Scope question.

[TinMan]

A question about my scope.
Having a common ground with my scope is becoming a real pain in the butt.
Can i float the ground by disconecting the earth in the 3 pin plug.or from the scope chasis itself?.

[MrMag]

i have disconnected the ground on my scope. 

[ion]

It is a highly not recommended practice to lift the ground on a scope that is measuring mains voltages.

Nevertheless we did this for as long as I can remember in the good old days of the big tube TEK scopes and somehow survived.

We usually had a big cardboard sign leaned upon it that said "Hotta Scopini" everyone knew what it meant and stayed clear.

Nowadays an isolation transformer would be highly recommended. Also note that the scope transformers may not have the dielectric withstand above 1500 V and could arc over.

[Centraflow]

Brad not sure what exactly you are referring to, the "mains ground" or the "probe grounds", the latter are mostly common, so you have to be careful how you connect the second probe ground as in across a CSR for example with reference to using another channel

regards

Mike

[TinMan]

@ION
An isolation transformer would still have a common ground-earth.
I know the two grounds on the probes would still be common,but thats not a problem. What i need to be able to do ,is measure the voltage and current across the input of a mains transformer,but cant due to the common ground situation-good thing the RCD works in the meter box.
I'm really not sure as to why you would use the ground as the reference point in a scope anyway,as the ground and neutral both are 0 volt points.there is no leakage either,as i get no voltage at all between neutral and ground-which you shouldnt.

The whole front panel of the scope is plastic,and I'm more than happy to put the rest of it in a plastic box-just in case.
Sure,we shouldn't do it-but we all do things we shouldn't lol.

I guess i could just make a short extention cord,and leave the ground disconnected in one of the plugs-just to try it.

[poynt99]

Brad,

Sometimes it's the only way. I had to use a ground lift for my scope while testing the RA circuit.

[TinMan]

Brad,

Sometimes it's the only way. I had to use a ground lift for my scope while testing the RA circuit.

Well i'll make up that short extention cord tomorrow,and give it a try.

[ion]

Brad,

Sometimes it's the only way. I had to use a ground lift for my scope while testing the RA circuit.

What I said was just the legal disclaimer. My scope is operated with a ground lifter nearly all the time.

For line measurements, I use a CSR on the low side (with ground lifted) so that at worst case the scope is only a few volts above ground.

Regarding the isolation transformer, I use it without a secondary ground to energize the DUT.

I built a box with built in 4 wire 0.01 Ohm CSR and 10 to 1 mains voltage divider that has BNC outputs, direct connection to the scope for quick testing anything plugged into the receptacle.

[BEP]

I must disagree, and so would any scope manufacturer. This post isn't intended to anger folks but SAFETY MUST prevail.

A scope is not a hand-held voltmeter. Attached is a document you should find available from any reputable scope manufacturer, at a minimum, in the operator's manual.

I've used a scope on likely every possible measurement task where a scope is applicable and there is no case where you should lift the ground. In fact, doing so is a violation of safe work practices and has been reason for more than one person to loose their employment where I work.

Before you explode, please read the portion of the attached document titled "A minus B Measurement".

Having suffered severe electrical burns because a former colleague decided to 'lift' the scope ground, this is a pet peeve.

[ion]

I agree with you WW, scopes should always be grounded.

I have nearly welded my scope case to another piece of grounded equipment that was a bit too close when I floated it while probing mains circuits.

Floating is extremely dangerous, but sometimes you have to live dangerously, but never foolishly....ask the Wright Brothers.

[poynt99]

Actually Brad, I must correct myself.

I lifted the ground of the function generator, not the scope.

So may I suggest you leave your scope grounded, but isolate the grid supply by using an isolation transformer, and do not connect to the earth on its secondary. If that works for you.

[gridbias]

The solution to this is to get a battery operated device .

Or use a clean inverter/battery system . Not easy !

Grounds are a pita for  experimenters especially in this field where we might not want to shunt everything to earth.

Lifting the ground on a mains connected device will introduce other misleading artifacts because the circuitry assumes that we have a clean earth refrence .Of course differential mode solves most  of the problem except options of  where to put the shield( ground) .



The other is that a battery operated unit is usually digital and almost usless in that it knows what it wants to see  ...
We are blind and misled by these wonderful devices  if we are not very careful. and even then ... our calculations are always based on assumption at some level.

This is a good time to congratulate  Poynt agian in the Rosie cro ordeal ..  Well done ! and like a gentleman .

[BEP]

Floating is extremely dangerous, but sometimes you have to live dangerously, but never foolishly....ask the Wright Brothers.

ION,

Just so all are clear that I am no scope-master I'll admit that I've blown my fair share of equipment and DUTs using connection practices that seemed to make sense or even give me the results I expected.

That said....

I'm very curious what measurement situation required floating the scope ground clip and/or the chassis.

There have been many of my measurements and results done with the ground pin removed that simply led me astray. Low frequency or fairly soft transients show fairly accurately with a lifted scope ground but I've found that any signal other than those wind up being modified by the addition of the scope chassis circuit to the DUT.

IMHO, the ground clip and connection to ground is simply part of the signal shielding mechanism. While it can be used as a signal return path for some measurements it isn't intended for that purpose on anything but the oldest tech (vacuum tube technology?).

This is probably why almost every (non-differential) input scope has two channels. Two are required to make a single floating measurement.
Even with my hopped-up ancient Tek7904A I can only make accurate measurements of two separate floating signals (4-channels).

[ion]

WW quote:

I'm very curious what measurement situation required floating the scope ground clip and/or the chassis.

I only do this to eliminate a ground loop problem when using a CSR on the low (neutral) side of the 120 Volt mains and needing the other probe for voltage waveform (2 channel scope) e.g.

Scope floating, ground clip of scope probe to one end of CSR, other end of CSR to probe tip. Other probe tip on high side of mains.

The scope chassis now rides slightly above neutral by fractions of a volt, but no ground loop.

I use this for current / voltage phase relationship when testing loads with PF less than 1.

[BEP]

Thanks.

Ah! That makes sense. Either that way or fork out for a scope with enough channels to look like an octopus or even more for the differential probes.

Such is not a problem when one knows what they are doing and what to avoid touching 

We always notice mistakes made in measurement technique or measurement interpretation by the many scratching the surface of this field. One of the things that bites folks is adding the scope chassis to the DUT and thinking it is only the DUT shaping the waveform. At Mains freeks this is not usually a problem and you obviously knew that.

[ion]

Thanks.

Ah! That makes sense. Either that way or fork out for a scope with enough channels to look like an octopus or even more for the differential probes.

Such is not a problem when one knows what they are doing and what to avoid touching 

We always notice mistakes made in measurement technique or measurement interpretation by the many scratching the surface of this field. One of the things that bites folks is adding the scope chassis to the DUT and thinking it is only the DUT shaping the waveform. At Mains freeks this is not usually a problem and you obviously knew that.

I would never probe a high speed circuit the way I described in the former post , the loading effect of the ground and the scope chassis acting as a big antenna would effect the circuit (depending on where you were putting the ground lead), not to mention the capacitive coupling of the power supply transformer.

[BEP]

I didn't think you would.

However, there are plenty of folks in this game who do - and see the wished magic as the result.

[TinMan]

What i need to be able to do,is test Lucs transformer circuit-as in current across the CSR and voltage across the transformer.
Now ,i have no problem hooking the ground of the scope to the neutral of the main's,but when i hook the active to the other side of the transformer,it flips the RCD out in the meter box.
So i would only be measuring a 50Hz signal in this case.