Electronics > Beginners
OScope Safety Question
David Hess:
--- Quote from: bhishmar on September 04, 2018, 01:50:26 pm ---
--- 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.
--- End quote ---
Neutral is nominally at the same potential as ground because they are connected together back at the distribution panel so it seems like you can connect the probe ground to neutral without getting in trouble. The problem is that the current going back through neutral raises its potential compared to ground and adding the probe ground just makes another path for the neutral current to return to the distribution panel. That may not cause damage, but the ground going through the test instrument is now carrying part of the neutral current so it has different voltages across it.
The safe thing is to treat neutral the same as hot. Nobody would ever ground hot, right? RIGHT?
--- Quote ---2. Some Ground continuity path thru a hi-impedance for differential-probe's (internal ground), to limit common-mode voltage at differential amplifier inputs.
--- End quote ---
The differential probe has three inputs; two signal inputs and a ground (more properly called common but whatever) whether it is made available or not. Modern differential probes are grounded through their connection to the test instrument. But if they are connected to an isolated input on the test instrument, then this ground connection is missing and the common mode voltage may drift anywhere within the limits of the high impedance signal inputs. If the common mode voltage drifts out of range, then the probe will not work correctly and may be damaged.
This becomes a significant hazard to the user if the isolated different probe is used to make a measurement on something which has a high common mode voltage. For instance if the differential probe is used to make a high side measurement on rectified 240 volts AC which will be 340 volts DC or slightly higher, then the differential probe will charge to 340 volts DC without its ground connection through an oscilloscope which has an isolated input. 340 volts storied in the common mode capacitance of the differential probe and across the input impedance of the differential probe is not much but imagine the same situation with 1000s of volts of common mode voltage which high voltage differential probes may be specified for.
--- Quote ---3. Saftety
--- End quote ---
It comes down to just being a bad idea to connect any low impedance point to the non-isolated DUT. Treat neutral the same as hot.
The differential probe issue with isolated inputs is not commonly encountered but it is important to be aware of it just like using AC coupling with common x10 and x100 probes.
Navigation
[0] Message Index
[*] Previous page
Go to full version