On Nov 16, 2005, at 5:22 AM, Jim Brown wrote:
> On Wed, 16 Nov 2005 04:05:51 -0800, R.Measures wrote:
>
>> My 240V, 40A circuit is wired 2-hot and 1-neutral. I assumed that no
>> "safety" ground wire was needed because neutral is connected to
> safety
>> ground in the circuit breaker box. Under normal operation, virtually
>> zero current flows in the neutral wire.
>
> NO, NO, NO, NO, NO!
In a SB-220 or a TL-922, it's Yes, yes, yes! because there is no
connection between the 120v fan and either side of the 240V input.
Instead, the fan is powered by one of the 120v primaries of the xfmr.
>
> Neutral is intended to carry current (although it is not used in the
> 240 volt circuits in homes).
When an electric clothes drier or oven has a lamp for lighting the
interior when the door is open, current for the lamp normally flows in
the neutral wire and in one of the hot wires.
> Ground is a PROTECTIVE conductor that
> should NOT carry current except in the case of a fault. (A fault is a
> failure of some component or wiring that results in an equipment
> enclosure being hot).
> The function of the safety ground is to provide a path for fault
> current that blows the breaker and protects the user from shock (and
> fire).
Bonding the enclosure to the neutral wire insures that the enclosure
can not become a shock hazard if the safety-ground wire is severed by
an anomaly.
> Electrical codes require that the safety ground (from the
> breaker panel) be connected to the equipment enclosure (via the green
> wire).
>
>> In my opinion, the folks who got paid for updating the NEC
>> probably realized that if they didn't make at least some changes,
>> eyebrows would be raised by bureaucratic bean counters. Another
>> factor may have been that they knew little about circuit analysis and
>> Ohm;s Law.
>
> You are quite mistaken. First, people who write most codes and
> standards are often doing so on a voluntary basis. As vice-chair of
> the EMC Working Group of the Standards Committee of the Audio
> Engineering Society and a member of a half dozen other working groups,
> I can tell you that the people who write standards are usually the
> brightest and best.
They apparently were a bit lacking in the area of Ohm's Law.
> I know a few of the people who contributed to the
> part of NEC that addresses grounding and bonding, and they are VERY
> good EE's.
>
When I worked for Raytheon, there was a brilliant EE who determined
that it was absolutely impossible to make solid state devices work if
they were floating at 5000V above ground potential. Do you agree?
end
> I suggest you study the Power and Grounding "White Paper" on my
> website, which is a free download. Mike Holt (www.mikeholt.com)
> teaches good electrical practices and the NEC to electricians, and has
> written excellent materials to go with his classes. He sent me his
> excellent document on "Grounding and Bonding." It is not free. :)
>
> Two other excellent references are the IEEE "Green Book" and "Emerald
> Book," both of which are IEEE Standards. They are published and sold
> by the IEEE.
>
> Jim Brown K9YC
> http://audiosystemsgroup.com
>
>
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>
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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