Oscilloscope mains probing

[GeorgeTsak]

Hello, EEVblog!
I have seen many videos / posts including Dave's video on how not to explode your oscilloscope, saying that the oscilloscope's ground is directly connected to the earth ground and that if you connect the ground lead of the probe in the wrong place a short circuit could occur. They also stated that if you wish to measure a device that is earth grounded you should use an isolation transformer, differential probes or if you are more extreme remove the ground pin from the oscilloscope power cable.
My question is, since the oscilloscope is earth ground referenced shouldn't you be able to measure mains by just connecting the positive lead of the probe, leaving the ground lead disconnected without any issue?

[Richard Crowley]

In theory, yes.  Making many assumptions, some of which could be hazardous or fatal depending on conditions and factors that may not be obvious to the inexperienced.  You could start by asking WHY you want to measure the power mains, and WHAT you expect to see, and HOW you intend to protect yourself and your test gear from the many hazards.

[GeorgeTsak]

Thank you for your reply!
I do not wish to measure mains at all. I was just curious.

[tggzzz]

In theory, yes.  Making many assumptions, some of which could be hazardous or fatal depending on conditions and factors that may not be obvious to the inexperienced.  You could start by asking WHY you want to measure the power mains, and WHAT you expect to see, and HOW you intend to protect yourself and your test gear from the many hazards.

Precisely.

The OP could start by looking at the praxis/safety references at https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

If the OP is ever tempted to disconnect a protective mains earth, have a look at https://www.eevblog.com/forum/projects/isolation-transformer-for-scope/msg2259465/#msg2259465

[Gyro]

I do not wish to measure mains at all. I was just curious.

Sadly, that is how a lot of new scopes meet their doom. Beginners (not making any assumptions about your level of experience) buy them and realize that the only signal sources they have are the probe cal output and the mains. It's a really good time to breadboard a 555 timer oscillator or an opamp sinewave oscillator to experiment with.

Kudos for being honest about your reason and for asking before you try probing the mains.   

[tautech]

Hello, EEVblog!
I have seen many videos / posts including Dave's video on how not to explode your oscilloscope, saying that the oscilloscope's ground is directly connected to the earth ground and that if you connect the ground lead of the probe in the wrong place a short circuit could occur. They also stated that if you wish to measure a device that is earth grounded you should use an isolation transformer, differential probes or if you are more extreme remove the ground pin from the oscilloscope power cable.
My question is, since the oscilloscope is earth ground referenced shouldn't you be able to measure mains by just connecting the positive lead of the probe, leaving the ground lead disconnected without any issue?

Never float the scope, keep it grounded always !

Yes for quick and simple mains measurements on a mains ground referenced DUT you can remove the probe ground (reference) lead and often it's a good idea to in order to prevent accidental shorts. If room is tight for probing you should also apply one of the little probe condoms to prevent the probe ground ring from connecting with something it shouldn't.
All that said it's not advisable to use ordinary scope probes for mains work instead get 100:1 probes that offer higher working voltages and greater safety for the user and the scope.

Play safe !

[bd139]

If you want to probe the mains, buy a step down transformer and probe the secondary. Low voltage, fully isolated, no scope boomey boom boom 

You can work out the attenuation with a DMM and measuring the primary voltage (safely) then the secondary and dividing.

[jh15]

I was in a plant where there was a lot of emi (electromagnetic interference) coming in on test equipment on the production line. To start, I used a floating scope, and saw stuff.

I was spanked by coworkers. Don't use that 2 to 3 prong adapter hidden in everyone's tool bag.

Being mostly administrator and (the best part 50/50 technician) on the floor tech, I obeyed.

used a filament transformer and it was clean.

This is in '83 so no computer stuff of much concern around.

Told my techs something is on the line power. I saw it. (but  not how I did it at the moment).


Called tektrronix.

They sent a legal float scope item. This was basically a battery (nicad) and inverter with a handle.

I had fun walking down the hall with a facilities owned drop light on my belt 100w bulb.

So hooked up a floating scope and found the problem.

It was a bog standard magnifying fluorescent light. It's transformer (ballast) was making a corona discharge to its base. no 3 pin plug I am sure. 

So it was a 60hz xfmr did not show the higher freq I saw on scope