Proper oscilloscope connection to prevent ground looping

Thread Starter

DMoss

Joined Feb 13, 2019
3
I have a question about proper oscilloscope connection for doing work on vintage stereo equipment (120v AC input to +-76v DC board voltage). Our house wiring is a 1950’s 2 wire system with no ground wire to the outlets. The only earth ground in the house is a bare copper wire from the breaker box to water main. There is also an earth ground at the service transformer in the alley. I need to know how to properly connect an oscilloscope and DUT in this environment to prevent ground looping. Obviously, neither the scope nor DUT is earth grounded when plugged in? What’s the potential difference between the two (are they both floating)? If I’m probing, am I the only earth ground in this situation? I don’t know how isolating the DUT would accomplish anything? Would a battery operated scope be an option (still needs an earth ground) and what about probe isolation? Is a differential probe the only real option? I want to be able to do this safely, but reasonably too. Thanks
 

dl324

Joined Mar 30, 2015
9,321
Welcome to AAC!

If you're working with line voltages, floating your scope (i.e. no earth ground) isn't safe for a novice.

For making measurements just connect scope ground to circuit ground.

To add electrocution protection, install GFCI outlets. Since the won't have earth ground connected, code requires that they be labeled to indicate no earth ground. If you install at the first outlet on a circuit, the whole circuit is protected.

Another thing to check is that outlets are polarized and wired correctly.
 

Thread Starter

DMoss

Joined Feb 13, 2019
3
Thanks. Don't plan to test line voltage, I can do that with the DMM. Mainly want to use the scope to do work on the circuit boards. The transformer takes the line 120v AC and rectifies to DC, so if I just put the probe ground on the DUT chassis am I safe? Are the boards isolated?
 

MaxHeadRoom

Joined Jul 18, 2013
19,406
Sounds like the boards are isolated, so you could earth ground them, or at least the 'scope.
Do you have the ability to set up a ground rod near the work space?
Max.
 

BR-549

Joined Sep 22, 2013
4,938
Measure and see if you have a potential. You can always measure the potential between the DUT ground or common and the testing equipment ground or common.

And IF and WHEN you do.......if you have a dual channel scope with add channel feature......you can safely measure without isolation with the proper procedure.
 

dl324

Joined Mar 30, 2015
9,321
The transformer takes the line 120v AC and rectifies to DC, so if I just put the probe ground on the DUT chassis am I safe? Are the boards isolated?
Anything with a transformer would be isolated from line voltage and it would be safe; though you can still get hurt if DC voltages are high enough, so caution is still warranted.

Anything with a switching regulator is probably not isolated; they tend to avoid the expense of a bulky transformer.

If you don't have earth ground at your receptacles, your scope is floating. In theory, you could make differential voltage measurements with a single channel, but it's best not to get into the habit of doing that until you know what you're doing.
 
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