Safeguards to preventing damage to a scope?

[george graves]

I've asked someone this question once before - and it must be my thick skull keeping me form understanding it....

But form what I gather, you can damage an oscilloscope by current through the ground connector on the probe?  Is that right?  Could someone explain what kind of set up would cause a situation like that that would cause damage to the scope?

As I said, someone briefly described it - but I just didn't quite get it.  So....a quick drawing of the set up would be really appreciated!

[free_electron]

the ground terminal ( outer shell of the BNC connector ) is connected to the circuit board. that same ground is also connected to the ground pin of the scope ( chassis earth )

make a fat short with the ground lead of the probe and current will flow to the earthing . blowing the ground fault interrupter , but possibly frying the thin connection point as well.... circuit board damaged = dead scope.

the same goes for shorts between ground wires. one scope ground is at 12 volts , the other ground lead connected to 0 volts of the same source.

now you again send current across the board ... if that current is large enough it may damage that connection. damaged circuit board = dead scope...

simple.

[george graves]

Thanks! 

So if you connect the ground to anything that has current to give(that is above or below ground potential) - you could fry it?  How does floating supplys factor in?

The reason I'm asking is I want to measure some signals (RPM signal from the ECU specifically) on my car - to see what voltage and frequency it is.

[Psi]

How does floating supplys factor in?

If the supply is truly floating then connecting the ground clip to either side just moves that side to ground potential.
No current is needed to do this (ok, maybe fractions of pico amps) anyway, its fine.

The problems occur when you connect the ground clip to something you thought was floating or already at earth and it turns out not to be. One common way this happens is when you have a floating circuit but you have connect it to some other device. For example an audio cable from your device to an amp/PC, the audio shield is grounded through the amp so now your circuit isn't floating any more.

If it's a low voltage thing you're measuring, and you short that to ground, you may only see a small spark when touching the clip to the circuit. You should always watch for this spark when connecting the ground clip.

If you try and connect the scope ground clip to something at full mains potential you'll get a large bang/spark and it will probably blow a fuse or circuit breaker somewhere. If you're lucky the scope will still be ok.

The reason I'm asking is I want to measure some signals (RPM signal from the ECU specifically) on my car - to see what voltage and frequency it is.

How are you planing to run the scope in the car? an extension cord inside or an inverter off the car battery.

The car is floating so if you run the scope off an extension cord to a mains socket in the house everything is fine.
You could then safely connect the scope ground clip to any part of the car since it's all floating.

However, if you run the scope of an inverter that's powered by the car battery things maybe different. It will depend whats inside the inverter. I can't remember how they do earth if at all. (it may/maynot be fine as well)

[george graves]

Well I have an old APC Uninterpretable power supply thing with a dead battery(12 volts) - I was thinking of hooking that up to the battery for power.  Not sure if the noisy voltage oif the car would fry it - so I might pick up an inverter(not sure if I trust them either)

Here's another question:  Why don't scope makers just make the scopes themselves floating - and not tie the ground to anything?  Like a multimeter would be?

[Psi]

Here's another question:  Why don't scope makers just make the scopes themselves floating - and not tie the ground to anything?  Like a multimeter would be?

Some scopes are, like portable battery powered scopes.
But when the scope runs off the mains it really should be earthed for safety.
I seem to remember a scope company wanting to make a mains powered isolated scope but their lawyers wouldn't let them.
It's the sort of thing that the companies are afraid to do in case someone dies and they get sued.

[free_electron]

simply put an isolation transformer in the power line to the scope.
but beware !!! NEVER EVER EVER connect the probe ground to a live wire ! even with an iso transformer. you could kill yourself ! the scope may be floating but if you touch that terminal you become the 'bridge' across the iso transformer .. and then the current runs through you.. and 20mA across the hart is enough to make it lock up , cramp your fingers so you cannot release the wire and you die because your bloodflow stopped..

That's why we have things like differential probes and isolated probes. Those solve the problem on the probe side.

[alm]

Here's another question:  Why don't scope makers just make the scopes themselves floating - and not tie the ground to anything?  Like a multimeter would be?

