240VAC + ISO Tranny + GROUND?? NEC vs. ??

[W1RKW]

Don,
Navy doesn't like hull currents.

The USN does use hull currents but for other reasons.  And yes, 120VAC is balanced making neutral hot. Both conventions are used on subs, not sure about surface ships.

[WA1GFZ]

Yea, they don't want the poor little fish to get a shock when they are sucking crap off the hull.

[N4LTA]

I had a very serious lightning strike about 20 years ago. It hit the power line serving my home right at the transformer and destroyed the transformer. It entered the service and tripped every circuit breaker in the main panel.

The best ground in the house was  - guess what - my tower which is 150 feet from the house - the pulse went down the 120 volt branch circuit to my tower which ended at a 120 volt duplex weatherproof receptacle. It literally blew the receptacle to pieces and the metal "inerds" of the receptacle were stretched out on the end of the solid wire 12 " from the receptacle as if someone had grabbed them with pliers and pulled them taught. I should have taken a picture.

All the ham equipment was destroyed as well as every piece of electronic equipment in the house that was plugged in.

Pat
N4LTA

[KA2QFX]

FWIW...

Every place I've lived I set up my station with a local earth ground whenever possible to drain off RF noise. I would NEVER open the ground return to the panel!  But to isolate it for RF I would (do) installed a heavy RF choke (air core) on the return. About 50uH of conductor rated for the same or better current than the grounding conductor. There's no violation of NEC with this. Back in the day several lightning protection systems required inductors to ground. I don't think they do that anymore.

I have used isolation transformers with floating neutrals as Don described but I haven't seen any advantage to this in practical terms, but then again, I've never had any hum/ground problems to speak of.

Also, the last time I visited the sub panel issue it was pretty clear that NEC recommends only ONE panel (the service entrance) have it's neutral strapped to the local ground. This includes outbuildings! Unless fed from a utility service drop the panel does not get strapped to neutral.  I can have it's own earth ground however.   In the event of a nearby lightning strike there will inevitably be a ground potential gradient between the two earth grounds. That differential will be seen across your equipment (ground to line/neutral) unless you provide some break-over protection. 

Given the drop any transformer will exhibit I don't see what advantage you expect to gain. I have to agree with John JN in his comments in that regard.  I am a fan of Constant Voltage Transformers for load regulation but that's another issue.   The independent "RF" ground and choke isolated "safety" ground thing has always worked well for me.

Mark

[N3DRB The Derb]

I like these guys, they wanna sell you copper but everything they sell is good quality stuff.

http://www.gacopper.com/index.html

[N3DRB The Derb]

good pdf of how the broadcasters do it.

[WBear2GCR]

FWIW...

Every place I've lived I set up my station with a local earth ground whenever possible to drain off RF noise. I would NEVER open the ground return to the panel!  But to isolate it for RF I would (do) installed a heavy RF choke (air core) on the return. About 50uH of conductor rated for the same or better current than the grounding conductor. There's no violation of NEC with this. Back in the day several lightning protection systems required inductors to ground. I don't think they do that anymore.

I have used isolation transformers with floating neutrals as Don described but I haven't seen any advantage to this in practical terms, but then again, I've never had any hum/ground problems to speak of.

Also, the last time I visited the sub panel issue it was pretty clear that NEC recommends only ONE panel (the service entrance) have it's neutral strapped to the local ground. This includes outbuildings! Unless fed from a utility service drop the panel does not get strapped to neutral.  I can have it's own earth ground however.   In the event of a nearby lightning strike there will inevitably be a ground potential gradient between the two earth grounds. That differential will be seen across your equipment (ground to line/neutral) unless you provide some break-over protection. 

Given the drop any transformer will exhibit I don't see what advantage you expect to gain. I have to agree with John JN in his comments in that regard.  I am a fan of Constant Voltage Transformers for load regulation but that's another issue.   The independent "RF" ground and choke isolated "safety" ground thing has always worked well for me.

Mark

I agree about what NEC seems to call for.
I'm trying to fathom the thinking behind it.
To me it seems counter intuitive.

----------------

Actually the potential gradient between the two ground points is the issue!
Or a big issue anyhow.

With a long ground run, and an indirect hit (a direct hit, all bets are off on any system with a direct hit?) I'm saying that the vdrop along that ground is going to be a big problem.

Otoh, with the (what was it called?) separately derived service (??) using a large iso transformer and a local ground, the only opportunity for a voltage differential is if there is breakdown across the iso tranny. Then the opportunity for damage comes if that differential can not be dissipated or otherwise shunted to ground. I suppose if the breakdown voltage of the iso transformer is exceeded at a sufficiently high current then damage could occur - but imo it would be worse with a solid connection running 100ft or so back to the panel. Especially if the inception of the "hit" is somewhere at the far away end of the 100ft run, since the self inductance of the wire itself is going to resist the pulse of the lightning! So, it will "look" for a better path to ground.

As far as line loss, I don't see that at all in practice.
I have a large iso tranny now in service, but running the ground back to the panel, and the 120vac taps out there are far stiffer than they were with the very same wiring split to 120-0-120 with no iso tranny. With the iso tranny running off 240vac and (for example) using just one single 120vac outlet one gets the benefit of having a 240vac line feeding it, stepped down AND the inductive "kick" of the large core. It works nicely and it kills common mode noise afaik.

Another big positive with the use of an iso tranny is the ability to kill transient pulses that often take out solid state microprocessors and cmos! A patent I read on a very excellent and advanced design method to protect gear from line transients made specific note that their system was approximately equivalent to using a large isolation transformer! The benefit they had was smaller size and lower cost! In my case, iso transformers can be gotten inexpensively (whenever they can) and the "large size" is not a big worry, so that works for me!!

