Original poster: "Gerry Reynolds" <gerryreynolds@xxxxxxxxxxxxx> Hi Gary,The continuing story... This one will never go away, I think :o))) The strays thru the NST are real and worst if the NST is grounded to mains ground. This is one reason I like to ground the NST to RF ground. RF coming into the HV bushings will see a lot more capacitance to the NST chassis than the primary winding. The chassis will (in a sense) shunt the RF back to RF ground where it wants to go anyway. Any remainding RF that does get on the primary can be helped by the EMI filter.
In the context of OUTDOOR USE, I still like to think of the NST and the TC as a single system, not to be approached while operating. Yes, you can make the NST chassis safe during operation by doing a green wire ground to it, but then again, what about the greatly more hazardous NST bushings, TC primary windings, and TC secondary topload. Seems like the NST chassis gets "lost in the forest" of hazards. Even in a fault condition, the system is not approachable during operation. The worst kind of fault I can think of (off hand) is a NST HV secondary to NST primary short. This fault would put the NST into current limit and the primary voltage would win. If a counterpoise is used for outdoor use and connected to RF ground, then a yes it could be see a hazardous voltage if it were to be shorted to NST primary or secondary. I would declare the counterpoise a hazard from the start also not approachable during operation.
Also, I said in a previous post that the safety gap ground should always be grounded to the NST chassis cause it is the NST it is to protect. Someone else mentioned about tying it to RF ground so a secondary to primary hit can be shunted back to the base of the TC secondary. Im thinking both statements have elements of truth to them and both can be met only if the NST chassis is grounded to the RF ground (and not the mains ground). For the case where the NST is green wire grounded and the TC is RF grounded, perhaps it is best to protect for the secondary to primary streamer hit by tying the ground terminal of the safety gap to RF ground. This would keep the strike off of the mains at the expense of some overvoltage protection of the NST. The caps and MOVs in the Terry filter, I think, should continue to be grounded to NST chassis.
For INDOOR USE, I think one has no choice but to ground the counterpoise to mains ground and maybe use the RF choke as you say. This case requires a lot of consideration and safety precautions and is generally not a desirable configuration.
Gerry R.
Original poster: "Lau, Gary" <Gary.Lau@xxxxxx> Hi Gerry: I think the point of the choke is to prevent the stuff tied to RF ground from being elevated to a hazardous voltage should any number of fault conditions occur. I know that I have a very substantial RF ground - two very long buried copper pipes and a copper sheet, but the DC (or 60 Hz) impedance to my green-wire ground doesn't even register on my DMM's high resistance range! I've thought about such a safety choke, but it's on my to-do list... I agree that parasitics in most any choke will provide some capacitive path back to the mains ground. Unresolved is how to minimize that, and just how much inductance is needed. But one must also bear in mind that there are comparable paths through the NST to hot & neutral. I dutifully use a commercial EMI filter to minimize that, but have no idea just how effective that is - probably not very at my 82KHz resonant frequency! Regards, Gary Lau MA, USA > Original poster: "Gerry Reynolds" <gerryreynolds@xxxxxxxxxxxxx> > > Hi Terry, > > This has been mentioned before as a solution to indoor operation. I > really dont think the choke would help if there was an impulse on the > RF grouind. Like you say, an arc could go around the choke. Chokes > of any design have their resonances and become capacitive above > certain frequencies. I think an impulse would go thru the parasitics > of the choke even if it didn't arc over.. > > Gerry R > > > >Original poster: Terry Fritz <vardin@xxxxxxxxxxxxxxxxxxxxxxx> > > > >I have never mentioned this before, but I have often contemplated > >using a choke between the RF ground and the AC line ground. The AC > >line ground could then still provide 60Hz safety but the choke would > >block the RF from the AC ground. My DRSSTC has this but I have > >always used an RF ground too... One problem with running only the > >AC ground is that if the RF really wanted to arc across the choke, > >it easily could... > > >