Some are, the Tek TPS2000 series is the only one in a desktop form factor as far as I know. Isolating multiple high-bandwidth scope channels is a lot harder than isolating a DMM that in many cases only does ~50 S/s, as opposed to 1 GS/s for 2-4 channels in an entry-level DSO. Both isolating a DC-100 MHz signal and isolating the ADC/memory from the FPGA is hard. I believe most designs take the former route.

simply put an isolation transformer in the power line to the scope.

I hope you're not suggesting to interrupt the ground connection. This is strongly recommended against by scope manufacturers, and is a firable/fryable offense (you may either get fired or fried) in some workplaces. Even if you consider floating a scope safe under some very limited circumstances, than it's still a bad idea to recommend it to someone that does not have a firm grasp on electrical safety.

but beware !!! NEVER EVER EVER connect the probe ground to a live wire ! even with an iso transformer. you could kill yourself !

That's all fine, except that said circuit may not be expected to be live, either due to operator error or due to a fault. I'd rather have my scope emitting smoke when I make a mistake than my head. A scope with ground connection is a lot more foolproof than one without: it may show that your power supply is actually not short-circuit proof and kill the regulator, but it is less likely to kill the operator.

[vk6zgo]

BWD in Australia  made a special Oscilloscope for use in these situations.(they are in fact mains rated)
You still see them on EBay at times for a much higher price than you would pay for any other BWD or anything in fact with its otherwise
modest specs.

In the car stuation,it is easiest to run your 'scope from a mains extension cord.
DO NOT damage the power cable with the car door or similar,as you do not want the car to be "HOT" at mains voltage!
Yes,I know it is clumsy & inconvenient,but it is floating with repect to the car voltages,if it is really that important to you (but see my next point).

In a car,the best procedure is to connect your probe earth clip  to a car "earth" point (the car chassis/body & anything electrically bonded to it is  by convention.regarded as "earth" although the whole car is insulated from the actual Earth).
Any data regarding ,waveforms,dc voltages,etc on the Internet or elsewhere will follow this convention in any case.

[george graves]

Excellent - thanks all.

Any thoughts on using an off-the-shelf consumer inverter?

[EEVblog]

*scurries to the whiteboard*...

Dave.

[sonicj]

Excellent - thanks all.

Any thoughts on using an off-the-shelf consumer inverter?

I've done it. The output of the inverter was completely square & my Rigol DS1052 didn't seem to mind.
-sj

[grumpydoc]

> simply put an isolation transformer in the power line to the scope.

No, please don't - you'll risk making the entire chassis of the scope live. I know that you then go on to point out that connecting the 'scope ground to the live supply is a bad idea but it's too easy to connect to a point that you thought was OK but isn't.

It might not be you that touches the scope chassis.

When using an isolation transformer it feeds the device under test, NOT the 'scope. The 'scope can then stay grounded and not be a nasty surprise for anyone.

[EEVblog]

I just shot a video on this, editing now...

Dave.

[mikeselectricstuff]

The correct method is to isolate the thing you are testing and then ground the scope to the  signal ground.
e.g. if looking at the mains side of a SMPSU you run the PSU in an isolation transformer and ground the -ve HVDC rail.
For safety it's good practice to  make a seperate connection from the unit ground to mains earth, so you're not relying on the scope ground lead.

[free_electron]

There is nothing wrong with using an iso transformer PROVIDED YOU KNOW WHAT YOU ARE DOING.

There are even fully isolated bnc cables and probes for such circumstances.  There simply are situations where you can't go without. Take an H bridge in a motor drive: 400 volts dc( im talking frequency drive ).
Your task is to visualize the gate drive voltage of the top transistor.... You cant do it because the scope does not have enough dynamic range to show a 2 volt signal superimposed on a 400 volt offset.
The solution ? There are two : use a high voltage differential probe or float the scope...

Tektronix makes a scope that has separated and isolated inputs. 4 channels. The input bnc is not connected to chassis ground. Each input channel has its own psu digitzer and memory and communicates over an optolink with the main processor.

[EEVblog]

I just shot a video on this, editing now...

As usual, was going to be 5 minutes, but ended up being 25min!

Dave.

[george graves]

You rock Dave! 

[T4P]

I just shot a video on this, editing now...

As usual, was going to be 5 minutes, but ended up being 25min!

Dave.

Informative videos are good .... 

