Hi I Just read a post on the Eev blog Forum where somebody was asking about the oscilloscope probes and how they can be potentially dangerous. If you hook this ground lead up to the wrong point in your circuit, you can blow up your circuit, you can blow up your scope bang. And it really is a big trap for young players. and I've mentioned it before, but they wanted to know exactly under what circumstances that could happen if I could do some diagrams to explain it and things like that.

Well, glad you asked. Let's try and sort it out. So what's actually causing the problem here? Well, it's to do with the fact that the BNC connector the outside ring of the that metal outside ring of the BNC connector on almost all uh, benchtop oscilloscopes like this are what's called Main's Earth grounded their Main's Earth reference. In other words, this metal shell on the outside of the BNC is connected directly via a very low impedance path dead short through to the Main's Earth on the back of the oscope on the IC connector, and hence through to the Main's Earth in your system.

So if I've got a power cord, here's an IC Power cord is plugged happens to be plugged into the same power board as this oscilloscope. It doesn't have to be. It can be on the opposite side of the room, the opposite side of the house or factory, or whatever. The Main's Earth will be common and tie together throughout the whole system.

So I've got my multimeter here set to Ohms. Let's make sure we probe the right thing. I'm going to probe Earth the center Earth pin there and I'm going to probe the B andc and there it is 0.9 Ohms, It's effectively a dead short. There's some there's some DC resistance in there, but it's a very low resistance path.

effectively. Well, it is a dead short through to Main's Earth and that can be problematic. as I'll explain. And because that outside shell of the BNC connector is connected directly through to your Earth pin here, you're going to get the exact same response on your Earth pin.

There it is. It's a dead short look at that. And because it's a very low impedance path, this path is capable of actually carrying a lot of current if you hook it up to the wrong point and it can do a lot of damage. This little uh ground attachment lead is probably the weak Link in the whole system because the Uh Shield of the uh of the coax going back and the internal connections all going back to Mains Earth That's very, you know, very chunky bits of copper in there, but this is probably the weak system so this is going to uh, blow up or vaporize.

If you attach this to the wrong point and you have enough energy in your system, you can blow this probe apart in your hand. Bang! And just as an aside, on any multi- channel scope like this, be it two or four Channel Like this one, these both of the BNC connectors are shorted together. So there you go. They're shorted together and through to Main's Earth.

So if you've got your two probes like this, they're both connected to your oscilloscope. These two grounds are shorted together. You can't just put these willy-nilly anywhere in your circuit because if you put them to anywhere that is not the same voltage potential I.E both ground or both plus 5 Volt or whatever. then if there different potentials, you short out that thing.

Here's a power supply. I'll show you if I connect one ground lead to the positive output, another one to the negative bang I short the output that can ruin your day. Don't do it. And not only that, if that particular, uh, point in your circuit that you're shorting out has a large amount of energy in it and you get a huge current through this by shoren it out bang, you can blow up your probes and Sparks can fly everywhere.

Real nasty stuff. So let's try and take a look at this and figure it out. What's happening on a diagram here? Please excuse the crudity of the model. Didn't have time to build it to scale or to paint it.

So what I'm going to show here is three different scenarios. This is scenario number one and I'm going to assume a single probe, not the Dual probe thing I showed you before. That's a different thing We're talking about this: Main's Earth referenced problem. So what we've got here is, we've got our scope.

Okay, and it's got the BNC connector here. and it's got the IC Mains input connector going down to earth down here. and note the different symbols. The Shazzy Main's Earth symbol here is different to internal ground symbols.

That's just an a little aside. It's not really important for the purposes of this, but as you saw with the measurement before that, BNC is shorted directly through to Main's Earth and it's also shorted directly through to the alligator clip on your probe here, or your crocodile clip whatever you want to to call it. Now here's your widget, your circuit that you're probing, and it for Simplicity sake. Let's say it has a positive Rail and an internal ground uh, rail as well, and various points in your circuit that you're probing.

Okay, now let's assume that it's battery powered or what's called floating. It's completely floating. It is not referenced in any way back to Main's Earth. It's just isolated in its own little world.

It's not connected to anything else now. Uh, that means in this scenario, here, you can connect this alligator clip through to any part of your circuit. It can be connected to ground, could be connected to, uh, your positive rail, or any other part of your circuit, and you won't do any damage at all. Why? Because there's no current flow, there's no way there's no Loop here.

