[Gllug] OT: Merging UPS outputs

[Anthony Newman]

Russell Howe wrote:
> On Mon, Aug 22, 2005 at 09:58:01AM +0100, Chris Bell wrote:
> 
>>   That sounds as though they actually generate and distribute a 3-phase
>>supply, then use a 3 to 6 phase transformer to give a local 6-phase supply
>>as you cannot earth more than one point at a time.
>>   Standard practise here is to generate and distribute 3-phase, with the
>>centre tap tied to earth (via earth spikes, etc) at every transformer and
>>every main switch.
> 
> 
> Hm.. this would be a single earth point per circuit, right? (with a
> transformer joining circuits together, so there was no direct current
> path between them). Otherwise, it would sound like a bad idea.. I was of
> the impression that multiple earthing points on a circuit was something
> you strived to avoid, due to ground loops etc.
> 
> Then again, I'm certainly no electrician and won't even pretend I know
> what I'm talking about, so please point any misguided musings.
> 

Bulk distribution of 3-phase power is usually done in delta 
configuration (i.e. with no neutral) as it saves redundant cable and 
earth current problems.

If we're talking about last-mile consumers, only the local low power 
distribution is done in star (3 phases + neutral), so hopefully the 
local earth loop distance and minimal neutral voltage drop will prevent 
any major earth current grief, and the fact that the incoming phase 
voltages may be a bit floaty is irrelevant.

In principle the neutral current at your local step-down transformer 
star point should be zero (algebraic sum of equal 3 phase currents is 
zero), as should the earth current for single-phase consumers, although 
the crude way in which single phase supplies are balanced (and non-unity 
power factors if we're getting pedantic) makes this something of an 
academic point.

Anthony
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