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Transformers, Grounding, Cable questions.
---------- Forwarded message ----------
Date: Sat, 04 Oct 1997 22:12:17 +1000
From: Phil Chalk <philoc-at-ozemail-dot-com.au>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Transformers, Grounding, Cable questions.
Hi all,
Due to a few lucky scores recently I am currently
building a 'mains power box' for general service as
well as TC work. It will essentially be an isolation
transformer, variac, voltage & current meters + optical
current waveform metering output, filtering & MOVs,
switching, fusing etc in a box.
Later I will build a High Voltage box containing a few
NSTs, filtering, safety gaps etc; & after that maybe a
HV DC supply - I'd love to run a TC on DC.
Then all future coils will be self contained with
everything except power supply which will be connected
by leads to the TC unit. My first & only 'complete'
coil had NSTs & everything in the box, making it one
big heavy unit for a relatively small coil.
Anyway, I have a few questions.
1. Does anyone have any preference (& reason) for
which order to connect the isolating transformer &
(auto) variac ? i.e. mains-isolation-variac-load, or
mains-variac-isolation-load ? Assuming the in-between
connections are 'finger-proof' are there any pros &
cons ?
2. I'd like to have remote indication of safety-gap
firing. I'm thinking either some kind of optical
detectors looking at the gaps, possibly hall-effect
devices, or a small torroid as a current-transformer on
the lead from the centre gap electrode to ground as
sensing devices, followed by appropriate amplification,
indication etc. Anyone have any ideas, or ever tried
to do this before ?
3. Those of you that use RG8/RG213 as high voltage
connecting cable, what sort of connectors (if any) do
you use ? Surely not PL259s or 'N's ??? There seems
to be varying opinion on whether or not to ground the
shield. (& if so, I guess at which end ?)
My spec for RG213 rates it at 5.5kv max DC & 3.7kv RMS
max working voltage, but I'd be happy to push it. The
Uniradio equivalent UR M67 is rated at 40kv & 6.5kv
respectively. With the shield grounded, I think the
100pF/m capacitance would be a good thing with cable
lengths of around a convenient 5 metres. I'm still in
two minds as to which end to ground it - i.e. unless
I'm shot down in flames, the NST cases will be grounded
to mains ground at the HV box. The safety gap & filter
ground will have to connect by cable to the bottom of
the secondary & to a separate RF gnd terminal at the
'TC box' end. When an RF gnd is not available (I live
on 3rd floor) it will have to go to mains gnd.
Grounding shield at HV box end makes the cable like a
giant feed-thru cap of a few 100s of pF & turns my
L-section filter into a Pi-section, no ? Grounding it
to RF gnd at the TC end 'feels better' somehow though.
Any comments most welcome. Thanx,
Phil Chalk.