[saturation]

Most automobile waveforms, daresay all, are < 100kHz, and easily viewed with cheapo scopes like the Velleman class.  Rather that struggle with your bench scope, extension cords etc., buy a true floating DSO that is capable of reading auto waveforms, the Vellemans' will run you between $100-200 US, tops.

Thanks! 

So if you connect the ground to anything that has current to give(that is above or below ground potential) - you could fry it?  How does floating supplys factor in?

The reason I'm asking is I want to measure some signals (RPM signal from the ECU specifically) on my car - to see what voltage and frequency it is.

[EEVblog]

Dave.

[george graves]

OK - totally makes sense now!  Now I see says the blind man.

So you really think that the probe ground lead would blow before a PCB trace on the scope does?

[sonicj]

So you really think that the probe ground lead would blow before a PCB trace on the scope does?

The BNC has a low resistance connection to the metal chassis. The metal chassis is the connection to ground. The clip lead will eventually pop like a fuse, but likely not before you **** your pants.

We should ask photon to test this for us. 
-sj

[george graves]

Got it - thanks!

[free_electron]

USB isolators are not that rare. analog devices has a single chip one . little 14 pin thing. I used one a couple of weeks ago on a design i'm working on....
Maybe an idea for a product to add to the eevblog store. I can send you the layout and schematic for it.

[MikeK]

Thanks for the video, Dave.  Quite sobering.  I'm now wondering if I've done some stupid things with my scope.

[alm]

There is nothing wrong with using an iso transformer PROVIDED YOU KNOW WHAT YOU ARE DOING.

People who knew what they were doing have been killed doing exactly this. Granted, this was with the big clunky analog scopes with much more exposed metal, but they had actually built a plastic enclosure for the scope for insulation. An engineer screwed up and electrocuted himself.

There are even fully isolated bnc cables and probes for such circumstances.

These are designed to work with isolated BNC connectors, as found on the Fluke Scopemeters and Tek scopes with floating channels. Not much point using with metal BNC connectors and thin plastic knobs that are not designed to protect the user from dangerous voltages.

  There simply are situations where you can't go without. Take an H bridge in a motor drive: 400 volts dc( im talking frequency drive ).
Your task is to visualize the gate drive voltage of the top transistor.... You cant do it because the scope does not have enough dynamic range to show a 2 volt signal superimposed on a 400 volt offset.
The solution ? There are two : use a high voltage differential probe or float the scope...

Yes, this is a hard problem. The slew rate of many differential probes may also be too low. Capacitance to ground will be an issue for a floating scope. It won't have any guarding if it's designed to be grounded. Still, even if you as an experienced engineer decide that floating the scope is the only solution and take precautions to reduce the risk, then it's not a good idea to suggest the same to unexperienced hobbyists.

Are you able to pull this off without having the OSHA people scream at you?

USB isolators are not that rare. analog devices has a single chip one . little 14 pin thing. I used one a couple of weeks ago on a design i'm working on....
Maybe an idea for a product to add to the eevblog store. I can send you the layout and schematic for it.

Yes, these work fine (I don't have the p/n memorized, but searching for iCoupler should find it). I would put such a circuit in an enclosure for obvious reasons if you're using it with high(ish) voltages (> SELV). I believe there are two second source manufacturers, but I keep forgetting their name. These are all limited to USB 1.1 or USB 2.0 full speed, last time I checked. USB 2.0 high speed is much less common and probably much more expensive.

[vk6zgo]

There is nothing wrong with using an iso transformer PROVIDED YOU KNOW WHAT YOU ARE DOING.

There are even fully isolated bnc cables and probes for such circumstances.  There simply are situations where you can't go without. Take an H bridge in a motor drive: 400 volts dc( I'm talking frequency drive ).
Your task is to visualize the gate drive voltage of the top transistor.... You cant do it because the scope does not have enough dynamic range to show a 2 volt signal superimposed on a 400 volt offset.
The solution ? There are two : use a high voltage differential probe or float the scope...

Tektronix makes a scope that has separated and isolated inputs. 4 channels. The input bnc is not connected to chassis ground. Each input channel has its own psu digitzer and memory and communicates over an optolink with the main processor.

