Probe Oscilloscope Ground Blow up issues USB

[Mr. Scram]

When you attach the ground clip at that point, this is what you get.  You're shorting to ground, because the ground clip is, well, grounded.

So how would you get the voltage drop across that resistor?

[tronde]

When you attach the ground clip at that point, this is what you get.  You're shorting to ground, because the ground clip is, well, grounded.

So how would you get the voltage drop across that resistor?

[Mr. Scram]

So putting the ground/neutral lead behind the load, is what you're saying? Wouldn't you also get the load resistance included?

[tronde]

My drawing is showing exactly the same as the one you posted, only slightly re-arranged.

If you move the shunt into the neutral wire, and connect the cround clip to the end of it most close to the source (lowest voltage potential), you will not get a short. But, as you can see from some of my previous posts, you can end up with an unwanted current in the ground clip / wire.

The best choice to measure the voltage drop across the shunt is to use a differential probe or use two probes without ground clips and the math function in the scope.

Measuring mains with a normal scope is challenging, and it is easy to forget a tiny detail that will ruin everything.

[Zero999]

• Differential probe (of suitable CAT rating).
• Isolating transformer to float the device under test, NOT the scope, note that the right thing to do with the safety bonding connection on the output side of the transformer is highly situation dependent and I would always use an RCD on the OUTPUT of the transformer.
• Math mode (With two high voltage probes, of suitable CAT rating, and with the ground clips treated appropriately. This may or may not be workable depending on the details of how your circuit under test is referenced).
• Battery powered scope with all plastic case (Fluke Scopemeter or the like), no good for anything fast and differential, as you end up adding way too much cap to the signal you have connected to the 'ground' clip, but useful for things like having a look at power harmonic currents.

All good but I don't see why you'd use an RCD on the secondary side of an isolation transformer? It won't do anything! If one side of the secondary is earthed, there will be no leakage and the RCD won't trip.

A better solution would be an insulation monitoring circuit. This could be made with a neon lamp and some high voltage resistors, so the lamp glows, if one side of the transformer is earthed, thus warning you the other side is live, with respect to earth.