Oscilloscope Connections

Thread Starter


Joined Oct 31, 2020
Got a new oscilloscope to play with. 126 page user manual but with no advice as to how to properly connect the test probes.
Would someone take a look at the illustrations below and let me know if this will work? Thanks in advance. It would be appreciated.



Joined Jul 24, 2013
Since one lead of scope meter is grounded you have to be careful during measurements. If your ground and neutral are connected than you may create a short circuit.


Joined Mar 30, 2018
If it is a class II scope with no earth connection, then it won’t matter whether the battery or vehicle ground is connected to real world earth/ground. Even then, it will only be a problem with the 2nd of the three scope connection shown.

Important: if it is a class I scope with an earth connection via the mains lead – then the probe croc clip is earthed – so wherever you make a connection with the croc clip will be connected to earth/ground.

Thread Starter


Joined Oct 31, 2020
Thanks for the replies.

The alternator is not in a car. It is on a bench. Being driven by an electric motor.

The test leads from the scope are grounded to the same ground that is used for the 120 volt power source.


Joined Oct 2, 2009
You have to be double or triple careful when using any bench test equipment to measure line voltages.

#1 Assume that your oscilloscope is grounded to earth ground. Check this.
#2 Assume that the grounding clip on the probe is connected to earth ground also. Check this.
#3 Don't connect the probe grounding clip to any part of the AC line receptacle. Since the probe and scope is already grounded you can prevent a disaster by keeping the ground clip away from any thing that is live.

Testing any other piece of equipment that is not grounded is another story.


Joined Dec 13, 2021
Its unlikely that your scope earth on the scope clips is not connected to ground some where
one of those pictures you show the scope ground clip connecting to one side of a coil.
thus that part is now grounded through the scope,


Joined Jul 10, 2017
If you have a dual channel scope, the safest way to make measurements on high voltage power circuits is by using both inputs to make differential measurements. First check the manual and make sure that your scope and probes can handle the voltage. Connect the ground clips of both probes to ground on the device. With both input attenuators set to the same range and the vertical mode sat to A-B, connect the two probes to the points to be tested. If your scope doesn't have a differential mode, set it to A+B and reverse one of the input polarity switches. The displayed waveform will be of the voltage across the component you are testing.