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Re: grounding NST's
Original poster: Sparktron01@xxxxxxxxxxx
Hi Phil
Comments below...
-------------- Original message ----------------------
From: "Tesla list" <tesla@xxxxxxxxxx>
> Original poster: FIFTYGUY@xxxxxxx
>
> In a message dated 3/21/06 8:04:53 PM Eastern Standard Time,
> tesla@xxxxxxxxxx writes:
>
> > I would also put a line filter on the primary side
> >of a MOT or NST and a RFI choke (low impedance to 60Hz, high
> >impedance to RF) on the ground between the case of the NST and a
> >Terry filter on the HV side.
>
> Dave, what kind of filter would you feel appropriate for the RFI
> choke here? Would a ferrite bead/ring type as used on power cords,
> with a couple turns of the wire connecting NST case and Terry/RF
> ground be sufficient? Or are you thinking a larger air-core coil?
> What value range?
---------------------------------
Excellent question! First Terry Fritz had the idea first of placing an
RFI ballast choke between the "safety ground" (grounding conductor)
and the case of a NST or MOT farm. Lets assume a 120V, 20A branch
circuit capacity and you want the touch voltage to be less then 30Vrms
(not making these numbers up, anything below 42V peak or 30V rms
is considered "touch safe" if exposed). Also assume maximum let through
current is 40A before OCD opens (fuse, CB or PCB trace your pick).
This calculates out to a 60Hz impedance of 0.75 ohm (30V/40A), and
a theoretical calculated impedance (if all inductive) of 1.25k ohms at
100kHz (linear increase with frequency , Z = wL). This calculates out
to ~ 2mH inductance, which could be easily built using a ferrite ring
or toroid, and high current wire.
---------------------------------------------------------
> So let me throw this *new* question out there:
> Even if you put the NST at the coil proper (in "high voltage
> land"), you still have to run power (LV) wires to it. So how do you
> ground things in case of a secondary strike to these LV wires?
------------------------------------------------------------
In that case, I'm __much__ more aggressive. I would suggest a
horn safety gap (0.005 - 0.010" or less) directly across LV power
connections of transformer (mini-self clearing Jacobs Ladder if you will).
I hear the audience... "but DAVE your going to blow the OCD!!!"
INDEED. If I have HV on the LV primary (or control circuits),
_irregardless_of_the_source_ , requires immediate system
shutdown, or operator injury (or DEATH) is probable
(HOUSTON, we have a MAJOR problem!!!)
I submit this would work even on the largest system to protect
against a HV fault somehow appearing back into the primary
(LVAC side). Yes, you will trip the system OCD and probably
blow the SG up, but it is better then the alternative hypothesis
consequences (the SG is expendable, YOU"RE not!)
--------------------------------------------
> I'm running my LV wires in a flexible metal gooseneck conduit,
> so a secondary strike will hit the conduit instead of the power
> wires. These power wires include the "hot" to the NST from the
> variac, the "hot" to run the SRSG from the phase control, the "hot"
> to run the gap cooling fans, and a neutral common to all three.
> Obviously, a secondary strike to any of these would be bad. But the
> conduit will be a relatively large piece of tubular metal laying on
> the floor, so I'm hoping it will have low impedance to RF ground and
> thus protect the wiring inside.
------------------------------------------------------
I've done the same thing on my early 5kVA high power system using
3/4" UA to shield (ground plane) power wires up to transformer.
The conduit is shielded, the UA is run through a straight strain relief
in an angle bracket with a plastic cable bushing over fitting. These wires
are then run to primary of transformer bank.
-------------------------------------------------------
> So far so good? But how do we bond it to ground? After all, a
> short of any of the hots to the conduit will make the whole thing
> hot, and a long piece of flexible metal conduit on a concrete floor
> isn't necessarily going to trip any breakers or blow fuses to
> indicate the fault. And it isn't safe, either, since the conduit is
> connected at the panel end and therefore accessible during normal
> (plugged in) operation. So we can't leave the conduit floating.
> One end (at the coil?) to RF ground? Remember, the theory is to
> have low impedance to RF ground. But we have other concerns as well.
> So other end (at the control panel?) to house ground? But then what
> if it takes a streamer hit?
---------------------------------------------------------------------------
In my case, the UA shell takes the hit, and it runs to ground (and indirectly
the RF earth ground) via local ground paths. In my case, I've got a
supplemental ground grid outside my shed. The UA is just like a metallic
conduit, an conducting ground plane (shield)? This condition of power path
shielding is preferable then arcing over 300V rated power cable.
----------------------------------------------------------------------
> And we can't bond it to *both* house and RF ground, because that
> would be like tying both together. Maybe if the conduit was itself
an RF choke?
> Perhaps the conduit should be nonmetallic up to the point of the
> maximum expected reach of RF and streamer hits, but metallic and tied
> to RF ground in the vicinity of the coil?
>
> -Phil LaBudde
>
Best Regards
Dave Sharpe, TCBOR/HEAS
Chesterfield, VA. USA