EEVblog #279 - How NOT To Blow Up Your Oscilloscope!

[nitro2k01]

09:23 It may very well be that your Dell supply disconnects itself from mains earth when the secondary doesn't have power, or even when the supply is not connected to the computer, to prevent the sleeve of the supply from touching anything and making sparks fly. You should do some more testing on this. Science!

19:25 If that actually cause the computer to reboot, that is indeed very nasty. USB ports are usually fused and/or current limited. I believe they even have to be according to the USB specification.

A bit after 20:0 I think I did actually hear slight static in the audio. Did you by any chance have the camera hooked up to mains power in the same plug pack as you powered the gadget or scope from?

[Bored@Work]

The comments on youtube just prove there are a lot of idiots out there playing with electricity

What? if you disconnect the earth ground on the oscilloscope?

what? if you dont have the earth-ground on your outlet or NOT USING IT on the electrical plug

I once worked in a lab where all the? scopes were plugged into isolation transformers for this very reason.

and why my lab gear is? on a isolation transformer

You can also disconnect your scope mains earth pin so it's floating,

[T4P]

19:25 If that actually cause the computer to reboot, that is indeed very nasty. USB ports are usually fused and/or current limited. I believe they even have to be according to the USB specification.

I did hear before that over current protection occurs by rebooting your PC ...

[McMonster]

What this video lacks for completness is clearly pointing out not to hook up your ground clips on both sides of the transformer. I know it is clear from the video that they're connected internally, but it wouldn't hurt to say this anyway. A blown Rigol on the other hand would hurt at least your wallet.

[Monkeh]

09:23 It may very well be that your Dell supply disconnects itself from mains earth when the secondary doesn't have power, or even when the supply is not connected to the computer, to prevent the sleeve of the supply from touching anything and making sparks fly. You should do some more testing on this. Science!

It won't. Why would the earthed side of the connector touch anything not earthed? There shouldn't be anything but earth potential it can reach.

19:25 If that actually cause the computer to reboot, that is indeed very nasty. USB ports are usually fused and/or current limited. I believe they even have to be according to the USB specification.

He meant the Arduino. You can hear the 'new device' ding in the background.

[free_electron]

The comments on youtube just prove there are a lot of idiots out there playing with electricity

Darwin Awards !

NEVER EVER disconnect the ground of your testequipment. it is the itum under test that needs to be put behind the isolation transformer !

[Bored@Work]

The comments on youtube just prove there are a lot of idiots out there playing with electricity

Darwin Awards !

NEVER EVER disconnect the ground of your testequipment. it is the itum under test that needs to be put behind the isolation transformer !

I found the five quotes when there were approximately 50 comments on YT. That makes a 10% rate of suicide candidates. And despite the fact that Dave mentioned not to do it in the video.

[EEVblog]

19:25 If that actually cause the computer to reboot, that is indeed very nasty. USB ports are usually fused and/or current limited. I believe they even have to be according to the USB specification.

I used the wrong word, I meant the USB device detection "ta-da" sound.

A bit after 20:0 I think I did actually hear slight static in the audio. Did you by any chance have the camera hooked up to mains power in the same plug pack as you powered the gadget or scope from?

No, battery. The (very long, 5m) lapel mic lead was probably picking something up.

Dave.

[EEVblog]

What this video lacks for completness is clearly pointing out not to hook up your ground clips on both sides of the transformer.

There is a ton of stuff this video lacks for completeness!
There are countless scenarios that aren't covered I'm afraid.
Just look at the Youtube comments BAW posted.

Dave.

[Bored@Work]

There is a ton of stuff this video lacks for completeness!
There are countless scenarios that aren't covered I'm afraid.
Just look at the Youtube comments BAW posted.

But that part was not lacking. You mentioned around the 20 min or 23 min mark to not float an oscilloscope. But 10% of the YT commenters didn't get it. They either comment on something they didn't watch or are really not up to it. Maybe both. And does one really need to be told not to disable a safety feature?

[McMonster]

What this video lacks for completness is clearly pointing out not to hook up your ground clips on both sides of the transformer.

There is a ton of stuff this video lacks for completeness!
There are countless scenarios that aren't covered I'm afraid.
Just look at the Youtube comments BAW posted.

Dave.

