Proper oscilloscope connection to prevent ground looping

Question

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.

Answer 1

Grounding loops are not your problem. The real problem is the unknown potential between the scope and the DUT. Without an Earth GND yes both are floating. (Does your scope even accept a 2 prong input without an Earth GND? Usually, most scopes have the chassis connected to Earth GND.)

If you just probe your DUT with the scope floating, you will attach the scope GND to the DUT GND, making that the common ground. The problem is that if the two are at a different potential then you are not protected from shock and may damage the equipment.

Read this document by Tektronix.

"The four safe solutions for these issues fall into four categories: battery-powered oscilloscopes, differential measurement systems, isolated-input oscilloscopes, and monolithic isolation amplifiers." source

Alternatively, if you can guarantee the Vrms of your DUT signal you could do a differential measurement using 2 single ended probes by connecting their GNDs together and putting one on DUT GND and 1 on the signal. Then doing a math function on the Oscope to subtract the two.