How to safely use oscilloscope when measuring mains voltage

Question

I have a P5120 voltage probe that is rated up to 1,000 V RMS. I have 2 oscilloscopes, the MDO 4034 from tektronix and the TPS 2024 also from tektronix. The PCB board that I am measuring is directly hooked up to mains voltage.

Now when I was measuring on the TPS 2024 oscilloscope all was going well and I was able to get my measurements. When I switched to the MDO 4034 oscilloscope I got a massive spark when I connected the ground clip to my circuit. Thankfully nothing happened to the oscilloscope, but I don't understand why this happened?

Why can I measure anything I want to on the TPS 2024 oscilloscope, but when I measure the same thing on the MDO 4034 oscilloscope it creates big sparks?

Someone help me understand how to safely use this probe so that I do not damage my oscilloscope.

Answer 1

The Tek TPS2024 is a very special and therefore expensive (albeit older) oscilloscope that has four channels that are completely isolated from earth. It's also okay for general-purpose use, and it's my daily driver.

Most oscilloscopes have their chassis and the outer part of their BNC input connectors directly tied to earth. A typical and common newbie mistake is to connect the ground lead to some voltage wrt earth and damage something or other.

If your probes are rated for mains connection (at least CAT II or CAT III, depending on what manner of mains you're probing) you can use two in differential mode using two channels (subtract one from the other). That's not so great for measuring small voltages relative to the mains (for example, the voltage across a current shunt to measure the current through a MOSFET) you might need a special probe if you actually need to do that. On the other hand, maybe you just get an isolated current sensor or current transformer of adequate bandwidth and use that, if your pockets are not deep enough.

It is not recommended to float the oscilloscope wrt ground, that would be unsafe as you could get a shock or damage something that might be plugged into a 'scope port- it could get blown to smithereens (or perhaps just killed)- for example the USB port.

Sometimes it's possible to power what you are measuring through an isolation transformer, allowing the reference node in question to actually be safely tied to earth.

Answer 2

When I switched to the MDO 4034 oscilloscope I got a massive spark when I connected the ground clip to my circuit.

Can't do that unless the ground of the scope connected to ground on your PCB. Either you were connecting the probe to ground and your ground is not grounded OR you connected it to Line or Neutral which is also very bad as that is shorting either one of those to ground (and shorting line or neutral to ground is bad bad bad, lots of current, enough to melt metal).

If you did indeed connect it to ground, then there is a problem with your PCB grounding, because it should be close to the scope ground. If you saw a spark, it's clearly not. The best way to troubleshoot these problems is before you connect ground, get a DMM and measure the AC and DC voltage between the scope and the PCB.

So what to do? Get a fully differential probe one that can handle high voltages, then you can connect the probes to whatever you want assuming that they don't go above the spec of the probe. Cheaper 100MHz diff probes that can go to 1300V are available at everyone's favorite distributor named after a South American river.