Maybe you coukd make regular "how not to blow yourself up" section on the blog? One detailed description once per 1-3 months perhaps?

[JuiceKing]

I found out about this the hard way the first time I hooked up an oscilloscope the wrong way to the output of an audio power amp! Fortunately, the amp was well-behaved and nothing was damaged.

This blog also answered a great puzzle of why just the other day my signal generator, which I was SURE had floating output, suddenly seemed to be tied to earth. It was, I understand now, attached to an Arduino, which was in turn attached to my computer through the USB port. Thank you so much for including that tidbit at the end of the blog!

[NiHaoMike]

If you can find one, a wireless USB kit is a cheap way to get isolation for USB. I got one for $20 from Fry's. Another option is to use a battery operated computer.

For short term testing, a power inverter and lead acid battery or isolated power supply makes a cheap alternative to an isolation transformer. Or wire two power transformers into an isolation transformer.

As for the scope, if the DUT cannot be unearthed, apart from using an expensive isolated probe, a battery operated scope can work. Since that's still an expensive option, maybe we should design an open hardware isolated probe? Maybe modulate the input signal to RF, isolate that with low value, high voltage capacitors and/or a RF transformer, then demodulate it. For simplicity, use separate batteries to power each side and mechanically link the power switches. (Back when switching power supplies were expensive, TVs ran directly from rectified mains and isolated the antenna with capacitors.)

[Architect_1077]

Well, this episode has raised a question for me regarding Earth grounding. I've always made it out to be merely a safety feature, however, after watching Dave's video I realized that electronics devices can be grounded directly to earth ground. I've actually gone around the house inspecting a couple of devices and, sure enough, it seems that earth ground is sometimes used for more than just a safety feature. So I'm sitting here wondering what is the purpose of that?

Going back to the topic on scope safety, would it be safe to say that the best bet would be to always connect the ground probe to a known grounded point, just to be on the safe side?

[ejeffrey]

It *is* a safety feature.  All large exposed metal objects in your house (water pipes and fixtures, ducts, and structural members, and metal appliances) are connected to the electrical ground.  The purpose is to prevent large potentials from forming between two conductors, even in the case of a wiring fault.  The idea is that if a live wire accidentally touches a grounded surface, a large fault current will flow through the ground connection causing the circuit breaker to trip.  The same thing applies to any electronics that has exposed metal bits like a metal enclosure, or any connectors that have exposed metal shields (RCA, D-sub, USB, HDMI -- pretty much anything).  The circuit ground is connected to the electrical ground so that in the case of a wiring fault such as a primary to secondary short circuit in the power transformer the exposed metal can't become high voltage relative to other objects in your house.

The other alternative is double isolation.  This requires that the insulation be designed so that a single fault cannot generate an unsafe short circuit.  You do this by some combination of plastic enclosures, barriers between the primary and secondary transformer winding area, self-healing capacitors, and in extreme cases epoxy potting.  Almost all 'wall wart' style power supplies are double insulated, and you will find that they and the electronics they power have no ground connection.

[Architect_1077]

All large exposed metal objects in your house (water pipes and fixtures, ducts, and structural members, and metal appliances) are connected to the electrical ground.  The purpose is to prevent large potentials from forming between two conductors, even in the case of a wiring fault.  The idea is that if a live wire accidentally touches a grounded surface, a large fault current will flow through the ground connection causing the circuit breaker to trip.

That's what I already knew. What I didn't realize, however, was that active circuits could be directly connected to electrical ground as well, when I always thought of electrical grounding to be something completely independent from active circuits, only meant to protect from exposed metal, if you get what I mean.

[SeanB]

I will add to that that TV antenna leads should be bonded to ground, mostly for lightning ( not a direct hit, that will ensure everything is blown in the house, but for dissipating static charge that can build up on the wiring, or for induced current from nearby strikes) purposes, but it improves safety. This means that often you will get a mild shock if plugging a TV set into the antenna, and might see a small spark, as this is due to the capacitors that isolate the live chassis from the antenna connector.

[Trinity]

In the second case where the circuit under test is connected to isolated mains and floating, the video said that it was perfectly safe to work with.

