A knowledgeable friend says:

Russell,

Don't really have time to critique anything at present (ask 
me why in a
couple of weeks time), but from the brief description the 
reliance on the
ground-neutral bond at the distribution panel would be the 
greatest
potential weakness (source of error) since any voltages 
developed between
the local earth point and the remote earth-neutral bond 
point would distort
the true phase-neutral voltage that he is trying to measure.

The achievable accuracy would be highly dependent on the 
electrical
environment.  In a relatively quiet domestic (residential) 
situation it may
work OK most of the time, but in an industrial environment 
(especially one
involving PWM AC variable-speed drives) the earth-neutral 
noise can be
horrendous  - I've measured RMS earth-neutral noise on a 
400V 3-phase (230V
phase-neutral) in a local machine shop at over 70V with 
spikes having
substantial energy in excess of 500V.  I was called in to 
find out why MOV's
were dying, and serial communications from PC's to CNC 
machine tools were
unusable  -  and it soon became obvious why.

I ran a similar system at home for 18 months or so (before I 
bought the
Centameter) and the results I was obtaining seemed 
believeble most of the
time  - but I was really only interested in overall usage 
trends and not the
absolute readings.  I did however observe significant 
"glitches" in the
logged 5-minute averaged power readings whenever I used my 
large Hitachi
circular saw extensively.

Regards,

Ken Mardle


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