This that famous harmonic distortion?

[tom66]

Re: This that famous harmonic distortion?

Then what exactly is the point of isolating scope like that? You have accomplished nothing...? The power supply inside the instrument is already isolated from live and neutral...

BTW, on my scope, every screw is metal and earthed, probe test point earthed, USB port earthed, RS232 earthed, etc.

You CAN and WILL make mistakes, saying you WILL NOT is NOT an excuse for defeating a SAFETY SYSTEM.

[Behemot]

Re: This that famous harmonic distortion?

Now also the primary is isolated…and having the crazy fistortion there, it may help it a little bit with smoothing.

Well I am counting on the breakers, hopefully they won't let me down

Anyway, what you suggest: as UPS on battery is probably floating, but carying power, to plug isolating transformer behind the UPS and probe from there, directly grounding the neutral and looking at phase-to-neutral (grounded) peaks? What exactly is the difference between measuring phase-to-ground directly?

[rievax_60]

Re: This that famous harmonic distortion?

The outlet socket on some isolation transformers is earthed. This sort of transformer will not isolate test equipment from mains earth, which is not safe to do anyway.

[tom66]

Re: This that famous harmonic distortion?

I can confidently say if your scope is well designed it will do absolutely nothing to the accuracy of the readings.

[Behemot]

Re: This that famous harmonic distortion?

The outlet socket on some isolation transformers is earthed. This sort of transformer will not isolate test equipment from mains earth, which is not safe to do anyway.

Exactly, I don't really want to get isolated from earth…thats the last salvation if something goes wrong

[rievax_60]

Re: This that famous harmonic distortion?

A safe way to measure mains related waveforms is to use the probe tip only. Dont connect the probe's ground lead to anything. If there is no or very little voltage present on the Neutral then whatever is seen on the Active is likely to be real.
A multimeter reading should be taken also to avoid being mislead by a wrong DSO reading.

[rievax_60]

Re: This that famous harmonic distortion?

For example, If I were to connect my DSO's probe ground lead to the Neutral of a mains outlet, a Safety Switch trip would occur.
Also it is not too uncommon to find Active and Neutral connection reversed at power outlets. This is extra dangerous because the outlet is always live even with the switch in the OFF position.

[Behemot]

Re: This that famous harmonic distortion?

Yes, I already found that - I have these branch joints and the old/cheap ones have phase switched with neutral on one side.

As for the safety, you mean when there are currents flowing through neutral, they may go through ground than as it has lower impedance?

[rievax_60]

Re: This that famous harmonic distortion?

Yes, I already found that - I have these branch joints and the old/cheap ones have phase switched with neutral on one side.

As for the safety, you mean when there are currents flowing through neutral, they may go through ground than as it has lower impedance?

If I have understood properly, if current is diverted from the Neutral circuit to the Earth wiring, there is no real safety concern, just a nuisance Safety Switch trip. The Safety Switch responds to the current imbalance between Active and Neutral.
I wont say any more as its a bit difficult to follow what is being done.

[Kiriakos GR]

Re: This that famous harmonic distortion?

The reason it is not safe to isolate a scope is because it's a single insulated device, which means one failure is permitted, a live-to-chassis fault. With a properly earthed instrument, this will simply trip the RCD/GFCI or blow the fuse. Isolating it removes this protection. In addition, if you happen to connect the earth of the scope to a live circuit, the scope chassis is now floating live. If you touch it, you could be electrocuted.

So, always isolate the load/DUT only!

a) There is no external metal chassis in all latest DSO
b) There is no electrocution chances other than live phase to contact osciloscopes ground alligator and blown the fuses at the electical wall panel of your home or lab.

c) we have two basic teams of people using osciloscopes, the rule of thump of isolate the load does not apply to electricians.
Therefore when some one is playing with power quality measurments with out training and far spesialised tools he is at risk covereted by CATII enviroment.
At the same risk is even your mother when her irroning system mains cable gets damaged by all this pulling around and heavy handling which leads to a shortcut and blown fuses.

