[Antennas] Statement of fact about power system grounds.

[Eric Lemmon]

 David,

I daresay that your linear amplifier  is equipped with a NEMA 6-15P or 6-20P
plug, meaning that the two flat blades connect to two "hot" phases, and the
round prong is ground.  The neutral is not needed.  This arrangement is also
seen on larger window air conditioners, portable air compressors, and
electric welders.  If the appliance needs only 240 VAC, a neutral connection
serves no purpose.

The electric clothes dryer and electric stove are different, in that the
heating elements require 240 VAC, but the interior light and the timer run
on 120 VAC.  For many years, NFPA 70, the National Electrical Code, allowed
the grounding circuit to also serve as the return for the relatively small
currents drawn by the light and the timer- but no more.  Several
electrocutions occurred due to a poor ground connection allowing the dryer
or stove cabinet to become energized by the light or timer currents, so the
cognizant CMP (Code Making Panel) took the pro-active step of eliminating
the grounding circuit as a permissible return path for dryers and ranges.
This was especially important in a mobile home, where the entire vehicle
could become energized if the grounding quality was impaired.

The NEC now requires a four-wire connection:  Two hot leads for the 240 VAC
circuit, a neutral for the small 120 VAC loads, and a separate equipment
grounding conductor.

73, Eric Lemmon WB6FLY


-----Original Message-----

David Ring wrote:
> Does that mean that all 240 VAC devices wired with 3 prong plugs also 
> violate the rule?
> 
> My stove and amplifier are three prong plugs - what is happening 
> differently with a clothes dryer?  And where does the "fourth" prong 
> go?  Are these new plugs two phases of ac voltage (240 V between them), 
> neutral and ground?