Back in the day,Tektronix made plugins for the 545,etc,which had an offset control,so you could look at quite small signals superimposed on,from memory,up to around 400 volts dc.
This was a much more common requirement in the days of tube equipment,when anode voltages were anything from 200 volts upward,depending upon the application.
I assume this would work in the application you referred to.
It is a shame that they have stopped making things like that,although the special isolated design you described does the same job.

What I can't quite understand is the OP's requirement for a floating device for automotive use.
All the automotive waveforms I have ever seen were expressed  w.r.t. the car chassis.
It is fairly easy--if you stick your earth clip on a car body,engine block,etc.you can use the probe to look at waveforms with no chance of damage to your 'scope.
In any case,most circuitry in cars is protected by fuses,so you will probably blow the fuse & protect the 'scope.
Even if you are careless enough to put the earth lead on the "hot" side,the 'scope body still has to be touching the chassis to cause problems---stick a newspaper under it,& you have a "floating" instrument as far as the car supply knows.
Under normal conditions the car "earth" & the mains earth have no connection.*

Looking at mains voltages direct is another story altogether,& special precautions need to be taken.

*If you are running your Oscilloscope off the battery,using an inverter which connects  the earth pin of the output socket back to the car chassis,(which is fairly uncommon) the 'scope body will be at the same potential as the car body/chassis,so it would be important to get the probe earth lead & probe tip the right way around.
If you have a DMM,you can check this at the outset.
If you are paranoid,you can also use the DMM to check any points you wish to probe with your 'scope,

[EEVblog]

USB isolators are not that rare. analog devices has a single chip one . little 14 pin thing. I used one a couple of weeks ago on a design i'm working on....
Maybe an idea for a product to add to the eevblog store. I can send you the layout and schematic for it.

That's actually been on the to-to list a long time...

Dave.

[EEVblog]

So you really think that the probe ground lead would blow before a PCB trace on the scope does?

The BNC is usually bolted directly to the chassis which has a proper big hunking 1sqmm short cable to the IEC connector earth pin. Teardowns like my Agilent one should show this.
I've seen scope ground leads vaporise!

Dave.

[amspire]

  There simply are situations where you can't go without. Take an H bridge in a motor drive: 400 volts dc( im talking frequency drive ).
Your task is to visualize the gate drive voltage of the top transistor.... You cant do it because the scope does not have enough dynamic range to show a 2 volt signal superimposed on a 400 volt offset.
The solution ? There are two : use a high voltage differential probe or float the scope...

I would not try and float a mains powered scope to solve this.

High quality current probes are brilliant, and if you know the circuit, you can often work out what is happening via the current waveforms only. I have also used a current probe to measure voltage by adding a resistor across the switching circuit and measure the current through the resistor. There are issues with this method. Otherwise you have differential probes or battery powered scopes.

The problem with floating a mains powered the scope is if you connect a floating scope to a circuit with the kind of slew rates you can get with MOSFET switching circuits, you can easily get current pulses through the scope supply circuit via the transformer capacitance of hundreds of amps peak. These are currents the scopes were never designed for. Even if the scope survives, the spurious waveforms near the switching point will be very misleading. Just as an example, say if you have  rectified 240VAC which is 336V switched in in 20nS and you attach a scope earth that has 25pF transformer capacitance to the mains. That calculates out to be 420A peak current going through the scope circuitry.

The Australian-made BWD Powerscopes did solve this problem brilliantly with four 600V differential channels if you can live with the 50nS rise time.

Richard.

[pickle9000]

The early TDS 210's had an undersized grounding strap on the chassis. They found out about it after it was in service (and melted down) and had a recall on them. Of course they found out the hard way and there was no indication on the scope that it had failed. Could have been a big problem.

...mike

[DJKA]

OK, so what about connecting the ground clip via some fuse like 200mA? If something goes wrong the circuit and/or scope will not be hurt...

[Bored@Work]

OK, so what about connecting the ground clip via some fuse like 200mA? If something goes wrong the circuit and/or scope will not be hurt...

200 mA is still enough to kill you. 200 mA the wrong way is also enough to kill a lot of circuits. And it ruins high frequency measurements.

[DJKA]

How should the fuse ruin HF measurements? Please explain me as for a beginner. Although I've never had a scope before on my hands, I presume that the ground lead is a simple piece of wire, so is the fuse(a thin wire made of lead). What about say 10mA fuse?

