 internal low
> voltage windings even with ground at the center tap. Same voltage across, 
> same
> inductance across, same current, same output. In the event that the 2 low
> voltage windings are paralleled, then you have a condition where you would 
> only
> allow 120v from the center tap to one of the end windings. But even in this
> case, neutral would go to the center tap and could be tied to ground. In 
> either
> case, I don't see why the strap "must" be removed. Maybe I'm missing 
> something?
>
>
> Take care, 
> Bart 
>   >>
>
> Hi Bart,
>
> True, if the strap is left connected you will have +120 volts to one outer
> LV bushing and -120 volts to the other, relative to the grounded "strap"
> on the center terminal. It will effectively be two 120 volt sources, out of 
> phase (or is that in phase) Let's don't get started on the "phase" thread
> again :-) But most pig coilers are going to opt to power their pig trannies
> through a 0 - 240/280 volt variac and here is where the problem would ex-
> ist by leaving the strap connected. A 240 volt variac basically has infinite
> range of output voltage between 0- and 240 or 280 volts. So that is what
> you want to feed into your pig. If the center strap is grounded, that will
> shunt the inputs from each leg of the variac to ground. "I" personally see
> that as a problem.  And if you don't  "ground" the strap, then you would 
> have the entire external tank of the pig floating at 120 volts in reference
> to
> ground :-(  Maybe I'm wrong here, I certainly have been before :-) Maybe
> it would still work if you left the LV bushing grounded to the case (and
> made sure that the case and the strap were actually connected to ground)
> but leaving the center LV bushing floating has certainly worked for me
> without any problems and I'm not going to try changing it (if it works,
> DON'T fix it) ;-) Maybe others have comments on this?
>
> Coili' in Memphis,
> David Rieben
>
>