The output coil of the IT must be fully insulated from ground, that's it.
You are right, the neutral and ground is tied at the circuit breaker
box (or elsewhere), but that is why you use the transformer with two
isolated coils, it removes the ground reference at the output side
(after your mod that is).
There is one small detail regarding the modification of the
tripp-lite, with the output grounded as sold the regulatory
requirement for the insulation strength is much lower than a typical
double insulated transformer. This may or may not be reflected in the
construction.
Since you will be using the transformer for scope measurements
(grounded through the scope ground), and not for safety, it will
probably not matter. Wouldn't hurt to Hipot test this transformer.
As for connecting the earth connection through or not, that is a
matter for debate. If the transformer does not comply with the
requirements for double insulation, you need to leave the earth. If it
is to be used as a safety isolation transformer the standards even
demand the earth is removed, in some countries.
I would leave the earth conductor connected through, personally, since
it guarantees the case of a DUT will not be dangerous no matter what.
Apart from potential Y capacitors (EMI) there should be no connection
between earth and neutral in any DUT and the earthed chassis should
not interfere with any measurements.
ST
On Sun, Dec 7, 2014 at 7:28 PM, louis.wilen@... [TekScopes]
<TekScopes@...> wrote:
I'm trying to verify that my Isolation Transformer (IT) is really going to provide "tech" isolation. The goal is to use the IT between the mains and the DUT. (I will definitely NOT be using the IT to float the scope. No need to even go down that path of discussion at all.)
I looked through the previous discussions on this topic and found this comment from Tom Miller, which pretty much sums up the main goal of using an IT:
"The goal is to separate ground from neutral for the unit under test. Do not isolate the test equipment. Think of it as making a transformer isolated power supply by adding the missing transformer. Now, you can connect the scope ground clip to either side of the AC line in to the TV (or whatever)."
So I obtained a Tripp-Lite IS-250 and disconnected the internal neutral-to-earth ground on the output side of the transformer (as explained in ) so that I'd have a "tech" Isolation Transformer.
The earth ground is still connected to the IT chassis and IT third prong on the input and output.
But considering the entire electrical path, I'm still not convinced that I have full isolation. As we all know, the neutral is connected to earth ground where the power enters a house (typically in the circuit breaker box). Therefore, I'm thinking that there is still a current path between the IT output neutral and earth ground. Hence, when plugging a grounded "three wire" DUT into the IS, shouldn't I use a 2-wire to 3-wire cheater plug between the IT and the DUT to achieve full isolation?
(I submitted this question to the guy who produced the YouTube video (Todd), but I don't have 100% confidence in his answer. I have a lot more confidence in the answers that are provided on this forum.)
Thanks for any replies.
Louis
W3VVV
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