Question: I have questions about the limitations of using a mains-rated power transformer (e.g. a small 6VAC 1VA PCB mount transformer) as a cheap differential probe, so that waveforms can be safely observed on an oscilloscope.

My questions refer to this schematic, which is not intended as a real circuit but to allow me to refer to the various elements:

![schematic](https://i.stack.imgur.com/iGwrJ.png)

[simulate this circuit](/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2fiGwrJ.png) – Schematic created using [CircuitLab](https://www.circuitlab.com/)

Vin1 and Vin2 are any two points in a mains-powered circuit across which the differential measurement is desired. R1 represents a standard scope input impedance. For now let's ignore R2, C2, C1, R4, and R5.

My understanding is that with an ideal transformer, this is a safe way to accurately display a typical mains voltage waveform. However, a real transformer will have some common mode leakage represented here by C1. This should mean that the transformer's "CMRR" (as it were) deteriorates as higher and higher frequency content appears across it.

1. My main question is: will this common mode issue present a danger to an oscilloscope when probing a circuit like a phase angle controller with fast edges? Will the high voltage, high frequency content be capactively coupled straight across the transformer, resulting in a short via the shell of the scope's BNC connector?
2. If this coupling could be a problem, would the addition of a simple filter like R2 C2 suffice to solve it, assuming that the user did not mind the loss in measurement bandwidth?
3. Are resistors like R4 and R5 a sufficient way to minimize loading on the circuit being measured?

Answer: 
This is country specific and standards specific

GREEN is universally accepted to be a protective earth connection 

White, Blue, Brown... is odd... if it wasn't for the white it would make sense. 

[![enter image description here](https://i.stack.imgur.com/HKi0x.png)](https://i.stack.imgur.com/HKi0x.png)

<https://en.wikipedia.org/wiki/Electrical_wiring>
The US/Canada standard is black=hot, white=neutral and ground is green, green-with-yellow-stripe or bare. The only other colours I've encountered is the standard I believe is UK/EU in which brown=hot, blue=neutral and ground is green with a yellow stripe. A lot of Chinese-made cables follow the EU standard.

Even with standard colours, I always check continuity with the plug pins to make sure it's right.

Your cable is very odd, in that it has no green (or bare or green/yellow) wire, and has white in combination with blue and brown. This is not anyone's standard for line voltage (120 or 240 Volts AC) cabling.

You should consider that this cable might not be intended for line voltage, even if Kingwin has molded an AC plug onto it. It may be intended for low-voltage DC in some specialty application, maybe low power lighting or communications. Look for printed specs on the sheath of the cable itself; it should say it's rated to some AC voltage higher than 120. If it doesn't, you might consider removing and discarding the AC plug and putting the cable in your parts bin for some future low-voltage project.

If it does appear to be an AC cable, then we have to assume that the folks at Kingwin used whatever colours they had handy. Make no assumptions on how this is wired; check it several times for continuity, preferably once before you have a coffee and once after.

When you're sure it's right, use electrical tape to mark the wires with the colours that will make sense to the next person that sees it. In the US that'd be black, white and green.