Oscilloscope 'floating' measurements

It's not clear what you're after (echo).

For most technicians, the term "floating" in connection with scope use, refers to 'floating the scope' which means: Read This Part > separation of the scope power cord's 3rd pin from earth ground.

I didn't look at Tek's cautions/recommendations, but I know scope manufacturers are familiar with the (nearly always) ill-advised practice of Floating A Scope.

Are you unfamiliar with proper and safe practices, or unfamiliar with using a scope, or unable to express what it is you want ask about? All of the above?

I'm fairly certain that all of Tek's recommended procedures are safe.. maybe you're just not understanding what's presented in the literature.. or maybe you're trying to apply the wrong procedure for your intended purpose.

As far as various types of electronic gear/equipment go, it's very common that there are floating voltages present inside them.

No one was placed here to try to understand what you meant, even though you didn't say it.

Maybe you were thinking of sci.telepathic.electronics.

-- Cheers, WB .............

Thanks. No question about the scope being grounded, no one suggested otherwise. Tektronix maintain that floating measurements, taken in a DUT powered by an isolating transformer, is bad practice.

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Agreed on the dangerous aspect but I have had issues with ground loops between an audio generator and the scope when testing audio amplifiers. I carry a ground lifter in the scope probe bag and use it when needed - not very often. I NEVER connect the scope ground lead to any elevated supplies when using a ground lifter. There are other better and safe ways to measure high Voltage circuits.

G=B2

Agreed on the dangerous aspect but I have had issues with ground loops between an audio generator and the scope when testing audio amplifiers. I carry a ground lifter in the scope probe bag and use it when needed - not very often.

A simple way is to use a 25 amp rated bridge rectifier and connect the resistor across the AC input terminals while shorting the " + " and " - " terminals - giving two diode drops in each direction with current capacity enough to trip a 20 amp supply breaker if necessary without damage.

The bridge and resistor can go inside the scope.

... Phil

Thanks, I think you are right.

You could be right

Try this:

-float/eng/safety.html

Phil's still right.

Yes they show an isolation transformer. But they show it isolating the scope, NOT the device being tested.

There's no mention of using an isolation transformer to float the device being tested.

Tek's app note is based on NOT being able to isolate the device being tested.

Jeff-1.0

seem

You are right, thank you.

"Jeffrey Angus"

What this achieves is ELIMINATING the need to make floating measurements.

Similarly, even with equipment that has mains isolation by virtue of a 50/60 Hz transformer - one can temporarily disconnect the secondary side from supply ground and float the circuitry. This again allows you to ground any part of the circuit you desire to facilitate scope measurements.

... Phil

The OP was very clear in what he wanted to do i.e. make scope measurements on the primary side of a non isolated switcher. The only 'problem' was that he misinterpreted what had been said in a Tek application note, which he now understands correctly for what it was. I have seen no indication that he is incompetent at using a scope or understanding safety procedures anywhere in his post or various replies. He has merely asked for what people on here who are involved in this type of work, recommend in terms of safe practice from both a personal injury and equipment damage point of view. There has been a consensus expressed, and as far as I can see, he accepts and fully understands what he has been advised.

Arfa

"Arfa Daily"

Nothing.

Both require new information to be punched into them ...

... Phil

"Phil Allison"

So, if the earth clip of your probe contacts mains active - the only result will be a tripped breaker, just as it would be without the above modification.

Also, diode bridges fail short when overloaded so in this case would fail safe.

... Phil

"Arfa Daily" wrote in news:9sfCq.209438$ snipped-for-privacy@newsfe05.ams:

Heh,I've seen the results of people connecting scope grounds to what they - thought- was an OK place to connect. line V can really do a lot of destruction. I've also replaced a LOT of scope power cords that had the ground pin cut off. I used to include a copy of TEK's publication on safe isolated scope measurements with each of those scopes;my own personal policy,not TEKs. I don't know if it ever did any good....

Tek has historical basis for its warnings, the cases of 'scopes used to = be metal and connected to probe ground. If every exposed (metal) part of = the scope becomes live there is a definite shock hazard involved.

?-)

Just the same, i always prefer floating the UUT. Sometimes this is not possible though, commonly with higher power equipment (>10 kVA).

?-)

Well they did want to sell expensive differential probes after all.

?-)

According to this part, it seemed possible that the OP (or possibly other readers) may be inclined to do something risky.. even though earlier replies recommended proper safe procedures (and the reasons for them)..

Using a scope in the conventional/safe manner (earth grounded power cord), on a Hot circuit (primary side of a SMPS) without proper isolation IS bad practice, no question. I know you knew that, Arfa.

So, that just takes the subject back to using a proper dual-trace model with suitable input capacity, and a Differential Mode or Add feature described in the scope's operation manual.. *or* using a proper isolation xfmr between the utility power source and the DUT (device under test).

My opinion is that using a proper I-T is the best overall solution.

BTW, it hasn't been mentioned yet, so I'll throw it in here.. A properly isolated DUT is safe in terms of there is no path to the utility power source for the test equipment or the operator/user to accidently come in contact with. That doesn't mean that the operator is fully safe from electrical shocks or electrocution, if one is careless.

The 'one hand rule' is safe practice.

There can be high voltage potentials and sources of current within the DUT to present hazards for the tech/equipment operator. Safe practices are still required, even though the threat of having line-sourced current pass thru the operator to earth ground is eliminated, the DUT when powered may still be capable of injuring the operator/tech.

Safety information is covered in the Repair FAQ

-- Cheers, WB .............

One of us is smokin' something. You're saying that you can hook the ground clip of your probe to the mains active, blow the mains breaker and still have no damage to the probe or scope????? Set up a webcam so you can record the experiment. I'd like to see it.

Whether the earth clip lead survives is moot since it is not related to the mod.

... Phil