OK, so what about connecting the ground clip via some fuse like 200mA? If something goes wrong the circuit and/or scope will not be hurt...

200 mA is still enough to kill you. 200 mA the wrong way is also enough to kill a lot of circuits. And it ruins high frequency measurements.

[free_electron]

USB isolators are not that rare. analog devices has a single chip one . little 14 pin thing. I used one a couple of weeks ago on a design i'm working on....
Maybe an idea for a product to add to the eevblog store. I can send you the layout and schematic for it.

That's actually been on the to-to list a long time...

Dave.

Right, expect mail... Altium pcbprj ok ?

[T4P]

How should the fuse ruin HF measurements? Please explain me as for a beginner. Although I've never had a scope before on my hands, I presume that the ground lead is a simple piece of wire, so is the fuse(a thin wire made of lead). What about say 10mA fuse?

OK, so what about connecting the ground clip via some fuse like 200mA? If something goes wrong the circuit and/or scope will not be hurt...

200 mA is still enough to kill you. 200 mA the wrong way is also enough to kill a lot of circuits. And it ruins high frequency measurements.

A fuse forms a RC filter with the scope .

[G7PSK]

What would be the situation if an elcb plug was used on the oscilloscope if you got the connection on the probe wrong it would be see as an earth fault and trip the breaker which normally are around 30ma.

[T4P]

What would be the situation if an elcb plug was used on the oscilloscope if you got the connection on the probe wrong it would be see as an earth fault and trip the breaker which normally are around 30ma.

ELCBs introduce additional resistance and an additional point of failure into the earthing system. < Taken from wiki, unless it's an RCD which is a detector of imbalance which wouldn't work either.

[bruce273]

This may be unrelated but from the video if a scope's mains ground was tied to neutral on the secondary side of the transformer and the circuit to be probed for some reason, like the plug was incorrectly wired, couldn't that cause a mains short through your probe? Also isn't this method of wiring often illegal, at least in Europe, or is that when the primary side is tied to ground?

[vk6zgo]

What would be the situation if an elcb plug was used on the oscilloscope if you got the connection on the probe wrong it would be see as an earth fault and trip the breaker which normally are around 30ma.

ELCBs introduce additional resistance and an additional point of failure into the earthing system. < Taken from wiki, unless it's an RCD which is a detector of imbalance which wouldn't work either.

Actually,RCDs do work!---In the mains case,& when both  the 'scope & the DUT are fed from the same RCD.

Back in the day when I was fixing a lot of things which needed isolation transformers,I got caught out one day,& plugged the DUT directly into the mains ,with the 'scope probe across the the 240v input to the switchmode-.
Yes,it wasn't my day------the earth clip lead was on the active side.
Zap!! followed by the desk RCD dropping out for my bench & the one in the next room.
Much good natured ribbing ensued!

The thing could have dropped out due to a current overload,as it incorporated that as well,but the RCD is usually much faster in operation.
The two benches were regarded as one device,as far as the RCD was concerned,so there would have definitely been an unbalance in this case.
It would be fairly unusual in a bench situation for the DUT & the 'scope to be  fed from different RCDs.

The first time I encountered an RCD,I was working on a piece of equipment,& wished to check for presence of mains.
Being a bit lazy,& because the neutral was a bit hard to get at,I hung the AVO8 between active & the earthed chassis.
Something went clunk! & the mains disappeared leaving me mystified,till I discovered that the "Sparkies" in my absence, had fitted the RCD.

So there's something interesting,an analog VOM draws enough current to unbalance an RCD & drop it out.
A DMM,or "scope won't,so for a quick check for the presence of mains,you can measure between active & earth.(With in the 'scope case,your probe earth clip firmly attached to a point connected to the mains earth .)

[T4P]

Alright thanks. Looks like the same RCD on the bench is the answer, anyways, my house's ELCB is getting pretty slow already to the point that the huge 100amp breaker outside will trip before the ELCB.

[CampKohler]

Dave just made a very detailed video on scope grounding: . Read the comments, too.

[george graves]

   Hehhe- ya don't say!  Yes, he made the video in response to my question(this post).  Heheheh -I'm the "young player" he mentions.