Re: [TCML] Single-ended NST, wiring

[BrianB <brianb@xxxxxxxxxx>]

It's important to be clear that we are speaking of two different grounding issues.

First is the bonding of the HV transformer case/core to the safety or earth ground. I've always done this on permanent installations per NFPA 70 (NEC 2011) and inspectors who don't know what to do with the Tesla Coil always check that the Power Controller and HV Trans are bonded. On temp installations such as TV or movie shoots I prefer to leave it ungrounded as it seems to lessen any RF getting back into the Power Controller but this is far from scientific just personal preference.

On pole pigs with one bushing the case has to be grounded to prevent core to Low voltage winding flashover (ie the max potential could only be 240vac).  It's also important that in the HV circuit, the case of the HV Trans is connected to the innermost turn of the primary.  If instead the HV bushing is connected to the inner turn of the primary there will be a high voltage gradient between it and the base of the secondary.

We have never tied the primary to the RF ground for previously mentioned "potential" safety issues.

I know this relates to pole pig coils but I hope it is of use as it is similar to other single and dual bushing transforms.

Regards,
BrianB


On Mar 5, 2013, at 10:34 AM, jhowson4@xxxxxxxxxxx wrote:

> Hmmm. I wonder why this configuration is so prevalent in the small kit coils available from information unlimited and in the larger ones used at resonance research and the like then. For the purpose of limiting that voltage rise perhaps? If the dangers are reasonably high, then one would think the kits would be engineered away from this configuration. Heck the little information unlimited BTC-30 I built years ago, does not even have an RF ground, it just uses the house ground, with no problems I might add. 
> 
> 
> 
> Thanks, 
> John "Jay" Howson IV 
> 
> 
> "Why thank you, I will be happy to take those electrons off your hands." 
> 
> ----- Original Message -----
> From: "Jim Lux" <jimlux@xxxxxxxxxxxxx> 
> To: tesla@xxxxxxxxxx 
> Sent: Tuesday, March 5, 2013 10:45:17 AM 
> Subject: Re: [TCML] Single-ended NST, wiring 
> 
> On 3/5/13 7:08 AM, Jim Lux wrote: 
>> On 3/5/13 5:42 AM, jhowson4@xxxxxxxxxxx wrote: 
>>> alright so apparently i missed something all these years. 
>>> 
>>> I thought you wanted to separate the two grounds and because the HV 
>>> was grounded you should only use the RF ground in that system, but 
>>> apparently not, you use both. 
>>> 
>>> You want to connect the ground on your transformer to the house 
>>> ground, make your circuit like you have shone, and connect the 
>>> bottom/inner turn of your primary to the base of your secondary.
>> 
>> No.. there is no good reason to connect primary to secondary. Some 
>> people do it for historical reasons, but for the vast majority of cases 
>> this isn't a good idea. 
>> 
>> If you are using a Neon Sign transformer, or anything with a grounded 
>> centertap, connecting primary and secondary as you describe is 
>> DANGEROUS. On a typical NST driven primary, both ends of the primary 
>> are "hot" relative to ground.
> 
> 
> 
> I missed the reference to the single ended NST with one end grounded. 
> In that case, the danger is not as high. 
> 
> However, you generally do not want the primary connected directly to the 
> secondary in any case: the spark from the secondary is a nice low 
> impedance, and you don't want to make it easier to have 
> someone/something hit by a spark connected to a HV source with more current. 
> 
> In typical cases, of course, the substantial impedance of the secondary 
> winding would be in series as well, but there are cases of injury (and 
> death?) when a connection was made between the primary winding and a 
> person via a streamer. So that pushes for making the primary winding 
> generally "isolated" from ground, so that if something "grounded" (e.g. 
> a person) happens to touch it, the hazard is less. 
> 
> OTOH, you want some non-infinite impedance from primary to ground to 
> limit the voltage rise between HV windings in the transformer and the 
> core. 
> 
>> 
>>> The NST (4kV, 50mA) for my SGTC is a single end type with one of its 
>>> secondary winding ends attached to the metal casing.
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