[TowerTalk] Grounding Multiple Buildings

[Jim Lux]

On 9/15/14, 10:02 AM, John Lloyd wrote:
> Jim,
>
> Many of today's switching power supplies with RFI filters in the AC
> input have these capacitor from the line input to ground.
>

The pi filters on the input?

Yes, but the capacitors are also pretty small, designed to keep the 10s 
of kHz switching currents from propagating back up the line, so the 
leakage current is pretty small.

They also make sure that they have sufficient ratings, etc., to not 
create a hazard.

For example
http://www.vicorpower.com/documents/user_guides/brick/UG_PFMVIBrickFilterEval.pdf

shows a typical input filter.. it's 4700 pF after the bridge rectifier, 
and with the balanced choke, I think the AC Line/Ground leakage will be 
quite small.

I think most EMI filters are differential mode (e.g. between the two 
line conductors), not "line to ground" (because of the issues with 
safety, etc.)

That's very different from "two 0.1 discs soldered across the line cord"
,or, 0.01 uF as shown on page 123 of my 1972 handbook "A 700-Volt 
General Purpose Supply for Transceivers"

Or the similar balanced Pi filter for RFI reduction shown in Figure 16-1 
on page 483





> John Lloyd, K7JL
>
>
>
>
> Message: 6
> Date: Mon, 15 Sep 2014 07:36:53 -0700
> From: Jim Lux<jimlux at earthlink.net>
> To:towertalk at contesting.com
> Subject: Re: [TowerTalk] Grounding Multiple Buildings
> Message-ID:<5416F985.6010304 at earthlink.net>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> On 9/15/14, 7:09 AM, David Jordan wrote:
>
>> Here is how the NAVY would perform the grounding:
>> http://www.hnsa.org/doc/radio/index.htm
>>
>> -
>
> How the navy would do it *in 1946*.
> And, there's actually nothing in there about grounding, or electrical
> wiring practices. The antennas they talk about are dipoles, for the most
> part.
>
> In general, old documents aren't always a good source of "good
> construction and engineering practice". There has been substantial
> change in grounding, bonding, transient suppression knowledge and
> practice in the last 70 years, particularly when it comes to safety.
>
> This is not to say that information in old publications is actually
> wrong, but that it might be incomplete or inappropriate in view of
> today's practices.  A nice example is old versions of the ARRL handbook
> which recommend connecting 0.1 uF capacitors between power line and
> chassis for filtering. Today, this would be considered very bad
> practice: capacitor failure would lead to a potential line/chassis
> short, leakage currents (0.1 uf = 5 mA @ 120V) that will trip a GFCI, etc.
>
>
>