That's my story and I'm stikin' too it!

And yes, I am still concerned about the ground differential possibility as well as the problems with a long run of ground wire.


Now what are you saying about breakover protection? Between what and what?? And by what means?

A potential problem with using an RFC in the safety ground return to the panel is that a hit out at the (shack) location will not have a low reactance path to ground - meaning it will seek a shorter path to ground, through your equipment! So, you still need a local ground (rod) at the shack???



                  _-_-bear

[k4kyv]

Actually the potential gradient between the two ground points is the issue!
Or a big issue anyhow.

To avoid the potential gradient problem between two ground points, use a "star" ground system, rather than a daisy-chain  ground.

What that means is that for a star ground, you choose one single centralised ground point that connects directly to the ground rod and to any wires that bond it to other ground rods or grounding points.  Then, every piece of equipment in the system has its own individual ground wire that runs directly to that same central grounding point.

With the star ground, every piece of equipment will be at the same 60~ a.c. potential as all the others.  Of course, that isn't necessarily true for r.f.; in that case, there will be a difference in potential between pieces of equipment unless each ground lead to the central point is of the same length.  At 60~, the length of the ground wire in as small a space as a ham shack will have negligible effect.

Daisy chaining means that you ground one piece of equipment to another one, then that other one to another, etc, until the ground wire eventually connects to the central grounding point. Or else, you have one long ground wire going to the central grounding point, and ground each piece of equipment to it along the way at the most convenient point.  In either case, if the ground wire carries even the slightest current, for example due to the capacitance between windings of a power transformer or an r.f. bypass capacitor at the a.c. line cord of one piece of equipment, there will be a potential gradient along  the ground wire, and the chassis of each piece of equipment will be at a slightly different a.c. potential from each of the others, and ground loop hum will be inevitable.

[KA2QFX]

Well, first let's clear up one thing. My choke in the ground return is to clean up RF from getting out of the station and noise getting in. It's not about dealing with lightning.  I also use various Pi filters on the mains and ferrites on power cords.   Beyond that, everything in the shack is tied to a single, common ground point.  

My experiences with lightning stem from interconnecting copper voice and data over several large campuses between dozens of buildings. Ultimately the best protection (as seen in the earlier attached PDF) is to keep everything near the same potential through the use of local breakover devices, with or without some impedance between distant sources. To answer your question regarding "breakover" protection; this means establishing a maximum differential voltage, between ANY and ALL conductors supplying power, beyond which a breakdown occurs to prevent any further voltage differential. This can be various devices such as gas discharge, MOVs, air gaps, etc.  

As shown in the prior attached pdf regarding broadcast installations, this requires minimizing the various potentials points (conductors) that need to be protected. This requisite prefers a single ground location be chosen as the "reference" or "salient" (now that's an ol' buzzard term) ground regardless of how far away it might be. The distance isn't particularly relevant since the accompanying mains wiring is just as long and generally runs the same path.  With a local ground supplying yet another potential, or at least different from the reference ground, another set of potential limiting devices will be required AND preferably isolated from the "reference" supply to the extent possible within NEC guidelines.  Aside from the chokes and filters in my applications I accomplish this by breaking (opening) my power lines when the station is not in use.  I tend not to operate during heavy lightning storms  And if I were to be caught off guard I trust my breakover devices would prevent any voltages above the local station reference ground from exceeding +/-310 volts long enough to preclude a lot of damage. In the event of a direct strike all bets are off, but I have little doubt that I have sustained at least one direct hit when I lived in NYC and I lost only one two meter HT. It's coax did not pass through the common ground panel!  

Bottom line: Think of your station like the a Faraday cage. Keep all the potential differences outside the cage and you're golden. Who cares what the difference is between your cage and the "ground"100' away. As long as the differentials across anything inside your cage is low your protected.  The iso-transformer offers NO protection in that regard if it's breakover potential from primary to secondary is exceeded.  In that event, your cage now has an external differential to deal with that must be clamped to your local (ground?) potential in any event.

Isolation transformers serve well to provide local applications such as phase reversal, floating neutrals, isolating hot chassis from test equipment, etc.  They are not well applied in any protective manner involving transients. Your referenced advertiser's claim of "Like having an isolation transformer" needs to go back to school and look at how high frequency pulses can very effectively cross a magnetic coupling unless specifically designed to attenuate high frequency. Ever hear rectifier noise come back through a DC supply xfmr?  

[WBear2GCR]

mark,

No "advertiser" this is drawn from reading multiple patents on transient suppression techniques, as well as multiple papers on the subject. If I get lucky I can find the patent number then you can read it for yourself... true, not everything in patents is gospel, but this particular outfit went into rather significant depth on the methods and prior art...

Breakover protection seems reasonably trivial to do. Spark gap anyone? Ok gas dishcharge tube - MOV is worthless. Transzorb maybe if they make them big enough...

The assumption of a faraday cage is nice in theory, but the practical part is that this is not the case as long as anything outside the "cage" has a reference point that is different than something inside and they are connected. FLOATING the cage makes more sense than tying it down on a 100ft long inductive Vdrop line, as far as I can see!

Don, not talking about a series daisy chain ground. Talking about running back to the alleged ground at the service entrance.

Anyhow, I'm not prepared to debate the issue much more, since we seem to be chasing our tails at the moment... maybe in a while...

                          _-_-bear


EDIT: I re-read some of your post Mark, I think we are in general agreement... I just want to use the iso tranny so the only issue then is "where is" the ground going to be.  And why. I'm going to research this more and think about it some more...

[KA2QFX]

Whatever 

[WBear2GCR]

Exactly      

              _-_-