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Re: [TCML] Ground Voltage Gradient
Dave
Bonding Xo (neutral-grounded conductor) to Ground (grounding conductor) is a
requirement not only of National Electric Code (NEC, NFPA 70 see Article 250
in entirety for separately derived power systems) and National
Electric Safety Code
(NESC for utilities). Neutral and ground systems are then separate
and independent
systems to load. If they are tied together downstream of ground bond point of
system, dangerous circulating ground currents can (and have) occurred; which
can result in a serious shock, arc-flash, or fire hazard, and can cause GFI
false trips or WORSE no operation in a ground fault condition. If you have no
ground reference/bond, and a power conductor starts "striking" (ground faulting)
to ground, up to 8X Vnom can enter the distribution circuits. ==8^O
It doesn't require much imagination to see what will happen if ~1kV is applied
to a 120VAC load; hint it won't be pretty.
I know of at least two cases where loss of distribution ground and
residential ground
(not installed) resulted either in catastrophic electrical damage
(main panel fireballing-
house consumed by fire) or arc-flash electrocution. In Tennessee, a
substation ground
was stolen, bonding a radial feeder to a residential area. Ground
rise (up to 1kV into
homes) resulted in more then $2,000,000 in collateral damage. This account was
provided at a utility IEEE seminar on substation grounding I attended.
Grounding systems are so critical, it is the FIRST thing I design on
any power electronic
or power electrical engineering systems for which I'm responsible. It
is truly a life
safety issue. Remember, the ground path is the only thing that
maintains system voltage
symmetry, prevents dangerous step or differential potentials, and
conducts DC charges
(lightning) to earth. I just upgraded the grounding system in my
house last February,
and ran a #2 CU THHN ring bus between my disconnects, ground rod /
UFER system, and
cold water line into home. A #4 conductor was damaged (puckered
insulation, flowed
into conductor) by a direct ground strike, no other electrical damage in house.
And the wife wonders why I over build everything ("Honey, shi... happens!")
Regards
On Thu, May 6, 2010 at 11:27 AM, dave pierson <dave_p@xxxxxxxxxxx> wrote:
> [I maybe trending too far off topic, here. Might be pursued off list?]
>
>> Often Tesla coilers connect everything on the power source side
>> (control cabinet etc.) to the power source ground, and
>> everything on the "business" end connected to a dedicated
>> RF Ground.
>
>> In the case of 240V 60Hz supply (assuming a dedicated service
>> with no other loads attached) both L1 and L2 would be referenced
>> to ground only through the secondary windings of the distribution
>> transformer supplying the service.
> Is this an NEC thing? I'm thinking local grounding of entrance
> and metering is notuncommon. May Be Rong.
>
>> Additionally, let us assume that:
>
>> The distance between the ground at the source/control point and the
>> load/TC is 1000 feet (makes impedance calcs easier for the wire used
>> for L1, L2)
>
>> The Tesla coil uses a charging choke on the secondary of the high
>> voltage transformer located at the base of the Tesla coil
>> (eliminates ballast)
>
>> The Tesla coil uses a SISG (eliminates additional wiring for RSG,
>> control wiring, etc.)
>>
>> The Tesla coil is powered by a "pole pig" and the center
>> LV bushing has been left floating because after all this is a
>> Tesla coil, and we don't want that connected to RF ground. The
>> case is bonded to the RF ground.
>
>> The "pole pig" is housed inside some sort of metal cabinet, also
>> bonded to RF ground (personnel safety)
>
>> Now we know that an operating Tesla coil can produce a fairly high
>> differential between RF ground and Mains ground. No question
>> there. That is the point of using a separate ground, to keep high
>> voltage spikes etc. off of the mains ground.
> Seems to me the 'high differential' characterizes the differnce
> at HF 'only'? Mains Freq differential s/b minimal?
>
>> So lets say that the demonstration or display went well, everything
>> is hunky dory and the coil has been disassembled and put into
>> storage. For some reason, which does not matter because this is a
>> hypothetical question, the power is restored to the now unloaded
>> "pole pig" all happy in its protective cabinet. Now the case of the
>> pig, and its cabinet as well, are bonded to the RF ground out in
>> the field, and the center low voltage bushing is still floating, and
>> should have a potential of 0V with respect to the ground at the
>> source, I.E. mains ground.
>
>> So if someone were to put a voltmeter between the pig case and the
>> center LV bushing, what would it read?
> If i'm following correctly, would concur with zero(ish). Varying,
> amongst other things, with internal details of 'piggy'
> construction, stray capacity, etc, foo, mumble.
>
>> Are there natural ground currents that might cause a potential
>> difference between two ground points separated by 1000 feet?
> Sure. Typically measured (by all accounts: I've not done so.)
> in millivolts, except under 'special cases'. researching
> 'earth currents' and 'stray currents' will turn up some facts,
> some i regard as blather. (I've not found much, would like a
> good reference. Some 'research monitoring' is done. Really
> Potent solar storms can goof up the mains system in obscure,
> off topic ways...)
>
>> If such ground currents exist, are they constant or do they vary
>> from time to time, and if so, what affects them?
> Lotsa things 8)>>
>
> 'natural' ones, dcish, vary with solar activity, local ground
> chemistry. Unbalanced Mains currents vary with load on system.
>
>> What happens if a thunderstorm were to pass nearby?
> Things can get interesting.... Usually not damaging to humans,
> can bend/staple/mutilate electronics of the delicate kind.
>
> There have been exceptional/rare cases of faulted 'distribution'
> and 'transmission' lines that have gone Really Lethal. Faulty
> 'user gear', can, in rare cases, create hazards.
>
>> TIA for any opinions, insight, etc..
>
> best
> New dwp: dave_p@xxxxxxxxxxx
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>
--
Dave Sharpe, TCBOR/HEAS
Chesterfield, VA USA
Sharpe's Axiom of Murphy's Law
"Physics trumps opinion!"
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