It's not going to flow anywhere. Current just can't magically flow down that alligator clip through the circuit unless there's a return path. If you've got a floating system like this, there's no way the current can flow through there. So you can hook that alligator clip in a floating circuit to any point in that circuit and you won't do any damage.

What you will do is actually put: if you connect it through to this point here in your circuit, then you've Main's Earth referenced that point in your circuit. What does that mean? Well, it just means that you've connected it through the mains. Earth It doesn't. It may not do anything in most cases.

It's not going to cause a problem at all unless there's some. You know you're working on audio circuits and there's harm and all sorts of things, but we won't go into it. Pretty much Scenario: Number one: Isolated, um, uh, widget. you're working on completely safe to hook up the alligator clip or the probe to anywhere else in the circuit.

And because the probe is connected to the tip of the probe is connected to your 1meg input impedance on your scope. It's not. talk about x 10 probes and things like that is a 1meg resistor you. The worst you're going to do is put a 1meg resistor across any point in your circuit.

Usually not a big deal. You certainly can't blow anything up. Not a problem, All right. Let's look at scenario number two, where we've got one of these isolated main supplies or a plug pack.

We've got our 110 or 240 volt Mains coming in here and there's no Earth pin. We've only got our active and our neutral there and we've got a you know isolation Transformer 50, 60 HZ And there's physical isolation electrical isolation between the primary and the secondary winding, so there's no direct electrical path. It's just a little bit of capacitance really. But you know there's no direct electrical connection.

so it's an isolated Supply So and I've just pitched here. It's a, you know, a halfway Bridge rectifier. But it can be a switch in, uh, power supply, uh, a switch in plug pack or a regular Transformer based plug pack. Makes no difference.

If it's got that electrical isolated Supply and no, uh, Earth no Earth connection at all, then it's exactly the same as scenario number one with our battery supply. No different. You can connect your alligator clip up to any point in your circuit, any rail, anything, and you're not going to blow anything up. No current's going to flow because current can't flow DC Current can't flow through this Transformer in any sort of loop at all.

Completely safe. Now, these isolated plug packs pretty easy to spot because they've only got the two pins on them, active and neutral. There's no third Earth pin. Some of them, might have a third Earth pin just for mounting, or they uh, might have it to connect through to the Uh internal Transformer as a safety thing, but they will still be isolated, and if you're unsure if it does have that third pin, measure it.

take the output here. use your ometer measure meure it between the ground pin or both pins just to be sure. and the Earth pin. if you don't measure that direct short, then it's an isolated supplyer and that can be a switching type or the old-fashioned uh, direct uh, Transformer type and the other type might be one of these switching plug packs.

Another dead giveaway. It's only got the two pins, the active and neutral. It doesn't have the third pin going in there. Now here's an interesting one.

This is my Dell Uh notebook power supply and as you can see, it's got three pins on the Main's input cord and it certainly has the Earth pin on the connector. So let's measure it. Let's see what we get. Look at that it's 1K instead of a direct short.

This one, for whatever particular reason, is uh, only 1K and that that's pretty darn low and that can ruin your day. That I would classify that as quite low impedance. not as low as a dead short, of course, but still, you would want to, uh, go uh, probing in such a system with your oscilloscope. If you had your notebook, you were trying to repair your notebook and you powering it from this, then well, you know you've got that.

1K through to Main's Earth I'd be disconnecting this when I worked on it if I had to probe it live and power it directly from its battery. so it was completely isolated. Now let's look at scenario number three: What's called a Main's Earth referenced power supply or a Main's Earth referenc product or Main's Earth reference system. It's the same as before.

It might have an isolation Transformer in there, but it has that third Earth connection which shorts out the secondary side. It's almost always the negative uh line, but you know it doesn't have to be, but it almost always is. The negative line is shorted out and could be shorted to the primary so you lose that isolation there. So this is now connected through to this.

Main's Earth back here, back at your PowerPoint in your house, wire in your office, wire in your lab wiring. Whatever. it's all connected right back and then that is connected through to your negative terminal and your negative terminal is connected up here. So effectively, your circuit ground is now almost always.

Once again, it doesn't have to be the negative point in your product, but it almost always is. So that is not only uh, just uh, a common ground inside your product, it's also referenced to Main's Earth Now, why is this an issue? Well look what happens now if we connect this alligator clip up to this positive rail up here. You know how it wasn't a problem before. no current flowed, no danger whatsoever.

