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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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In Reply to: Re: May I comment? posted by Soundmind on July 8, 2006 at 06:21:53:
You don't ever want neutral current flowing through the ground circuit, it's a real shock hazard.In case of deconnection of the link neutral-earth or your Service Disconnection Means. But I think the real danger is that an electrician who does such a crappy job as to interverting neutral and earth, and furthermore without further checking before delivery, is prone also to intervert neutral and hot on the same electrical system. Have fun, customer... However, it exists, and I wrote an artcle about it for an inmate who had exactly this problem as we put it in evidence.NEC proscribes that a voltage source (transformer secondary) can only have its neutral grounded at one point, usually at the transformer itself.Is it a point of disagreement, or a misunderstanding somewhere. I note the following:"Article 250 of the NEC (National Electric Code) sets forth the general grounding requirements for electrical wiring in a structure.
Present requirements of the NEC specifiy that the ground lead (green wire), in a single-phase AC power distribution system, must be one of three leads.
The other two leads comprise the hot lead (black wire) and the neutral lead (white wire).
The ground lead is a safety conductor designed to carry current only in the event of a fault. The hot lead is connected to the high side of the secondary of the distribution transformer.
For fault protection, the NEC specifies that the neutral be grounded at the service disconnecting means (main breaker). The safety ground (green wire) is grounded at that point as shown in Fig.5.1.
Grounding of a three phase wye power distribution system is done similarly to the single-phase system. The connections for a typical system are shown in Fig. 5.2.
The neutral lead, as in single-phase systems, is grounded for fault protection at the service disconnecting means. The NEC specifies that the neutral shall never be grounded at any point on the load side of the service entrance on either single-phase or 3-phase systems.
According to the figure, a ground connection is made at the transformer, and another one at the service entrance. Maybe the distribution transformer was part of the building (and so, a single grounding was to be done)?As for lightening strikes, it's interesting that a roof mounted lightening rod with ground wire network is not intended to absorb an actual strike but to discharge static electricity in the air just prior to a likely strike.And also to provide a path to follow would lightning strike. Which is why the path is done with bare copper. The di/dt is so high (several kA/uS) that, for the first microsecond, most of the current is rejected on the outside of the copper bar. Density of current is soon so high that thermal electrons are ejected into the air, which ionizes (helped by the high electrostatic field around). So the current keeps on growing through the created plasma. Soon after the peak, the di/dt decreases, and the bulk of the current (average 20KA, up to 100KA) flows back in the entire bar for tens of microseconds. The air plasma soon extinguishes. The after-current (several tens of amps for milliseconds) flows entirely through the copper bar.It is believed that if the wire is bent, the lightning could actually fly off the condctor and discharge to earth through another path.Just at the time a column of plasma surrounds the copper bar, as I explained above, current grows to very high level. Should the bar be straight, the Lorentz forces from the plasma current in the local magnetic field do tend to tighten the plasma symetrically around the bar; That's OK.
Should the bar be bent, then the Lorentz forces act asymetrically, and the plasma column is ejected from the bar neighborhood. Plasma is very light a conductor, and since it responds also to its own magnetic field,it soon goes back and forth like a sprinkler pipe and hose should you open the faucet with its end loose. Soon, this plasma whip is likely to touch another metallic path to ground, current increases, the column stiffens or on the contrary breaks up again, depending on the geometry.
BTW some even believe that, with the good conditions, the plasma will circle around itself until forming a kind of a donut, keeping iuts shape thanks to its own magnetic field, like a natural tokomak. Obviously,it decays, and after a while, the plasma ball extinguish (after some kind of noise), rather suddenly according to the witnesses of "thunder in ball" (bad englsih translation of french "foudre en boule") Have you heard about that? Maybe just an urban legend, not so sure. High currents and high di/dt have odd consequences.
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Follow Ups
- Bad neutral connections, lightning current paths - Jacques 10:45:35 07/08/06 (1)
- Re: Bad neutral connections, lightning current paths - Soundmind 13:00:15 07/08/06 (0)
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