[Amps] Re Direct rectification of AC mains to derive the amp, VDD, supply

[peter chadwick]

Bill,

We start off with a 3 phase star (wye) connected supply at nominally 415 volts per phase. Each house gets one phase and the neutral i.e. 240 volts give or take. The neutral is grounded (earthed) at the substation. Along the distribution, the neutral is grounded at various places. When distribution tended to be lead sheathed or steel armoured wire cable, the sheath or cable was the neutral. Clamped in cast iron junction boxes, in certain soils,  the neutral return resistance could be problematical. The argument was that to 'use higher technology cabling' - i.e. cheaper!  - where there were three conductors and a metallic sheath, said sheath would be neutral and earth. To avoid problems with neutral and earth being at different potentials and to make sure the 'earth' was earth, earth and neutral are tied at the entrance to the property. Now if neutral is ruptured, the neutrals connected to the various equipment float up in voltage. So all earths are tied to the neutral in the house. The problem then comes if you bring in an external earth.....

Fortunately, you can insist on having a local earth and no connection to the neutral. So the power pole feeding my house - right at the end of the line - has a ground wire and stake grounding the neutral. I have all the ground pins of my house taken to separate ground and as they are tied back to the station RF ground, it gives under 10% of the approved ground stake resistance.

But if you have a grounded neutral and you go outside and touch something connected to the internal earth pin when there's a ruptured neutral fault, you can be in very deep sh*t! So certainly here, it's safer no to assume you can ground the neutral.

Now in the US, if you get a ruptured neutral, the two sides of the line divide the voltage between them. The side with the lowest resistance will float towards 120 on both sides....not necessarily actually getting there, but maybe well above 120 volts. But the in-balance if you have a 'grounded'  neutral then depends on how good the ground is and what the load is.

Does this explain it for you? I regret it isn't easy...Hell, standards people are involved and making it easy mean they aren't needed. I know - until June, I was one - although not in this field.

73

Peter G3rzp







========================================
 Message Received: Sep 19 2013, 05:02 PM
 From: "Bill Turner" <dezrat1242 at yahoo.com>
 To: "Amps" <amps at contesting.com>
 Cc: 
 Subject: Re: [Amps] Re Direct rectification of AC mains to derive the amp,	VDD, supply
 
 ORIGINAL MESSAGE:          (may be snipped)
 
 On Thu, 19 Sep 2013 12:44:45 +0200, Peter wrote:
 
 >The 240 volt EU supply is a very different animal to the US 120-0-120.
 
 REPLY:
 
 Hello Peter: Could you elaborate on this a bit? I assume the difference is
 how the neutral is used or not used. Is that correct?
 
 In the US the typical wiring is two 120 "hot" wires, a neutral and an earth
 ground. In the proposed power supply, the neutral would not be used at all.
 Just the two 120 hots and the cabinet would be earth grounded. 
 
 Thank you,
 
 73, Bill W6WRT
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