On Mon, 29 Jan 2007 17:01:16 +0100, HBcubed <rjsimmons@...> wrote:
My main test gear is a 465B scope with it's case/frame NOW hardwired
to a water pipe.
It would be better to ground it through the electrical ground (ground prong or screw on an outlet). It is not always a good idea to use the plumbing for ground, since great hazards are created if the plumbing is not properly earthed, or some pipes are plastic (or are later replaced for plastic). It depends on your local regulation if it is still legal to use the plumbing.
My questions are:
If I run the 465B with AC should I run it through a Variac?
Why would you? A variac is used for VARIable AC. Your scope should be made to run off mains voltage.
Many scopes have internal adjustment for "low" "normal" or "high" line voltage, i don't know the 465B.
If so, is the variac considered an "isolation xformer?
Most variacs are not, they are so called autotransformers with no isolation between input and output. There are also isolated varicas, which have a second fully isolated winding under the outer output winding with the wiper. But often an isolation transformer will be put in front of a variac to provide isolation and adjustable voltage.
Should the device under test also be grounded to the water pipe?
Depends. Usually your DUT should be grounded, via the electrical system. Sometimes if the DUT is powered via an isolation transformer the ground is intentionally removed. You should not need that if you don't exactly know how or why.
Can I power the DUT with the same variac?
You can certainly power the DUT with the one variac, you do not need one for a functioning bench scope that is used as test gear and meant to run on your line voltage.
What is the "perfect" ( safest) bench test configuration
A bench configuration can not make it perfectly safe, although it can make it unsafe.
An universal bench configuration (which i use) is the scope grounded and powered by regular mains ("just plugged in").
The DUT is powered through an isolation transformer (ideally with isolation monitor so you are made aware of a fault), possibly combined with a variac to bring things up slowly and check for supply range etc.
On my own bench all connections are jumpered with 4mm jumpers, so i can remove ground from the DUT, but ONLY if i know what i am doing. I didn't need that much, if ever.
It should be said that a GFI protected outlet can actually be safer than using an isolation transformer, in some cases. If you are not using the isolation transformer you should _always_ use a GFI to power the DUT (i highly recommend using one for everything, test gear and all). Note that GFIs do not function with isolation transformers, it's one or the other.
I will rebuild my bench soon and then it will have V/A/W/phase angle meters for the DUT power. I have collected nice industrial panel meters and i will build my very own small "nuclear power station control panel", always wanted that ;-). I've also gotten hold of a nice isolation monitor which will be part of the new bench.
ST
