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Re: [TCML] Single-ended NST, wiring
On 3/5/13 5:11 PM, BrianB wrote:
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.
On a 1 bushing transformer, I think the "innermost is grounded" is the
standard configuration. 1 bushing transformers would almost always be
used in a Wye/Star configuration connected phase to neutral.
For the 2 bushing transformers, I think you'd have to consult the mfr,
look inside, etc. I don't know that there's a convention for whether
H1 or H2 is connected to the innermost winding.
(I did some googling.. IEEE C57.12.70-2000 says:
If a transformer has a two-terminal winding with one terminal grounded
and the other ungrounded, the subscript 2 terminal shall be the grounded
terminal.
and
The numbering of the terminals of the H winding and the terminals of the
X winding shall be applied so that when H1 and X1 are connected together
and voltage is applied to the transformer, the voltage between the
highest numbered H terminal and the highest numbered X terminal is less
than the voltage of the H winding.
I'm going to guess that H2 is closest to the core.)
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
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