Protecting Yourself and Your Scope

[jminassi]

Hi Guys,

I want to buy an oscilloscope for testing audio gear.  I've been watching all the youTube videos explaining how you could harm yourself or your scope by connecting the ground lead from your probe to the wrong point in the circuit. The solution appears to involve using an isolation transformer on the device under test.  Alternatively you could buy a set of isolation probes.  It appears the idea is not to have the device under test referenced to earth ground.  Wouldn't using a "cheater plug" effectively lift the ground from the device under test?  What is it that I am not understanding about this?  If an isolation transformer is indeed the solution, should I be looking at particular specifications?  The DUT would be high voltage tube amplifiers for audio or guitar amplifiers.  Any clarification would be much appreciated.

John

[Thermioniclife]

Not having the DUT grounded to earth can be safer for you because the neutral wire and the earth ground are connected to the same point in your breaker panel, so just using a "cheater plug" does not isolate the DUT or yourself from ground. Using an isolation transformer will protect you because there is no physical connection to ground on the DUT. Having said that be aware that most isolation transformers have a connection to ground from the primary to secondary windings which is not a truly isolated trans. This is most likely due to lawyers. Fore example I purchased a Tripp Lite iso that I had to disconnect that connection inside the transformer.

[jminassi]

Thanks Thermioniclife.  What do you know...Shakespeare was right about the lawyers (Henery VI Part 2 Act IV Scene 2)!  I will be sure to check the wiring when I acquire an isolation transformer.

Best,

John

[Paul Birkeland]

I've been watching all the youTube videos explaining how you could harm yourself or your scope by connecting the ground lead from your probe to the wrong point in the circuit.

For audio purposes, watching those YouTube videos is more dangerous than the problem you are describing.

The solution appears to involve using an isolation transformer on the device under test.  Alternatively you could buy a set of isolation probes.  It appears the idea is not to have the device under test referenced to earth ground. 

If you're testing audio gear, buy a bunch of clip leads.  Always clip your scope ground to either the chassis plate or the shell of an RCA jack (on a preamp use an output jack so the ground will be tied to the audio ground at all times), problem solved!

[mcandmar]

A lot of that scaremongering comes from the days of old AM radios that had a live chassis, hence the need to float the device on an isolated transformer.

[jminassi]

Hi PB,

Is there something I should be reading that would help me rather than watching youTube?  All my work is confined to audio amps, preamps, phono stages, disc players, cassette decks and guitar amps. It's all my equipment, and I have a lot of it.

I understand there can be a different potential between earth ground and a ground in the audio circuit.  But I thought the chassis and RCA shells were usually tied to earth ground?  What is it that I don't understand correctly?

Mcandmar,

Thanks for the information.  I have a shortwave, AM/FM radio that is probably 65 years old.  It has an ungrounded, non-polarized plug on it.  I imagine that is the kind of thing you are talking about.  But again, I thought any amp with an exposed metal chassis (like a McIntosh MC275) has it's chassis connected to the center pin of the mains cord.  I am going to get my miultimeter out and test my
hypothesis.

John

[Paul Birkeland]

I understand there can be a different potential between earth ground and a ground in the audio circuit.  But I thought the chassis and RCA shells were usually tied to earth ground?  What is it that I don't understand correctly?

If the piece of gear you're working on has a 3 wire power cord, you can assume that the RCA shells are adequately close to earth potential. 

If you're unsure, you can always get a scope that will run off a battery.  They can come in handy for other reasons too sometimes.  A USB powered scope hooked to a laptop that's running off its battery will also have no earth reference.

[jminassi]

Ok, thanks!