Well, what happens now? This positive rail current can flow through here through the shield of your BNC through through the oscilloscope, down through Main's Earth down here, back through your powerboard, back into your uh, the lead, The IC lead coming into your product, all the way through here and up through your ground. Bingo Congratulations, you've just shorted that point there to that point there with a very low impedance connection through that, uh, alligator, clip that crocodile clip in your scope probe and what happens? Bang It blows up. Or you know if if this rail up here has a lot of energy can supply a lot of energy. You can vaporize your Earth lead.

it can explode in your hand. Real nasty stuff. if it's a real low energy thing. Well, your product might just shut down and it's not inherently like.

not massively, uh, dangerous. If your power supply is not capable of actually delivering a lot of current, then, well, your product. you short out your power Rail and it doesn't blow up. but your circle is not going to work anymore.

Okay, so you want to avoid that. So that's why when you're probing around with a Main's Earth reference oscilloscope like almost all bench. uh, oscilloscopes are if you're dealing with a Main's Earth referenced product, be very careful where you put that alligator clip that crocodile clip in your circuit because if it's not at the same point which is equip potential is the uh, technical term or the same potential equal potential in there as your ground line, you're going to short out that particular either a voltage rail or a signal wire or something like that. It's going to blow up and ruin your day.

So there you go. That's what happens with the Main's Earth reference system. Beware. Now let's have a look at some bench power supplies here.

You'll note that any good bench power supply will have three output terminals: positive, negative, and uh, what's usually a green one which is a dead giveaway. uh, which they call Ground Okay, it might be called Ground that's actually Main's Earth back on the Main's input lead on this thing. So normally if you don't join these together, if you know if you haven't joined them together at all, these power supplies are floating. They're completely safe.

You hook up your positive negative like this to your positive and negative Terminals and you can power your circuit. You can probe around. You're completely safe. But if you strap these two terminals together like on this power supply, over here, you see a lot of them will come with one of these straps that allow you to strap the Main's Earth through to the negative terminal and that instantly turns that isolated power supply into a Main's Earth reference power supply and the product you're powering.

You need to be very, very careful where you probe with an earthed oscilloscope. Now, why would you want to join these together? I Won't go into the reasons cuz it has to do. it depends on the system you're uh, you're actually designing and things like that so it won't go into it. but if they are joined together and you Main's Earth reference it beware.

But I don't think I can leave it there without giving you a bonus. One number four USB products and it can be other connections as well. Not just USB, but USB is a common example these days. Let's say you have your widget and it's battery powered.

Okay, it's normally completely isolated and completely safe to probe any way you want with your oscilloscope. But once you connect the USB lead through to your computer, you're in deep trouble. Why? Because the US the ground pin on the USB connector is almost always connected unless you got an isolated USB which are very rare and very specialized. Uh, it'll be connected through to the ground point of your circuit in there and then that lead will go back to your the USB port on your computer down here, which will be connected through to the ground point on your circuit board and your processor and everything else but your uh, the ground point in your PC is also Main's Earth reference Almost all PCS are and it goes through to Main's Earth directly through and you can measure that.

So Bingo your isolated product has just become Main's Earth reference by the mere fact of connecting up that USB cable and you've got that dangerous Loop scenario happening again and you've got to be careful where you Pro where you connect the alligator clip or the ground lead on your oscilloscope. If you connect it up here, bang, you've just shorted the whole thing out again. Beware. Now I know you won't believe me unless I practically demonstrate it.

So I will. We've got an Arduino compatible board here. In this case, it's the Freetronics 11 board and if you're pairing your little Arduino board from your plug, your isolated plug pack power supply. remember no Mains Earth pin and you're powering it, then you're completely safe because it's isolated.

or you're powering it from a battery. Completely safe. it's isolated. But if you take your USB cable here and it's hooked up to my desktop computer here and we plug that in and we power our board.

There it is. It's turned on. We're powering our board from our USB what happens? We've got our Main's cable here. Let's measure Main's Earth and see what we get.

Here's the ground pin on our Circuit look at that 9 Ohms, There you go cuz it's got to go through all Dicky stuff in the computer and things like that. it's shorted to Main's Earth Bingo! So your sweet little innocent Arduino board that you thought was isolated has just become a real potential problem. And if you try and probe around here with your oscilloscope or if you try to probe your circuit or your Shield connected onto there with the Uh with the alligator clip on your Crow probe and you put it anywhere. Other than that, Ground Terminal You're screwed.

