
Measuring high voltage in millimeters and other HV probe tricks - segfaultbuserr
https://hackaday.com/2016/12/08/measuring-high-voltage-in-millimeters-and-other-hv-probe-tricks/
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h2odragon
> Most of my work these days is below 30kV,

Oh. Good. "Most".

I figured 2kv/mm back when, using scavenged neon sign transformers and coat
hangers for probes. The graph in the article is enlightening.

That i survived making and living with bare wire Jacob's Ladders says
something about my teenage years. To this day I cant decide if it says
something good or bad, however.

Further enlightenment:
[http://teslamania.delete.org/frames/longarc.htm](http://teslamania.delete.org/frames/longarc.htm)

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QuincyJazz
Curious... If I need to measure 70kV, but HV meters go up to 40kV, can I put
two meters in series?

~~~
segfaultbuserr
Theoretically, yes, but only if the HV meters are ideal resistors. In
practice, voltage ratings cannot be added for good reasons.

The safe and legal way is building a separate HV probe for the meter,
internally, it can use a resistor divider. The individual resistors will have
a much lower voltage rating, and still, they cannot be added, you don't get a
new rating by connecting them in series. But you can test and certify the unit
as a whole, so the probe itself gets a new input and output voltage rating.

Another example is oscilloscopes. Almost all scopes are only rated for CAT II
measurements, which meant it's only safe to use the scope to measure the
voltage inside an appliance plugged in an outlet that is far away from the
electricity entry panel (even when the working voltage rating is the same,
different Categories [0] have different overvoltage handling ratings for
unexpected transients, the closer to the entry, the higher it needs to be).
Technically, you cannot even use an oscilloscope to measure the AC lines
directly - it requires CAT III certification (yeah, most people keep breaking
this rule). However, what you can do is building a standalone, differential,
active HV probe as your "input front-end", and design and certify the probe to
handle the hazardous voltage, in this case the oscilloscope only samples its
safe output and acts as a data acquisition and analysis system.

[0]
[https://en.wikipedia.org/wiki/Measurement_category](https://en.wikipedia.org/wiki/Measurement_category)

