AC Isolation confusion....could use a little guidance

Let me preface this post by saying nearly ALL of experience is in DC circuitry, I have a BASIC understanding of AC (rewiring lamps, changing wall switch and outlets, etc) now would to get into repairing and/or maintaining my own solid state audio equip. (early 70's - 90's)

I have done a bit of research on this but don't fully grasp certain situations and would rather not risk wrecking equipment not to mention my own death. Since all of my high powered equip. (guitar amps, rack gear) are 3 prong plug (which I won't be fooling with anyway) and all of my audio gear are polarized 2 prong, this is where the fog sets in since I still know Neutral Line is still connected to earth ground anyway. Was considering pulling the trigger on pro level scope but the grounding situation is a bit off putting. So my question(s) are:

1) When DUT is under mains power, and you are using a handheld scope, since there should be no path to ground, this is the safest method for probing and signal tracing with having to worry about what you touch even when the scope is chassis grounded to the DUT?

2) If this is true? Does the situation change if that scope is powered by a DC wall wart? I assume no....am I in error?

EdStraker said:

Let me preface this post by saying nearly ALL of experience is in DC circuitry, I have a BASIC understanding of AC (rewiring lamps, changing wall switch and outlets, etc) now would to get into repairing and/or maintaining my own solid state audio equip. (early 70's - 90's)

I have done a bit of research on this but don't fully grasp certain situations and would rather not risk wrecking equipment not to mention my own death. Since all of my high powered equip. (guitar amps, rack gear) are 3 prong plug (which I won't be fooling with anyway) and all of my audio gear are polarized 2 prong, this is where the fog sets in since I still know Neutral Line is still connected to earth ground anyway. Was considering pulling the trigger on pro level scope but the grounding situation is a bit off putting. So my question(s) are:

1) When DUT is under mains power, and you are using a handheld scope, since there should be no path to ground, this is the safest method for probing and signal tracing with having to worry about what you touch even when the scope is chassis grounded to the DUT?

2) If this is true? Does the situation change if that scope is powered by a DC wall wart? I assume no....am I in error?

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HVDC is more unsafe than AC.

1) Like a DMM a battery powered scope on 240Vac can be safe if not during a lightning storm as indirect transients are generally common to both neutral and line. Many recommend 50:1 HV probes for safety or limiting Vin max..

There are a few methods to measure as long as there is no risk of a lightning storm..

I prefer PE grounded probes on both Line and Neutral then use Math A-B or CH1 - 2.

You can also measure Neutral and if < 10V then use a series resistor on the ground clip such as 100 ohms

2) yes it changes, Wall warts are known to leak a little ac current (uA) , maybe isolated at 50/60 Hz but more leakage occurs with impulses >3kV and some may be unreliable for breakdown or leakage and rely on varnish coating on thin magnet wires for protection. Better transformers add mylar tape. Yet we trust them all time when chargers are plugged into phones etc. The quality certificates matter on 100% factory testing and no damage. So most are safe but not all..
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EdStraker said:

When DUT is under mains power, and you are using a handheld scope, since there should be no path to ground, this is the safest method for probing and signal tracing with having to worry about what you touch even when the scope is chassis grounded to the DUT?

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But if you are in contact with anything that has a path to earth ground, then that can cause an electrocution if the chassis ground somehow gets connected to the mains hot wire.
Remember it only takes a few tens of milliamps of AC to kill.
That's why it is recommended that any AC equipment being tested be transformer operated with the output not connected to earth ground.
At a minimum, any AC powered equipment you are working on should be connected to a GFCI outlet so, at most, you may experience a shock to earth but it likely won't be lethal.

crutschow said:

That's why it is recommended that any AC equipment being tested be transformer operated with the output not connected to earth ground.

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This why I mentioned the fact that all 120v audio equipment (at least all that I own is polarized 2 prong) since Neutral is tied to earth anyway...... So if I understand this correctly it is safe to even chassis ground the scope (under battery power) without fear of earth grounding yourself anywhere on the DUT as long as you don't touch the BNC's on the scope itself while under live test? Or am I still in error here?

EdStraker said:

This why I mentioned the fact that all 120v audio equipment (at least all that I own is polarized 2 prong) since Neutral is tied to earth anyway...... So if I understand this correctly it is safe to even chassis ground the scope (under battery power) without fear of earth grounding yourself anywhere on the DUT as long as you don't touch the BNC's on the scope itself while under live test? Or am I still in error here?

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You are still in error, and as you obviously have no idea of the principles involved - unless you understand EXACTLY what you';e doing, and why, I suggest you don't go connecting oscilloscopes to mains powered equipment, particularly ones with two core mains leads (class II).

