Disconnect earth ground to oscillioscope -vs- earth ground to dc test circuit

Has anyone ever actually destroyed any of their equipment by bypassing the earth ground on their oscillioscope?

Or destroyed circuits or equipment by earth-grounding the dc ground?

I'm trying to teach myself the oscillopscope, and learn the best practices - and I read / watched some "theories" but I do not know if the dangers are practical, if I'm not testing 220, 3-PHASE or even working with A/C current mains at all.

It is confusing to deal with the noise in my speakers / scope readings, since I'm trying to make audio circuits - and have lines into my sound card, pc speakers.

I usually have my scope disconnected from ground, but always keep this in mind.
You 'must' remember the chassis is one of your connection points!

I have seen the result of someone else blow the ground lead off a work scope and destroy the components in the cct. by placing a scope ground on the wrong point.

Use scope differential input measurements if you need to measure voltages across components.

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See:

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Has anyone ever actually destroyed any of their equipment by bypassing the earth ground on their oscillioscope?

The 'scope, your stereo, or TV, etc., is earth grounded to protect YOU (and other humans), not to protect the 'scope or other equipment. As long as the chassis/ground is connected to earth ground you can't get a voltage on the chassis. If you disconnect that earth ground, you are defeating a safety feature.

On most products, that ground is there it protect you from internal faults... If a wire comes loose, or some component shorts the chassis to the power line, the chassis remains at ground potential, current goes to ground, safely blowing an internal fuse/circuit breaker or the mains circuit breaker.

On the 'scope, the earth ground will also protect YOU if you connect power line voltage to the probe's ground. The probe may be damaged, but you'll still be alive!

BTW - The negative "ground" probe on a multimeter is almost always isolated from earth ground, even if you have an AC powered bench-multimeter.

You might want to watch this video. The guy is an engineer in Australia.

He has a lot of good videos and tutorials on electronics. Well worth watching. Very informative.

Basic rule that kept me alive for 60+ years is:
For the workbench, ground your test gear, and float (isolation transformer) the device you're working on.
For fixed installs, use battery powered test gear.
Leo..

Wawa:
Basic rule that kept me alive for 60+ years is:
Ground your test gear, and float (isolation transformer) the device you're working on.
Leo..

And always use a volt meter to check for voltage potential between grounded devices. Before hooking up your oscilloscope. Note: in the US, the ground and common is connects together at the breaker box/meter. This ensures that the voltage potential between ground in common is close to being the same. it is always good to make sure these connections are in good shape, every once in a while.

The appropriate way to "unground" scope readings, is to leave the scope and probe shields grounded, and use both input channels. Most scopes have an "invert" and an "add" setting for the channels. So you have A and B channels, you want to measure the difference, so you switch to add mode. Now you have A+B - that's wrong but now you select invert for B, then you have A+(-B) = A-B.

This gives you a floating voltage difference measurement, as mentioned in post #1.

Wawa:
For fixed installs, use battery powered test gear.

We did that also.
Often worked on the likes of TWT power supplies, used battery in the workshop as well for those.

I want to thank everyone for their detailed and speedy replies.

Using batteries is not really an option. My circuits are linked to ground through the USB cable to my PC workstation(s), and also sometimes through MIDI cable to instruments, and other power supplies that drive motors. I think I would have to have about 20 amps of continuous battery power supply for that to happen.

The ground for my oscilloscope presented issues once I started learning how to test my circuits, and discovered that was tied to the ac-mains-ground. It's because of this design, I assumed it was safe to setup my other DC circuits to use the mains-ground as well, since some laptop power systems also adopt this type of design.

Ever since 'big brother' stepped in, they appear to have some kind of design rule where there is no external feature for disconnecting earth from the instrument ground.

The very old BWD 530a - which conveniently has an earth-disconnect feature that disconnects earth from instrument ground. I guess, these days, they just want people to use two channels and the digital mathematical subtraction feature to measure the voltage across a component. Obviously it takes 2 channels instead of one. As for the debate between safety etc....... there's always 2 schools. For me, I prefer to have the manual control, and be able to remove earth from the instrument ground (if I ever wanted to do it).

I haven't damaged anything (including myself) before by disconnecting earth from measurement reference. It is good to have the earth connected though in general.

Southpark:
Ever since 'big brother' stepped in, they appear to have some kind of design rule where there is no external feature for disconnecting earth from the instrument ground.

We used to have a big yellow isolating transformer for this, there was a switch or isolating link of some sort which enabled us to separate the earth.

Boardburner2:
We used to have a big yellow isolating transformer for this, there was a switch or isolating link of some sort which enabled us to separate the earth.

Thanks boardburner! Nice.

Something to watch out for though, we also had a red one which was oil filled for use on 5 KV stuff.
No one could pick it up though.

If you use one of the readily available yellow ones be sure to check the insulation rating if you work on high voltage stuff.
Good enough for almost everything i have ever worked on though.

At home i normally use a lead with the earth disconnected (painted red).

However again with high voltage stuff the insulation rating of the oscilloscope PSU needs to be considered as the utility earth is connected to neutral.

PC noises and hum in permanent speaker setups due to a groundloop can be fixed with this "ebtech hum x" device.

Used e.g. in home recording setups where a grounded PC and grounded monitor speakers are used.
I think it works by using two beefy diodes anti-parallel in the ground line. Could be a bipolar low voltage TVS diode too.
Finding out which part of the setup you need to ground and which part(s) have to be powered through the ground-lifter could be trial and error.
AFAIK this technique is only allowed in the US (115volt), not in Europe (230volt).
Leo..

DocStein99:
, since some laptop power systems also adopt this type of design.

I find laptop supplies useful as they normally are fully isolated and have no earth.
For the majority of stuff they are great.Generally good current ratings too although some do not like motor loads and will shut down.

Wawa:
PC noises and hum in permanent speaker setups due to a groundloop can be fixed with this "ebtech hum x" device.
Buy Ebtech Hum X Voltage Hum Filter Online at Lowest Price in New Zealand. B0002E4YI8
Used e.g. in home recording setups where a grounded PC and grounded monitor speakers are used.
I think it works by using two beefy diodes anti-parallel in the ground line. Could be a bipolar low voltage TVS diode too.
Finding out which part of the setup you need to ground and which part(s) have to be powered through the ground-lifter could be trial and error.
AFAIK this technique is only allowed in the US (115volt), not in Europe (230volt).
Leo..

My 12-volt speakers are connected to the same multi-use power supply I use to run lamps and fans on my desk. I also draw power from that to test my circuits. I prefer not to have a bunch of plastic a/c power supplies to convert into 12v or 5v for all the accessories, rather 1 supply broken out to a patch panel.

If my scope was affecting the testing of the circuits - I don't feel confident with the readings, trying to clean noisy audio circuits.

Can you post a circuit showing how all your bits are put together ?
It may be a simple connection problem.

DocStein99:
My 12-volt speakers are connected to the same multi-use power supply I use to run lamps and fans on my desk.

If in use at the same time this could just be interference