However, and please correct me if I'm wrong - but I was under the understanding that **ONCE** you connected the ground probe to any part of the circuit, you tie that part to ground - and now, if you were to touch any other part of the circuit that has a different potential difference, you could now get shocked through your own body's connection to ground.  Whereas before connecting the ground probe, the system was still fully floating, and you could touch any part of the circuit safely.

If my understanding is correct, than that was something that definitely should have been pointed out, and in that case, it was very misleading to say that the the user would be completely safe.

[MartinX]

Trinity you are correct, as soon as you connect the ground lead from your oscilloscope that is grounded to the circuit fed from an isolation transformer it is grounded and depending on the point you connected the ground lead to, your supposed safety could be gone.

I work in a development lab and we have all of our oscilloscopes on isolation transformers. If the test object is large, complex consisting of several parts connected to several computers it is not a realistic proposition to keep it isolated, someone will always mess up by connecting another instrument a programming tool or something that connects it to ground anyway, it is the test object that is messy and difficult to keep track of. After a couple of exploded ground leads and one destroyed Tektronix TDS 3014 we put all of our oscilloscopes on isolation transformers and we had zero incidents since. Modern oscilloscopes are encased in plastic and if you connect it so that its chassis is "live" it is not easy to get in contact with it in order to electrocute yourself, the ground leads are the biggest risk since they are handled the most and people don´t see that alligator clip as something risky, it is usually ground and therefore safe to touch. Older scopes with metal casing is another story.

Of course we are tying to keep everyone informed about our grounding strategy or what you want to call it, and educate them about the risks.

An oscilloscope is an universal test instrument used in so many different situations and claiming that is should always be grounded because it is the safest practice is over simplifying things.

[alm]

I work in a development lab and we have all of our oscilloscopes on isolation transformers. If the test object is large, complex consisting of several parts connected to several computers it is not a realistic proposition to keep it isolated, someone will always mess up by connecting another instrument a programming tool or something that connects it to ground anyway, it is the test object that is messy and difficult to keep track of.

Why don't you use the proper tool for the job, which would be scopes with isolated channels (Fluke ScopeMeter, Tektronix TPS2000) or differential probes?

After a couple of exploded ground leads and one destroyed Tektronix TDS 3014 we put all of our oscilloscopes on isolation transformers and we had zero incidents since. Modern oscilloscopes are encased in plastic and if you connect it so that its chassis is "live" it is not easy to get in contact with it in order to electrocute yourself,

Does the manufacturer specify an insulation rating of the case? Or do you do your own certification? Did you do anything to isolate the BNC connectors and connections on the back (eg. USB, GPIB)?

the ground leads are the biggest risk since they are handled the most and people don´t see that alligator clip as something risky, it is usually ground and therefore safe to touch.

So not only are you ignoring safety instructions regarding the scopes, you don't even use probes designed for floating measurements (eg. Multi-Contact IsoProbe series)?

Of course we are tying to keep everyone informed about our grounding strategy or what you want to call it, and educate them about the risks.

I would hope there are at least big orange warning labels on all floating scopes warning not to touch them while the DUT is powered on. What kind of voltages and source impedances are you talking about? Do you at least take measures to limit the voltage between scope ground and PE to a safe level, like the Tektronix A6901 ground isolation monitor which grounds the scope if the voltage exceeds a certain safe level?

[MartinX]

Why don't you use the proper tool for the job, which would be scopes with isolated channels (Fluke ScopeMeter, Tektronix TPS2000) or differential probes?

We do have a Tektronix TPS2024 but this was a big disappointment, we thought our measurement problems were solved but unfortunately there is capacitive coupling between the channels that made measurements of signals with  fast rise times unusable, the 200MHz bandwidth can only be used if there is not a big differential signal between the channels. The probes that came with it was big clumsy and they fell apart. The memory depth, screen resolution and features like trigger options takes you back to 1995. Scopemeters and such seems to have the same problems and the 200MHz bandwidth is on the lower limit of what we need.

Differential probes work but then again the bandwidth is usually not sufficient, if you want a high speed high voltage probe it will cost a fortune if you need 6 of them to make a measurement and still there are problems, the measurement leads that are not shielded can pick up disturbance from near by circuits.

We have asked people from Tektronix Agilent and LeCroy for solutions and we don't feel our requirements are very extreme but there seems no one can offer what we are looking for.

