[volt-nuts] Follow up

Rob Klein rob.klein at smalldesign.nl
Wed Dec 16 11:53:26 UTC 2009

Rob Klein schreef:

 > I'm going to dig out my GenRad megohmmeter this afternoon
 > and make some measurements. I'll post the results later.
I just couldn't contain my curiosity and dug it out a bit earlier. 
Literally dug out, 'cause it was in the basement
between a lot of boxes. Fortunately, the mice didn't do any damage to it.

As I have already made a front panel for my proposed standard, with 
binding posts for measurement and guard
in place, it should be relatively easy to measure how the insulation 
holds up with regard to leakage.

The meter is a GenRad 1644 
<http://www.tequipment.net/pdf/IET/IET_GR1644.pdf>, which I saved from 
the scrapheap at one of my former employers. It is a Wheatstone
bridge based instrument, with bridge voltage settable from 10 to 1000V 
and a highest range multiplier of 1TOhm (1E12),
enabling measurements of up to 1E15.

Basic checks on the instrument were made by first connecting it to the 
voltage input of a handheld multimeter and
observing the bridge balance at 10MOhm. Lead wires were then removed and 
bridge balance was checked to be
at infinity at the highest range multiplier and 1000V.

Silicone insulated test leads were then attached to the instrument, 
taking care not to let them touch each other. The
banana plugs on the far end of the leads were placed with their 
insulation on the edge of a plastic sheet, so that the
metal parts were not in contact with anything but the surrounding air 
(which should be pretty dry, as it is freezing outside).
Bridge balance, still at 1000V, was obtained at around 300 TOhm.

Next, the '-' terminal was connected to the guard connection on the 
panel (which is electrically connected to the panel) and
the '+' was connected to the nearest binding post; 15mm heart to heart.
At the same dial setting, the bridge was still pretty much balanced, 
just ever so slightly lower. This suggests a very high
insulation resistance indeed. Certainly well over 1E15.

It will be interesting to repeat these measurements in the summer, when 
the humidity is up and the binding posts have
aged a little.

For my next trick, I shall endeavour to repeat the above for the 
feedthrough capacitors, albeit at a lower voltage.


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