You're going to short out the power supply, the Arduino shut down. It won't blow up in your face because the little power supply on the Arduino can't uh generate. you know, huge amounts of uh current and and provide a large amount of energy. but it's going to shut down short out your power.

Supply You might actually damage your power supply on your Arduino or you might damage your plug pack or something else so that can be a real issue. Even if it's battery powered, you're powering this thing from the battery or from the U. Once you connect that USB lead up, you've instantly Main's Earth connected it and you've ruined your day. And don't try this at home.

I'll actually demonstrate it. We've got our Uino it's working. there's the LED is flashing away and it's hooked up to our Main's Earth reference computer via the USB And if I hook up the ground lead to the ground uh, the Earth lead on my oscilloscope up To the ground point on the circuit. No problem, still continues to work.

I can probe away. everything's safe. but if I connect this through to the power rail the ground Lead watch this ready. Don't try this at home bang and you might be able to hear the computer reboot because the device is shut down.

I Just I've just shorted out my power supply nasty. And if you do that to a high energy circuit, you can blow the thing up, blow up your scope probe. nasty. But if I do the exact same thing again, powering it from the isolated plug pack, it's not Main's Earth referenced anymore.

So I can plug my ground lead onto the ground and it's fine of course. and I can plug my ground lead anywhere where else on the circuit. that's the same point that was causing it to short out before. And it doesn't do anything because this board is floating.

It's not. it's isolated. it's not Main's Earth reference. I can put that ground lead on any point in my circuit and it's not going to affect it.

So what do you do if you've got one of these Main's Earth reference products and you can't avoid it being Main's Earth reference And you want to probit safely or you're working on, you know, a high voltage high energy switch Mo main switch mode power Supply and you want to safely probe everything. Well, the way to do it is with a a high voltage differential probe. and I'll show you some examples of this. Here's a BK Precision Uh Pr60 Uh probe.

We're only talking you know 300 and something. uh dollars for these Uh typ of probes. and as you can see, it converts the Um single-ended BNC input for the oscilloscope through to an isolated Uh differential Uh probe system Like this. so you can put positive and negative anywhere in any product at all Mains Earth referenced or not and they and it be completely safe.

And usually these Um probes are actually high voltage Uh probes. So they actually have a Time 10. So this one's switchable between time 10 and time 100 uh attenuation. So it actually replaces your traditional time 10 single-ended oscilloscope probe.

But now it's differential. completely isolated and they can be isolated to uh, you know, several. You know, many hundreds of volts or several thousand volts in this case, common mode plusus 700 Vols and the differential can be plusus 700 volts as well. And uh, there's other ones.

Here's an agilant. Uh, one. Same thing you know. single-ended oscilloscope input through to a differential Uh probe output.

There it is that's a high 100 mahz high voltage differential probe and there's lots of of other ones on the market. There's lroy, uh one, You know, there's a whole bunch Yuuga make them. Every man's dog makes these high voltage differential probes and I highly recommend you. Uh, pick one up.

It should be pretty much a standard kit, especially if you're working on switch mode power supplies or something like that. They're a little bit on the expensive side, but Safety First and there are other ways to get around the issue as well. and that is to use a Main's Isolation Transfer formal like one of the ones uh shown here. It basically it does exactly what it says.

It isolates the mains 240 Vol in Earth 240 volt out isolated and that's designed to power your product under test. And it tells you here: isolate test equipment like crows or protect switch mode power supplies and things like that. but this can be used to isolate your oscilloscope. Uh, so that your oscilloscope is not Main's Earth reference anymore.

but that's not really the recommended way to do it. Your oscilloscope should remain for safety reasons. A Grounded. Uh, A Main's Earth grounded product.

So you should use the isolation Transformer to power your particular product under test. and this can be an essential bit of Kit in any Uh, test or or repair lab. And if you've got a USB product and you want to isolate that you don't want your USB cable to cause any issues, then you can get one of these, uh, rather, an obscure product a USB isolator. There's not too many of them around, but A Basically, it's got a transformer in there with some highspeed to uh couple.

not only the power lines, but the data lines as well at full USB uh speed. These things aren't particularly, uh, cheap. They're a fairly, uh, fairly Niche thing, but it isolates USB in USB out. The output is completely isolated data and power.

So there you go I Hope you found that interesting. So just be careful and watch out for Main's Earth reference systems next time you're probing here. Osilloscope, You don't want to blow up your oscilloscope. You don't want to blow up your probe.

And most of all, you don't want to blow up yourself. So if you like the video, please give it a thumbs up and I'll catch you next time.