5 Feb
2015
5 Feb
'15
10:59 a.m.
From: Jon Elson
there is no part of the EUT that is at neutral
potential
From: tony duell
With most 'scopes one side of the input is
grounded -- you have a
coaxial cable with the shield connected to the 'scope case and true
ground.
Ah. That's the part I was missing! I (naively :-) assumed that the 'scope's
whole input amplifier would be 'floating', and the input signal would be taken
as referenced to the 'ground' on the probe, which could itself be floating.
Clearly not! (I'll have to look at the manuals/prints for my 'scopes -
Tektronix 465's - to see if they are this way. Anyone happen to know,
offhand?) Thanks!
In most, if not all, countries, one side of the mains
is connected to
ground too.
Yes, in the US that's code; neutral (aka 'return') is usually tied to ground
at the main panel. (Well, at least in the state I live in - technically I
suspect that it's under state law, not Federal, but I would assume the states
are all the same, the way they are with commercial law.)
Noel
5 Feb
5 Feb
11:20 a.m.
With most
'scopes one side of the input is grounded [...]
Ah. That's the part I was
missing! I (naively :-) assumed that the
'scope's whole input amplifier would be 'floating', and the input
signal would be taken as referenced to the 'ground' on the probe,
which could itself be floating. 
11:36 a.m.
It really depends on each individual scope and it's worth checking the
manual or spec sheet to be 100% sure of what it can or cannot do; this
could be particularly worthwhile if you have a scope that was sold more
into the field service market... My Tek 222, for example is
double-insulated with fully isolated channels and is designed to be able to
safely float up to 400 V p-t-p. The 222PS or 224 will withstand higher
yet...
Best,
Sean
On Thu, Feb 5, 2015 at 2:20 PM, Mouse <mouse at rodents-montreal.org> wrote:
That amounts to a differential (rather than single-ended) input, and I
would consider that the _right_ way to do it. But, of course, that
costs, and single-ended is plenty good enough for most purposes, so
that's what most scopes do. :-(
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With most 'scopes one side of the input is
grounded [...]
Ah. That's the part I was missing! I (naively :-) assumed that
the
'scope's whole input amplifier would be 'floating', and the input
signal would be taken as referenced to the 'ground' on the probe,
which could itself be floating. 
12:19 p.m.
That amounts to a differential (rather than single-ended) input, and I
would consider that the _right_ way to do it. But, of course, that
costs, and single-ended is plenty good enough for most purposes, so
that's what most scopes do. :-(
Even when a 'scope has true differential inputs the common-mode range
may not be large enough to include mains.
-tony
12:17 p.m.
Ah. That's the part I was missing! I (naively :-) assumed that the 'scope's
whole input amplifier would be 'floating', and the input signal would be taken
as referenced to the 'ground' on the probe, which could itself be floating.
Clearly not! (I'll have to look at the manuals/prints for my 'scopes -
Tektronix 465's - to see if they are this way. Anyone happen to know,
offhand?) Thanks!
I am darn sure the Tekky 465 does have the 'ground' of the input amplifier --
the outer of the BNC probe connector -- tied to mains ground (and the metal
chassis/case of the instrument).
-tony
12:44 p.m.
On Feb 5, 2015, at 2:17 PM, tony duell <ard at p850ug1.demon.co.uk> wrote:
Absolutely correct.
Ah. That's the part I was missing! I (naively :-) assumed that the 'scope's
whole input amplifier would be 'floating', and the input signal would be taken
as referenced to the 'ground' on the probe, which could itself be floating.
Clearly not! (I'll have to look at the manuals/prints for my 'scopes -
Tektronix 465's - to see if they are this way. Anyone happen to know,
offhand?) Thanks!
I am darn sure the Tekky 465 does have the 'ground' of the input amplifier --
the outer of the BNC probe connector -- tied to mains ground (and the metal
chassis/case of the instrument).
3608
days inactive
3608
days old
5 comments
5 participants
participants (5)
-
ard＠p850ug1.demon.co.uk -
jnc＠mercury.lcs.mit.edu -
mouse＠Rodents-Montreal.ORG -
roeapeterson＠gmail.com -
scaron＠umich.edu