Why do we need an isolation transformer to connect an oscilloscope?

My professor always insists that I provide power to an oscilloscope thorough an isolation transformer. What is the necessity of this? What is the risk if I don't connect it?

• – Adam Lawrence Jun 25 '13 at 12:51
• Your professor might want to review their understanding of earthing test gear. It's messy trying to convey this to a professor, I realize, but it's that or the possibility of toasted scope / roasted student. – Anindo Ghosh Jun 25 '13 at 13:03
• i think they are more cautious about the oscilloscope getting damaged (it costs around $12,000). – noufal Jun 25 '13 at 14:44 • It's a ridiculous assumption that floating the scope will somehow protect it, when user life is at risk and plenty of ways to kill the scope still exist. Isolated differential probes will keep both the scope AND the users safe. What short-sighted fools. – Adam Lawrence Jun 25 '13 at 14:48 • I think any scope that is so easily destroyed (beyond blowing a fuse) should cost a good deal less than$12000, or should be thrown in the trash. – Phil Frost Jun 25 '13 at 15:12

You should never float a scope with an isolation transformer! This is reckless and dangerous advice from your professor, and he/she needs a reality check.

The accepted procedure for doing work that requires isolation is to ISOLATE THE UNIT UNDER TEST, NOT THE TEST EQUIPMENT.

Why?

• It's much easier to remember that the unit under test is what's unsafe and needs cautious handling, not your oscilloscope
• If you hook a communication cable up to your floating scope (USB, GPIB, RS232), guess what - it's NO LONGER FLOATING. (All of these cables have earth-referenced returns)
• As soon as you connect that floating scope return to a potential, all of the exposed metal on the scope is now at that potential. Major shock hazard.

If you cannot float the unit under test, use an isolated differential probe to do your measurements, and keep both the UUT and scope earthed. No measurement is worth the safety risk.

A battery-operated scope may seem like a good idea in this circumstance, but only if it has dedicated isolated inputs. A battery-operated ordinary scope with non-isolated inputs will still suffer the problem of the exposed metal floating up to whatever potential you connect the ground to. That's why all of the manuals for the battery operated scopes clearly say "This scope must always be earthed, even if you're running off the battery" - if you choose to ignore this, it's at your own risk. A scope with dedicated isolated inputs should still be earthed as a good practice. It's essentially the equivalent of using external isolated differential probes with an ordinary scope.

I work full-time in power electronics and have tens of thousands of dollars of lab equipment at my bench. If anyone is caught floating their scope, the float is immediately corrected by the test engineering team, the means of float is seized (most often this is a line cord with the ground prong removed) - disciplinary action is a possibility. Numerous senior/principal engineers have fried their PCs and their entire set of GPIB-connected bench instruments by trying to float the test equipment and forgetting about the GPIB interface. (No one has died yet - thankfully)

• What about the battery operated scopes? – abdullah kahraman Jun 25 '13 at 13:09
• I'll add this to the body of my answer. – Adam Lawrence Jun 25 '13 at 13:45
• i think they are more cautious about the oscilloscope getting damaged (it costs around \$12,000). – noufal Jun 25 '13 at 14:45
• @noufal how much is your life worth to you? Your school probably only values you in terms of the additional money they'll get for your completing your degree; I asume your self worth is considerably higher. – Dan Is Fiddling By Firelight Jun 25 '13 at 15:27
• Many words about things, that engineers must not do, but no words about why... – user54579 Aug 9 '13 at 18:43

The alligator clip on the scope probe:

is connected, through the power cord, to Earth. If you clip it to something that isn't at Earth potential, you get a large current, and things go boom.

That said, an isolation transformer on the scope isn't the way to go. There's a reason the engineers built the scope like this, and it has to do with safety and noise performance. It's better to isolate the device under test, and let the scope work as designed.

Remember, that ground clip is also connected to the metal chassis of the scope. It's likely you will touch it. It's also likely you are touching Earth. So consider these circuits:

simulate this circuit – Schematic created using CircuitLab

scope1 allows dangerous current from the unit under test (UUT) to flow through you to ground. You die.

scope2 may be damaged, or may just blow a fuse, since the UUT has been accidentally shorted to ground. But, you will live, because you are a much higher impedance to ground than the ground pin on the cord. This is why it's called a safety ground.

If you just avoid clipping the ground lead to anything that's not at Earth potential (scope3), then nothing goes boom. Just be sure to not make any mistakes!

If you want to protect you, the UUT, and the scope against mistakes, then the right thing to use is an isolated, current-limited supply (scope4). Between the isolation and the safety ground, it will be harder (but not impossible) to kill yourself. If you do mistakenly short something out, the current limiting kicks in and probably avoids permanent damage to anything.

