Electronics > Beginners
OScope Safety Question
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David Hess:

--- Quote from: KungFuJosh on September 04, 2018, 01:03:03 am ---I've changed my setup, but I'm well covered. I returned the DS1054Z and bought a Siglent SDS1104X-E instead. I have Pico TA131 250MHz probes, which are rated 600V in 10X, and I have a BK PR2000B 100X probe that's rated 2000V. I did also buy a differential probe, but that's 1300V at 500X, which isn't always useful.
--- End quote ---

Keep in mind that the voltage rating of those probes only applies when the oscilloscope is set to DC coupling.  If AC coupling is used, then the full input voltage is applied to the oscilloscope input which is probably only rated to 400 volts.
KungFuJosh:

--- Quote from: David Hess on September 04, 2018, 01:29:38 am ---Keep in mind that the voltage rating of those probes only applies when the oscilloscope is set to DC coupling.  If AC coupling is used, then the full input voltage is applied to the oscilloscope input which is probably only rated to 400 volts.

--- End quote ---

Yup, 400. One of the benefits over the Rigol which only could handle 300. But even when I eventually figure out what the benefit of changing the coupling type is ;) then it's all good because most of my circuits don't go above 350.
David Hess:

--- Quote from: KungFuJosh on September 04, 2018, 01:46:26 am ---
--- Quote from: David Hess on September 04, 2018, 01:29:38 am ---Keep in mind that the voltage rating of those probes only applies when the oscilloscope is set to DC coupling.  If AC coupling is used, then the full input voltage is applied to the oscilloscope input which is probably only rated to 400 volts.
--- End quote ---

Yup, 400. One of the benefits over the Rigol which only could handle 300. But even when I eventually figure out what the benefit of changing the coupling type is ;) then it's all good because most of my circuits don't go above 350.
--- End quote ---

You might want to use your x10 or x100 probe to measure low ripples voltages which would require AC coupling to work at a higher vertical sensitivity.

They make, or used to make, special x10 and x100 probes which included an internal shunt allowing them to be used with DC or AC coupling without limitations.  x1000 probes always include this shunt.
bhishmar:
Part of the earlier confusion here is created by  non-standard or arbitrary  terminology  used in the posts here, by different poster's I think.
Same is the case with the other post referred by TauTech.

So let me define/ re-define  some symbols , & put forward a topology here, as I understood from earlier posts, to aid in any further discussion (if any).
==================================================================
IT    :  Isolation Transformer
DUT  :  Device under Test.

E1      :  Earth-Pin (3rd pin)  of Mains Power receptacle
L     : Phase (Live) point of Power receptacle
N    : Neutral point of Power receptacle.

L1    :  Output-Line-1 of   IT Output Power  receptacle.
L2    :  Output-Line-2 of   IT Output Power  receptacle.
E2    :  Earth-Pin (3rd pin) of  IT-output receptacle.

** I am not claiming above is standard terminology.  Just that some terminology aids in streamlining discussion & help in better understanding.
==================================================================

Standard wiring configuration inside Isolation Transformer (IT)

L    &  N   wired to primary winding of IT    (obviously)
L1  & L2  wired to secondary winding of IT (obviously)
E1 & E2   both wired to Chassis of IT. (so E1 & E2 are shorted inside IT)
==================================================

Typical Wiring outside IT   (Topology):-
DUT  is powered from a Bench power supply /adapter  which is powered from L1 & L2 of  IT.
E2 is connected to  the chassis of Bench PowerSupply. (or adapter if it has a 3rd pin Power-connector), thru power-cord

Bench Power-Supply chassis may or may not be connected to its DC-supply output -ve terminal (& hence DUT Circuit Gnd) - Tester's option.

If using a DSO directly powered from mains (not thru IT), its BNC-Gnd and any DUT-point  touched by black-alligator clip will be at Mains-Earth (E1) point.
If DSO is powered thru IT, then BNC-Gnd will be same as E2.
But since E1 & E2 are shorted, ultimately BNC-Gnd (DSO-Gnd) will be E1, in both cases.
==================================================

I am assuming in earlier posts  "Power-Gnd"   meant  E1,
& "DSO-Gnd" meant BNC-Gnd.

So "Power-Gnd" (E1)  &  DSO-Gnd (BNC-Gnd) will always be shorted & same, irrespective of  DSO is powered directly or thru IT !!
bhishmar:

--- Quote from: David Hess on September 03, 2018, 06:52:01 pm ---The isolation transformer allows moving the ground point from the circuit breaker box to any point on the isolated side of the transformer with some limitations due to capacitance coupling across the transformer and capacitive coupling into the environment.  Now the ground lead on the oscilloscope's probe can be attached at that point relatively safely without creating a ground loop.

--- End quote ---

Would you care to explain the above in the context of above terminology & topology?
I understand  there are 3 separate issues to contend with.
1.  Ground loop creation by  DSO-Gnd & associated noise introduction.
2.  Some Ground continuity path thru a hi-impedance for differential-probe's (internal ground), to limit common-mode voltage at differential amplifier inputs.
3. Saftety
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