All audio equipment has isolation from the mains, so there is no direct connection from the mains to the audio circuit, and there is always a transformer of some sort blocking the ac.

Whether or not the mains lead is polarised, both the live and neutral have to be regarded as possibly at mains voltage.

High power amplifiers have large voltages present, even on the audio side. An amplifier that can produce 500 W into 8 Ohms will have a peak - peak voltage of around 200 V, so there will be voltages that large on the audio side. Also there can be large capacitors which remain at dangerous voltages for some time after the mains power is removed.

Low power amplifiers and pre-amps use smaller voltages, so there isn't much danger of an electric shock from the audio side.

However, if you are measuring stuff inside audio equipment, you need to know which part is on the mains side of the transformer, and that is not always obvious. You're welcome to post a photo and someone here can say which bits are likely to be live.

Audio equipment is often made without an earth to avoid hum loops. https://en.wikipedia.org/wiki/Ground_loop_(electricity)
There is no danger of injury if you ground the chassis of audio equipment, possibly via the ground lead of a grounded oscilloscope. There is a possibility of a mains hum but that will have little effect on fault-finding.

In my experience, all mains powered oscilloscopes have the BNC grounds connected to mains ground.

Diver300 said:

All audio equipment has isolation from the mains, so there is no direct connection from the mains to the audio circuit, and there is always a transformer of some sort blocking the ac.

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Wow, that's an incorrect, misleading and highly dangerous claim.

MOST audio equipment has mains isolation, and I'd like to think that anything remotely modern has FOR THE SECONDARY SIDE ONLY - but in the past live chassis and non-earthing was pretty common.

But even with modern equipment, the primary side of the transformer is live, and if it's switch-mode that is a considerable amount of circuitry that is live to the mains, and EXTREMELY dangerous for inexperienced connection of an oscilloscope.

Yes, there were valve amplifiers and TVs with a live chassis. However anything with audio connections like speaker sockets or phono connections had to have isolation to live and neutral.

I also agree that the switch mode circuits on the input side of the high frequency transformer are live, and it takes experience to know which components are part of the switch mode circuits.

Even when there is a conventional transformer, the can be inrush limiting circuits on the live side.

When in doubt use a 120V GFCI or 240V RCD to work with line voltage. They are designed to trip a relay with less than 6mA rms AC unbalanced current but international safety standards and method of implementation can vary widely.


Two pronged devices must use "double - insulation" when not using the protective earth (PE) gnd pin. This could be the plastic box and the varnish coated magnet wire.

The 3 pronged devices that use a switching supply and line filter have intentional leakage current to suppress RF noise < 2 mA to reduce interference with AM/ HAM radios etc. The metal-cased hand tools and e-stoves must use 3 pin plugs too but don't necessarily have intentional LC cap leakage filter current to ground.

I find the worst common offenders for leakage are the laptop chargers that have leakage capacitance feeding thru the RF transformer to the metal cased laptop which is sure to give your bare knees a tingle if sitting outside with damp shoes or bare feet with the charger powered leaking at a couple mA and your knee happens to touch the metal edge. The smaller the skin surface area touching the laptop case, like your wrist or knee, the more burning sensation is felt. But normally, it won't kill you, unless the insulation breaks down in a lightning storm.

Nigel Goodwin said:

You are still in error, and as you obviously have no idea of the principles involved - unless you understand EXACTLY what you';e doing, and why, I suggest you don't go connecting oscilloscopes to mains powered equipment, particularly ones with two core mains leads (class II).

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So in other words, unless you are a 30+ year expert engineer you have no business asking questions or trying to learn anything. Not very helpful. And if it WERE the case I wouldn't be here asking.

Tony Stewart said:

When in doubt use a 120V GFCI or 240V RCD to work with line voltage.

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This part is a given and absolutely agree.

I would not be working on anything more than 60-100w RMS to begin with, more than likely less. Pre, Power Amp, EQ (maybe) possibly a turntable. More often a Receiver. My concern was understanding proper safety factors/procedure.
Even my KLH A/V receiver from the late 90's is 2 prong variety.

The question still remains, are my 2 previous assumptions correct or in error? If in error, how so?

My only interest is troubleshooting bad components and tracing signal / signal quality through the circuit without shotgunning the whole thing and hoping for the best. Which is a way unprofessional approach to start with.

EdStraker said:

So in other words, unless you are a 30+ year expert engineer you have no business asking questions or trying to learn anything. Not very helpful. And if it WERE the case I wouldn't be here asking.

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No, you don't need 30+ years of experience, but you need to start from the basics and work up, you can't just jump in at the middle, without understanding what comes before. As a hobbyist you can easily get to a decent standard in a year or two, particularly now with all the information on the Internet. I was self taught as a child, from Electronics magazines, but it's a lot easier now