Does the manufacturer specify an insulation rating of the case? Or do you do your own certification? Did you do anything to isolate the BNC connectors and connections on the back (eg. USB, GPIB)?

No, they do not specify anything about the case. We have insulated some probes near the BNC connector were there are metal parts exposed.

So not only are you ignoring safety instructions regarding the scopes, you don't even use probes designed for floating measurements (eg. Multi-Contact IsoProbe series)?

The probes with extra insulation on the outside does not solve the basic problem that ground is connected to the chassis of the scope, the bandwidth is not good, they are big and clumsy.

I would hope there are at least big orange warning labels on all floating scopes warning not to touch them while the DUT is powered on. What kind of voltages and source impedances are you talking about? Do you at least take measures to limit the voltage between scope ground and PE to a safe level, like the Tektronix A6901 ground isolation monitor which grounds the scope if the voltage exceeds a certain safe level?

Warning labels only work the first week then they are just a part of the background noise. We do not constantly work with the scope chassis connected to lethal voltages of course, but measurements like that have to be made now and then, limiting the voltage to ground is not an option, then we could not make the measurements we need. We make motor control products that often run on directly rectified mains voltage (3-phase 400V) so we have about 600V DC on a lot of our boards and that is always dangerous. During development and verification of products like that there is no way to avoid exposed lethal voltages, you need access to do measurements.

A grounded scope is not inherently safe, if you connect he ground lead to the wrong place the ground lead may blow up, that is not safe and a very unpleasant experience, you may also ground a floating circuit making it less safe to work on. A floating scope may become electrified making it a risk if you connect yourself between it and ground, but what is the most likely scenario? What kind of products are you working on? Choose your poison. Isolation transformers have worked very well in our case, they solved the measurement problems and have been safer in practice than grounded scopes, the nature of our products make people working on them cautious, everybody understands 6000µF charged to 600V is dangerous and safety thinking is not forgotten.

There is no substitute for knowing what you are doing, exactly what you are doing.

[cs.dk]

A little off-topic maybe.

Is it best to ground the scope, or having it floating (use a wall-plug without ground) when measuring or cars/engine-management systems?

[vk6zgo]

A little off-topic maybe.

Is it best to ground the scope, or having it floating (use a wall-plug without ground) when measuring or cars/engine-management systems?

It doesn't really matter normally,as far as the car is concerned, (BUT see the final sentence!!),because it is standing on four big black insulators,so the car"earth" is not at the same potential as the mains earth! 
If you have other stuff connected to the car which causes the car body/chassis/engine block to be at mains earth potential,you have to be careful not to get your earth clip on the wrong spot.
If the latter is not the case,all the voltages in the car,including the few high ones are only with respect to the car body/chassis.etc.

That said,I must repeat what I said in another thread,"There is no real reason to put the earth lead on anywhere else but a point connected to the body of the car."
If you put the earth lead on say,a +12volt point,& your 'scope was sitting on a car seat,& thus insulated from car "earth",all would be well,but if you did the same with the 'scope's metalwork contacting the bodywork,there would be big "zap",& you could damage your probe,your 'scope,or your car.
If you again insulated the 'scope,& placed the earth clip on one of the high voltage points in the car,you would elevate the instrument's
metalwork to that voltage,& perhaps damage yourself!
Your Oscilloscope mains earth would not protect you in this case,because,as I said in the first sentence,the car"earth" is not at the same potential as the mains earth!

To answer your question:DO NOT remove the mains earth from the Oscilloscope,as if an electrical fault occurred inside the 'scope,causing  external metalwork to be live,& if the 'scope is sitting on a metal part of the car it would elevate the whole car to mains voltage,so everyone working on the car would be at risk.

[M. András]

cable tv connectors bite me for a few times dunno how much voltage does it have but it bites and i manage all the time to touch the shield and the core together, same happened when tried to plug in the hdmi cable from the running pc to the tv which moundted to the wall and has 5cm place so searched with hand, nice shock, but i wonder why does it sparks when i connect it to the pc every time

[SeanB]

That is because the cable is grounded at the cable box and at the customer entry, or at the PC. The TV set is ungrounded, and, because of the interference suppression capacitors in the power supply between primary and secondary, the chassis floats at around half the mains voltage if nothing is connected. You plug in the cable and provide a path to ground for this high impedance source, thus the shock and the spark.