Stupid made videos on youtube by all sort of Gurus they do pass a very disformed represantation of the actual risks.

[tom66]

Re: This that famous harmonic distortion?

What's your definition of modern? Rigol DS1052E: screws, BNC, trig out, etc. are all connected to chassis. Yes, the case is plastic, but that doesn't mean the rest isn't! And you could easily touch the other channel outputs e.g. while adjusting the V/div.

This is an electronics forum so in general we deal with situations where both isolating the load is appropriate and possible.

In the situation where you decide breaking the oscilloscope ground is necessary you should instead consider a differential probe; or, a differential isolated scope with double insulation, or a dedicated power meter. Yes, they cost a lot of money, but cheaper than a funeral.

[budm]

Re: This that famous harmonic distortion?

The outlet socket on some isolation transformers is earthed. This sort of transformer will not isolate test equipment from mains earth, which is not safe to do anyway.

This is very true, if you have your scope hooked up to this kind of the isolation transformer hook up, you BNC, the Ground of the scope probe is still connected to the safety Ground. If you are working the PRIMARY side circuit such as SMPS that rectify the main AC line, when you hook up that scope probe to the SMPS primary circuit ground, you will see nice spark or trip GFCI if the DUT is plugged into the GFCI outlet. If you float the ground of the isolation outlet, then the Scope BNC/ GROUND PROBE will be no longer be at ground potential, it will be 60VAC or more (for US120V) (which we see people here posted the wrong voltage readings when they are working in the primary side of the circuit and use the Chassis as the ground ref instead of the negative leg of the main filter cap as the ground ref point) or higher depend on the X/Y caps used in side the scope and the stray capacitance of the primary side of the scope. Some scope may not have BNC directly tied to Gnd, but has 47~100 Ohm resistor/cap network between scope circuit ground and the safety ground. Just measure the BNC and safety Ground to see what you have.
Experiment and take some voltage measurement to verify what every one is saying here to proof it to yourself, that is how you learn, just be careful doing it, GFCI should be used for the experiment.

[Behemot]

Re: This that famous harmonic distortion?

Sure, it (scope ground) is conencted directly to safety/earth ground, I already stated that. The question is, besides sparking, as here is no current protection (usually they are used in bathrooms and so), is there any problem bringing the rectified ground to 0 earth-ground potential? Or should I just hook two probes to the O-scope and take differential reading between primary ground and voltage?

[Kiriakos GR]

Re: This that famous harmonic distortion?

What's your definition

As mentioned and above there is no funeral for the scope or the person who using properly hes equipment.
By also respecting basic laws as for example to NOT walk barefoot on the floor as many kids do in locations with hot climate.

How about lets start talking of the right shoes that a person should have when working with AC Mains ?

Money ? Yes technology cost money....... And electricians are forced to pay an ever higher price for isolated BNC inputs rather than differential probes which is not that practical for some one who moves around and makes out door inspections and measurements.

The non earthed oscilloscope is something that most people do with out admitting it in public.
They replace the mains (sealed) male plug with a replacement mains plug, and they bring out the earth cable and connecting in to a banana plug, and also modifying a common Mains female plug by adding a female banana over it.
So this connection/disconnection to be easy.

About safety I do not have much to say if you operate your scope that way, there is lunatics out in the street riding 1000cc motorcycles in a single wheel, its their choice.

But from the other hand if you do isolate just the load in electronics you are still in big shit.
EMF shields does not work, and you are measuring more noise than you should.
And high voltages shields do not work either, and there is more chances HV to arc on you ( Old glass tube televisions or paper copier machines).

In summary, you make a plan up ahead of the applications which your equipment can serve as is, and you stick to it.
And if you wish to involve with a risky applications, you are upgrading your tools OR just say NO to this type of repair and say good buy to this customer too.

No one becomes semi-God by getting just one Oscilloscope.