In normal AC system DC bias is VERY rare. anytime a transformer is involved the dc bias goes to zero. Any AC powered device with a transformer in the front end of the power supply will likely fail in a catastrophic way if any significant DC bias is present. (You drive the transfomrer magnetic material into saturation and it stops being a transformer and becomes an air core inductor... with poor coupling)
Most precision AC power meters use an inductive current measurement and probably an AC only voltage step down transformer to measure voltage. This will not sense any DC voltage/current in the circuit. The hall effect clamp on sensors that will measure DC have bias problems and are generally far less accurate than clamp on transformers in AC circuits. In past jobs I've both designed and used power meters and I would have to agree that if one is attempting to do fraud then putting DC bias on an AC wall socket would be one possible way to do this. This fraud is easily detected with a simple DC voltmeter or DC coupled oscilloscope... so it would be a risky thing to do.... it is however given the details of the Rossi independent verification a possible fraud that would not have been caught with the instrumentation used in this test. Paul On Wed, Jun 26, 2013 at 8:24 AM, Alain Sepeda <alain.sep...@gmail.com>wrote: > I express badly... > > It is only different models of clamp... > you seems to have the Hall effect clamp, which measure DC and not to high > frequency AC. > > Essen seems to have used inductive clamp. Moreover it seems the PCE830 > filter-out DC anyway, for current and voltage. > > I don't know why expensive instruments use "inductive" clamps instead. > maybe to have better precision, bandwidth, calibration... > Why the don't measure DC voltage is also not clear... > > > > > 2013/6/26 Jed Rothwell <jedrothw...@gmail.com> > >> Alain Sepeda <alain.sep...@gmail.com> wrote: >> >> about Clamp and DC, there are 2 kind of clamp. >>> >> >> There is only one clamp on my Radio Shack ammeter. >> >> I suppose it is not good for very low current. >> >> (I can't find it . . . I may have thrown it out or given it away, but >> anyway there was only one clamp.) >> >> - Jed >> >> >