• just to confirm with my understanding... you are saying that the alligator pin of the probe is at the same potential of the AC main supply ground trough the oscilloscope. If it gets touch with any part of the test circuit whose potential other than zero, there a chance of current to flow throw this path to ground. This will damage the CIRCUIT/UNIT UNDER TEST. So its better to isolate the the test equipment under test. Am I right ? correct me otherwise... – noufal Jun 26 '13 at 4:50
• No. The problem with floating a scope is that it will be forgoten and still be floating the next time it's used. It's a dreadfull fealling to realise you have left some thing floating and the next guy could get a deadly shock. Float your unit under test and then make it safe at the end of your fault finding / measurements. It's a good habit to get in to. – Spoon Jun 26 '13 at 8:02
• Maybe it should be noted that the right thing to protect against equipment destruction here is a current-limited AC supply. Then, instead of vaporizing something (a fuse, if you are lucky), you just hear the relays in the supply clicking as the current limiting kicks in, you think "oops!" and remind yourself to be more careful about where you attach your scope probe. – Phil Frost Jun 26 '13 at 13:11
• @noufal If your unit under test has a bridge rectifier, hooking the scope ground up to the 'primary return' (the low side of the bridge output, which is usually the reference for all primary circuitry) without isolation means that the scope ground, the probe wire and the scope are shunting your bridge rectifier between neutral and primnary return, letting hazardous AC get in via the instrument (which isn't meant to handle power like that). – Adam Lawrence Jun 26 '13 at 15:50
• @noufal Hooking a scope up to a primary return w/o isolation is (in my experience) the #1 cause for scopes and test equipment getting fried. – Adam Lawrence Jun 26 '13 at 15:53

Both approaches are possible to apply (with different pro and contra). It is sometimes difficult to arrange an isolation transformer for a device under test, when the device consumes lots of power (power electronics driving a big motor XY kW). In such cases it might make sense to isolate the oscilloscope, since the isolation transformer can be very small and cheap.

LMA

While what you say is possible, you and your prof are not seeing all of the other damgers involved with that setup. The others are not wrong and they have told you how to do it a safer way in general as a habit if possible. As a general practice, your prof is incorrect or maybe you are misunderstanding your prof....most likely but cannot be proven.

I was taught the same thing... When it comes to AC/RF current, isolate the source from the O-Scope. WHY?

A scope is to:

1. View the wave form.
2. Many scopes can not handle the high voltages, you would blow up the scope.
3. You do not want to load down the circuit.
4. RF into a scope! Not a good thing at all.
Side note: Some scopes may have an isolation circuit built into the scope.
5. YES, you will have to do a bit more math when the isolation transformer is inserted - Not all xformers are exactly 1:1.
6. Remember the signal will be inverted if that is a consideration for your work.
7. Oh, read the scope manual and it will / should, tell you what the maximum input voltage can be.
8. Impotence, whoops - Impedance - Think back to way before transistors and the ability to simply construct a VOM with high input impedance: we had VTVMs to have high (100,000,000 Z). These devices were used so as not to load down the circuit being tested.

OK - I need to finish my coffee. More input please.

• On point 2: use the correct probe for the voltage. On point 4: scopes often go up to 1 GHz input or higher, which I think you will agree is RF. Or did you mean something other than radio frequency by RF? – Andrew Morton Jan 22 '17 at 15:48

The professor seems to be right, sometimes during circuit testing if it supposed to happen that we have to analyse the input mains waveforms and the system has 2pin power cord, then the phase and neutral orientation may change, since dso has its BNC ground connected to earthing the change in 2 pin cord may connect circuit neutral/phase used for reference of probe to earthing, leadind short circuit

• The professor is wrong. You ALWAYS use the isolation transformer on the device, NEVER on the scope. – JRE Jun 27 '17 at 4:49

Isolating an oscilloscope turns it into a battery powered device. Now think... Would you use a battery powered scope? Let me answer that for you.... YES!!!. So isolating a scope is completely harmless.

• Did you read the rest of the answers? A battery powered scope is designed differently than a mains powered scope so as to be safe despite floating. – JRE Aug 17 '20 at 15:31
• Luis, have a look at some high quality devices such as the Fluke Scopemeter series. Everything is isolated including the mains adaptor jack which is completely finger-proof. A regular oscilloscope has earth points, BNC connectors and now USB, etc., and perhaps a metal case. Connecting the probe earth-clip now makes the whole 'scope live. "So isolating a scope is completely harmless." Very wrong, I'm afraid. – Transistor Aug 17 '20 at 16:14
• @Luis Welcome to SE EE. While your answer has some merit it is very wrong from a safety point of view, which is the main point here. If you read the other answers you will see that using an isolating transformer increases safety in some areas, but can add unexpected dangers. – Russell McMahon Aug 18 '20 at 6:52