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Re: [TowerTalk] Fwd: shack wiring

Subject: Re: [TowerTalk] Fwd: shack wiring
Date: Mon, 02 Aug 2010 10:07:29 -0400
List-post: <">>
Hi Jim,

Yes, you are right, a three phase system aggravates the problem but it is also 
present on on split phase feed.

Worst case probably never happen, but my house in Brooklyn receives its power 
from a gigantic, three phase transformer, buried in the street. This 
transformer is feeding several houses on the block. An easy method to see if 
you have a three phase feed is to measure the voltage across the hot legs. 240 
volts probably a split 120/240 transformer, 208 volts and you most possible 
have a 208/120 volts three phase feed.

Now, if you have identical power supplies, the harmonic are also identical. You 
may have have different power supplies with different phase relationship 
between the harmonics.

Bottom line is, if you large power supplies, us 240 volts feeder and supplies. 
I believe most large supplies can be configured for 240 volts.

Hans - N2JFS





-----Original Message-----
From: Jim Brown <>
To: <>
Sent: Sun, Aug 1, 2010 12:56 pm
Subject: Re: [TowerTalk] Fwd:  shack wiring

On Sun, 01 Aug 2010 11:43:25 -0400, wrote:

>When you run 120 volts supplies on outlets on different phases but common 

>return, be aware that if they are of the "old" type, diodes feeding directly

>into capacitors, so called "top-" or "pulse" rectification, due to the 

>harmonics in the currents through the supplies, the current will add up, not

>subtract in the return line. You can end up with 40 amps in the return line

>with both lines loaded to 20 amps. I have seen this happened in computer 

>rooms in schools where they had many computers loading 2x120V outlets. 

Your observations are partly right and partly wrong. 

The classroom example is correct, because it is almost certainly a 3-phase 

system. The home example is not, because virtually all homes are single-phase 

(120-0-120). Here's why.

Virtually EVERY electronic power supply, linear or switcher, has a capacitor 

input filter, and the CURRENT to recharge that capacitor flows primarily in 

short pulses at the top (and bottom) of the AC waveform. As a result, that 

current is rich in harmonics. In a single phase system, all of that current 

cancels in the neutral (assuming equal and identical loading of both sides of 

the power line). 

In a 3-phase system, any harmonic whose number is divisible by three will ADD 

in the neutral rather than cancel. That's because the three phases are 

displaced by 120 degrees at the fundamental, and their third harmonic is 

displaced by 3x120 degrees (360 degrees). Likewise, the sixth is displaced by 

720 degrees, the 9th by 1080 degrees, etc. Those harmonics add in the neutral 

(AND in leakage currents on ground conductors). And it is those harmonics that 

we hear as "ground buzz." 

Now, almost none of us has 3-phase at home, but the mains power wiring in the 

alley or underground that feeds us IS 3-phase, and a form of 3-phase power 

distribution called High Leg Delta is widely used in cities, towns, and even 

some rural areas to feed both residences and businesses from the same lines. A 

center-tapped transformer on one of the phases feeds residences, while 

businesses that need 3-phase power get all three phases (but no neutral). The 

catch is that residences DO get a neutral, and all that harmonic noise from 

the businesses goes to ground on our neutral. If you hear "ground buzz", 

that's what you're listening to! 

If you can look at the buzz on an audio spectrum analyzer, you'll see those 

harmonics. I've got screen shots an FFT analyzer and an extended discussion of 

all of this in a couple of tutorials that are on my website. See either the 

Ham Interfacing tutorial or the White Paper on Power and Grounding for Audio 

and Video Systems.

73, Jim Brown K9YC



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