Re: Freaky Amazing DMM?!

"Michael A. Terrell" <mike.terrell@xxxxxxxxxxxxx> wrote in
news:ab-dnX9LJ9G4suDUnZ2dnUVZ_jOdnZ2d@xxxxxxxxxxxxx:

Ohm's law.

And common sense.


Notoriously rare, allegedly. :)
In something like 'phantom' voltages in unconnected lines in power
distribution, it might be considered that it goes way beyond ohm's law, as
inductance and capacitance are involved, but my 'common sense' tells me to
try to reduce to Ohm's law where possible. So for simple tracing of lines, a
light load is enough to reduce the values of indirectly coupled voltage. I'd
rather have more control of what load I choose than leave it up to whatever
is supplied in a meter, but that's just my choice. At least this way I have
the option of putting it in series with the meter and measuring current.

And a bit of awareness of insulation strength when high volts are
involved.


Er work with HV armored cable? You can't see the insulation, because
it is made like Heliax, with a polypropylene filled insulator. A real
bitch to cut, strip and terminate. Do it wrong and you have a
spectacular fire.


No, never done that. But I try not to walk blind into situations like that.
The only time I did get a copper vapour deposition of some strength across my
eyeballs was after someone else had gone in and royally blown 100A cables
apart first so the resulting mess made it very hard to figure out what the
state was, on a circuit that could not be isolated within reach. All it did
was confirm what I already knew: that my own assumptions are the best place I
can fix an error before it goes bad. (in this case I assumed that two
separate lumps of brass could not have been welded into one through a thick
thermosetting plastic layer by the actions of a guy who had shorted them with
only a momentary contact with a 2.5mm square wire. To this day I don't know
what demonic persistence he used to manage that much destruction, all I know
is it can't have been momentary, he must have been feeding it in like a
welder).

If you're using a low resistance input you might have to take it into
account for accurate measurements but on mains, the error is small, so
it's worth keeping inputs resistance low for meters dedicated to such
systems, for reasons plenty of posts have explained, so I won't flog
that horse now.


Only for those who don't understand how to read a real meter. That's
most users, or they wouldn't make so many of the crappy things.


True. Electricians do have a point though, I tested this last night with a
Fluke 79 series II. Its input resistance is high, but not the type of high
that prompted the OP to post. I knew that when you have a moderate length of
corridor, say 6 metres, with a lamp that has a switch at both ends, you have
a very standard situation where a line is unconnected in a cable when the
lamp is on. Which line it is depends on which positions the SPDT switches are
in to close the lamp circuit. I noticed that the capacitatively coupled
voltage on both lines was identical, at least, differences were lower by far
than line fluctuations. This is probably the sort of thing that angers the
electricians when someone claims a meter with high input resistance can be
used. In this case it can't, alone. But I'd still rather have a separate load
to apply, than have to use more than one meter. A 200K resistor seems about
right (for circuits up to 500V), as in this case there is only the earthed
switch box to reference to, and any more than a few milliamps would probably
trigger an ELCB somewhere...
.

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