Floating the 'scope

"George Hairoil"

It helps reduce the number of technicians getting electrocuted.

But YOU go right ahead and float yours - by all means.

.... Phil

That's a safety thing.

Our TPS2024 has four truly floating channels and floating trigger input. You can hook a scope ground clip anywhere.

John

Hi Phil, now don't yell at me too much, it's Friday night here and I'm half in the bag.

I've been floating 'scopes for.... say since the early '80's. I haven't been biten by it yet. (Though I've made plenty of other mistakes.) What are the obvious safety reasons?

In today's mistake I was looking at the voltage from a heater circuit. The heater runs off +15 and -15 but knows 'nothing' about ground. (Well nothing except for the bypass caps, but no DC connection.) Things weren't quite working right and I worried about some oscillation. I hooked up the 'scope to look.... opps.

George H.

(who believes that you make progress only as fast as you can make mistakes...)

"George Hairoil"

I've been floating 'scopes for.... say since the early '80's. I haven't been biten by it yet. (Though I've made plenty of other mistakes.) What are the obvious safety reasons?

.... Phil

Welcome to an entirely new class of stupid, Phil.

At a place I used to work at, the cro on my bench was floated by the previous "tech", he just forgot to tell anyone about it. And forgot to label it too. No clues at all unless you measured it, or took it apart to actually look.

Made worse by the fact it was a dual trace cro, and could have done the job without floating it.

I can only presume these idiots have been doing for so long, they stop caring about the fact you hadn't had to do it since two trace (or more) CROs appeared on the market. Hey, I don't care if they want to kill themselves, but their idiocy will continue to kill others well into the future.

I'm starting to think that being dropped as a baby isn't enough to explain this level of stupid.

Handheld and portable scopes often have floating inputs, and it is convenient when measuring circuits that have voltages which are referenced at other than earth ground. But line powered scopes are usually in metal enclosures that need to be at earth ground potential for shielding purposes, and thus the probes are connected to the chassis and the shields are an extension of this ground. You can float the scope by disconnecting the ground at the AC plug, and placing the scope on an insulated surface, but the capacitance of the scope enclosure will likely affect the measurement by inducing noise currents into the circuit where the ground clip is connected.

Hand held scopes have less material connected to the ground, but the ground clip can still inject noise and it will have a much lower impedance than the tip, which ideally will have a 9 Mohm resistor close to the tip so that minimal capacitance is applied. The best way is to use differential mode between two identical input channels, ie, A-B, but there is a limit to how much common mode voltage can be tolerated before the signal will be distorted.

Paul

Even if a portable scope is double insulated, that's not always safe.

If you clip the "ground" of the scope probe to a live line, that shield and any metal bits all the way to the scope are also live.

If the ground clip is exposed, it'll bite (I've frequently seen it, and on some probes, that's the way it's built) , if the insulation is damaged, it'll bite.

Also note that none of the cable insulations I've seen are rated for high voltage, only the centre conductor is good for the specified voltage, not the outer to the outside world.

Your only avenue is to completely isolate yourself from ground, which is going to prove to be a bit difficult, considering the capacitance you can't really avoid.. Forget about wearing an antistatic strap. I've worked in some environment where it's mandatory, so that's going to be a bit of a challenge.

Or use an earth leakage breaker, and have the thing cut out on you a dozen times a day even under normal use.

There are too many things that can go wrong. There are no procedures to ensure all the avenues are covered and enforced to prevent shocks from this method.

Tektronix has probes designed to solve these problems:

They are specifically intended for floating use with the TPS2000 series scopes, which, as John mentioned, have true isolated inputs.

Or, you can use these probes with an ordinary scope:

Ah, I stand corrected.

Though it still needs to be said, if someone is of the mentality that they can freely dismantle a piece of equipment that says "do not dismantle" to intentionally remove a safety mechanism (against the law the last time I checked), what is the likelyhood they're going to use the right probes for the job?

After all, if they obviously don't care about the now live metal bits of the casing, what makes you think they're going to worry about a probe in the first place?

What if they have to make an adjustment? Make sure they just touch the plastic bits and avoid the metal bits? Make sure they unclip the probe first?

More importantly, if they live through all those don't cares, what makes you think they're going to care enough to warn other users?

Yeah right, I see that happening.

Those are nice expensive but nice.

You should use an isolation transformer on any line powered device your testing. You can usually find a 500VA one for under 100 bucks. I got a TEMA 500VA one for 45 bucks from Newark. Well worth the money.

Its not really a good idea to float your scope.

Anyone use any of PICO's differntial probes?

This ones only $350.00 Canadian.

Unfortunately, it was the cost of the transformer that makes people do things like float their scopes.

"It does the same job, and it's a lot cheaper". Till someone dies. How much does liability cost?

Well the one I paid $45 on sale last year is still only $70. Cheaper then a new scope or a funeral.;-)

If you look around there's usually always a decent one for under $100.

"Hammy"

The horrendous fallacy in the thinking is way beyond the likes of retards like Hammy.

..... Phil

So does my cheap crappie OWEN 20 mhz hand held. That also includes isolation between the DMM inputs.

scopes,

I'd like to see a citation for the above. Sure, such modifications would leave one exposed to litigation and it may not be legal to SELL a modified widget in some jurisdictions, but I'd like to see a cite that says it's illegal for one to modify one's own property.

A competent engineer certainly would. These things *are* designed for engineers to use.

Who, exactly, is "they"?

When working with dangerous voltages, that's my plan.

Huh?

Other users should know something. Obviously not a good assumption in some cases.

Can I watch you?

So your legal advice is that it's ok to disable a safety device? More so, and to not replace it with something else after you've disabled it?

Use yes, butcher no.

Hacks, keep up with it, the questions are only going to get harder.

Good luck, it's been nice knowing you.

Yeah, thought as much.

So you're saying that it's MY fault that YOU'VE disabled a safety device?

Guessing I'll be living longer, that will be *me* watching *you*.

You have a real reading comprehension problem, don't you?

See above.

Your uninformed opinion.

Obviously they are, since you can't answer. Although, since you can't read it does make it hard to answer.

Luck has nothing to do with it. I've been around long enough so I'm not afraid of my shadow, unlike some here.

I can't say the same.

If you're not touching anything "hot", who cares if you unclip the probe?

Apparently you're too stupid to live - not my problem.

You must be DimBulb's brother.

Not such a new class.

I used to have a technician who had worked at a large equipment hire outfit. He told me that checking for disconnected ground was SOP when checking returned instruments.

They found several each week.