> -----Original Message-----
> From: towertalk-bounces@contesting.com [mailto:towertalk-
> bounces@contesting.com] On Behalf Of Jim Brown
> Sent: Sunday, July 25, 2010 12:25 PM
> To: TowerTalk@contesting.com
> Subject: Re: [TowerTalk] outlet ground problems and shack grounds
> updatinginfo needed please.
>
> On Sat, 24 Jul 2010 22:15:43 -0500, Fred wrote:
>
> >I have a grounding question for all you rf experts.
>
> Hi Fred,
>
> I suggest that you study the Power and Grounding tutorial that's on my
> website. http://audiosystemsgroup.com/publish.htm It's in the section
> on audio and video.
>
> Also study the piece on Ham Interfacing, and in the RFI Tutorial, look
> at the chapter on Solving Problems in the Shack.
>
> There's one big problem with your installation. Building safety codes
> require that the wire that connects the third round pin in the outlet
> to the breaker panel be run WITH THE OTHER TWO CONDUCTORS. There are
> three VERY good reasons for that. First, inductance -- if if follows a
> separate path, the inductance, which is proportional to loop area,
> increases the time that it takes a breaker to blow in the case of a
> fault. Second, if something causes the ground to break, it should also
> interrupt the hot and neutral. Third, when you bypass the hot (phase)
> conductor to ground with a capacitor, the resulting inductive loop area
> is very small if the ground is run with the phase and neutral, but very
> large if they are run separately. That loop acts as both a magnetic
> loop and an antenna to radiate power line noise, and it also picks up
> your transmitted RF and puts it on the power line. Most equipment has
> capacitance between the phase conductor and ground, usually in the form
> of a line filter and in the capacitance to ground of the power
> transformer.
>
> Because it can be so difficult to replace existing wiring like yours,
> older buildings are "grandfathered" -- that is, they are exempted from
> the legal requirement of grounded outlets.
>
> But you want to be safer and have your station work better. In your
> case, if you can run a ground wire up from the panel to your shack, you
> should also replace the existing power wiring in your shack with new
> power wiring that follows that path. You don't need to disconnect the
> existing outlets, but simply abandon them (or use them for lighting),
> and use the new wiring for your ham and computer gear. If it were me, I
> would use #12 copper, and pull in two circuits. This allows you put
> them on 20A breakers. If you need 240V in your shack, put those two
> circuits on opposite legs, and wire the two hots to a 240V outlet. You
> should put all of this in proper steel boxes that you can buy at any
> good hardware store.
>
> For your installation, I think you're moving in the best direction
> possible -- that is, new ground rods, bonded together, bonded to your
> shack ground. BUT -- they MUST be bonded to the power system ground at
> the breaker panel.
>
> 73, Jim Brown K9YC
Some good suggestions Jim, but I don't think that it is legal to pull 240
volts from two separate 120 volt lines.
However if a 3 wire plus ground #12 cable is run then two separate 120 volt
circuits can be used with a common neutral and the 240 volt circuit can also
be used. The two hots also need to be on a connected breaker (double pole).
73
Gary K4FMX
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|